Audi S4 LED Boost Gauge

Audi S4 In-Dash 0 - 25psi Boost Gauge (20 & 30 psi Also Available)

Before and especially after additional performance modifications have been completed and installed, it is imperative we know whether the Electronic Control Unit/Module (ECM/ECU) is providing the engine with enough fuel under high boost and/or load. You will want to know how much boost your turbo chargers are providing you with. For a start all the cash you spent on the ECU can be quantified if it lives up to the boost specs. Optionally, if you have gone the electronic boost controller route, you will want to see the boost information at eye level. Boost must be monitored closely when doing turbo upgrades and other inlet manifold pressure related performance modifications. Now, you can go and buy an "off the shelf" boost meter from your favorite after market parts store, and install it somewhere in the car where it is guaranteed to look like an "afterthought" and join the "Boy Racer" brigade. You could spend copious quantities of cash on an integrated system and loose those beloved S4 Logos in your instrument cluster, or you can do a proper job, with the satisfaction that you did it all yourself.  And hey, that is half the fun, isn't it?.  We chose to build and design our own Integrated 1-25 psi Boost Gauge integrated into the Tachometer of the stock instrument cluster. It uses the car's stock MAP sensor readings to give real time boost pressure readings on 20 1.8mm LEDs. LED number 1 is 1.25 psi and so on up to 25. It is accurate to +/- .125 psi when calibrated correctly. It can be easily adapted to read virtually any boost pressure, the only limitation is that of the selected pressure sensor. It can use any 5v pressure sensor. This article focuses on utilization of a selected computer grade 25psi sensor that we sourced from an electronics supplier. The parts we provide are for sea level operation but can easily be calibrated for other altitudes. Our Boost Gauge integrates into the Dash (Check out this page for a complete tested Boost Control Unit) so it looked as if it was there from the factory. This takes a lot of planning and measuring, in the long run it will look perfectly acceptable to the most discriminating connoisseur.

Disclaimer (Please Read it Thoroughly)

By connecting and using this Custom Gauge you agree that the designer, author, provider or re-seller is in no way responsible for damage to persons, a vehicle or vehicles as a result of installing this after market gauge or meter.

Liabilities: The designer, author, or re-seller assume no responsibility for the modifications you do on your car. Although to our knowledge the instructions are accurate, this includes any errors that the instructions may contain, any differences between your car and the one(s) showcased, as well as any errors in the instructions. If you don't feel comfortable working with automotive electrical wiring we suggest you let someone with some experience do these modifications. The installer should posses a good understanding of the electrical system of the vehicle being worked on.  Please double check all your connections and ALWAYS work with the car battery DISCONNECTED.  Always keep your safety and that of others as the highest priority.

Warranties: The modifications described in this document could potentially void any manufacturer's warranty you may have left on the car (not likely, but possible nonetheless). You assume all responsibility if your car is still covered by warranty.

Legalities: Laws in your country/state/region/whatever may not allow you to modify systems such as these on your car, the penalty being at least a fine. Please check with your local authorities to be on the safe side.

We assume NO RESPONSIBILITIES [emphasis added] what so ever for any accidents caused by the driver gazing at this tremendous addition to the stock instrument cluster while they should be concentrating on the road and their driving. We always endorse good and safe driving practices at all times.

License Agreement: By purchasing this unit,, the purchaser, Installer or builder (referred to as the purchaser) agrees to the following:

·        This license is for the purchaser of the unit only

·        The purchaser is NOT permitted to reverse engineer and/or re-sell any units conceived or provided by IMP Concepts or a certified re-seller to any other person or persons, individual or company without written permission from IMP Concepts.

·        The purchaser is strictly forbidden to publish any associated plans or diagrams for these or any of the designer’s or author's designs for any purpose whatsoever, without written permission from the designer and author (IMP Concepts)

·        Publishing any information about this unit, other than general descriptive information (with reference to the designer and author and their web site) is also strictly forbidden.

·        Additional units must be purchased for use by other individuals.

Any Violation of this agreement will be prosecuted to the fullest extent of the law.

With that, good luck, and have fun with this Custom Made Meter.

The first thing you need to do is acquire an electronic Boost Gauge as described in this article (And on this page).  You will need to be reasonably proficient with a soldering iron and lead solder, as you will be using them a lot. (Or you could get someone else to do it)

This article is for the real "Do It Yourselfer", we designed and built our own Custom In-Dash Integrated Boost Gauge, and that is what this article covers.

Complete Dash Mounted in the Car

Main Boost Control Unit

As you can see by the layout, there are quite a few components. The signal comes into the unit from the computer grade pressure sensor we selected. Remember to DISCONNECT the BATTERY BEFORE beginning any electrical work on the vehicle. We simply spliced into the car's vacuum line with a tee connector and connected a 1/8" NPT pipe which we ran into the car's drivers bay behind the dash. We used Pin 1 of the large plug of the ECU for our ground and Pin 3 for switched +12v. In reality the power can come from any location, but please ensure you install a dedicated in-line 1 or 1.5amp fuse in the main +12v power line.

The "Dimmer" wire needs to have +12v when in Dim Mode, and ground or floating when in Daytime or Bright Mode. This can be done in many ways. There is a Fuse in the fuse box that is marked "Headlight", you can tap +12v from here. Or, from under the dash you can take the dimmer wire from the power connector harness and splice it into the GREEN/RED or GREY wire on your headlight switch harness. This is the wire that activates the dimming circuit on the control unit when it receives a +12V signal from the parking or main light circuit.

The Programmable Peak Hold wire needs to have a switch and a pot or resistor connected to it. Grounding this pin will disable Peak Hold.

Boost Gauge Main Control Unit, Not To Scale

Please refer to the following layout diagram for all the interconnection and external wiring information.

Boost Gauge Main Control Unit, Interconnection & Wiring Layout, Not To Scale

Installing The Boost Gauge in the Dash

This is the part we were quite concerned about doing, and a quick call to our friendly Audi dealer, confirmed that to replace the dash/cluster was an expensive option, but it reassured us that the parts we were about to potentially destroy were available as replacement parts. Armed with this "insurance" we proceeded very cautiously. The following steps involve drilling and cutting the dash assembly. Unless you are pretty good mechanically, we recommend you simply read this for interests sake, and put a commercially available boost gauge somewhere in your car. If you do choose to remove the dash assembly for any reason, do not forget to DISCONNECT THE BATTERY.

Step 1 - Remove The Dash Complete With Gauges and Wiring

This is a very easy procedure. You can refer to the car's Repair Manual for details. We proceeded as follows:

• First remove the fuse box cover on the left of the car's cockpit. It simply pulls off. Put it to one side and store.
• Remove the two interior color coordinated covers that hide the lower driver's side trim retaining screws.
• Now there are 4 hex head screws that hold this piece of trim on. One is to the lower left of the fuse box, two are behind the color coordinated covers you just removed in the previous step, and the last one is tucked away under the dash to the far right rear of the lower trim panel.  Once all these are removed the trim piece lowers and comes off quite easily.
• Remove the small piece of trim just above the steering column, it just snaps off, revealing two Torx number 20 screws that hold the dash assembly in place.
• Remove the two Torx screws that hold the dash in.
• Put the cars steering column tilt in the lowest position and lock it.
• Gently pull the dash forward, just enough that you can get your hand in the left side, not too far because there are three plugs that need to be removed to get it out, and you don't want to stretch them.
• Remove the three plugs on the back, there are little clips holding them in but they are easy to do if you have a little bit of finger nail left. (After all the chewing you did contemplating this modification)
• Remove the dash assembly by pulling it forward and tilting it down to the left. Place it on a clean protected workbench. (We used towels to protect the dash form unnecessary scratches)

Now that you have removed the dash assembly, the fun can begin.

Step 2 - Dismantle The Dash To Remove The Tach Dial

This is quite easy, you just need a few nifty little pieces of plastic or metal. We used 1/16" plastic sheet.  We use a lot of this plastic sheet, where do we get it? you may ask.  Well, you know those three ring binders you use all the time for files, at the front and rear there is usually a sheet of plastic, either white or black used to support the pages.  That is what we use.

• Cut six pieces of 1/16" plastic about 3/8" x 1"
• Place the dash assembly face down on piece of cloth.
• Remove the two Torx Number 10 screws from the rear of the unit.
• Using a small screw driver, slowly  lift the six the plastic retaining clips and slide a single cut piece of plastic under each one.
• There are two additional clips on each side of the unit, using a small screwdriver release them very carefully and pop the cover off. Store it in a clean safe place.
• Remove the tachometer needle by putting a medium screwdriver under it and bopping it upwards. Be careful not to damage the dial.
• Now the Tachometer dial can be removed by turning it anti-clockwise and lifting it off the keyway.
• Get ready for the scary part coming up next.

Step 3 - Measure, Mark up The Tach Dial and Prepare for Drilling LED Mounting Holes

This is the very tricky bit. It can be done a number of ways, this step describes how we went about doing it. We chose to locate our 20 Boost Gauge LEDs (1.8mm ) in the Tach dial. We made a semi-circle of LEDs between the stock Tachometer numbers and the indicator lamps. The measurement was not that easy.  A template was created after many, many iterations and here is what it looks like. It includes the layout from another project (The Air Fuel Meter) we were installing, which is described in another article.

Sample dash Drilling Templates

Here is what we did:

• Print the template to scale on thick bond paper. (Best quality) Only print 1 per page to keep the scale.
• Cut the template's outer circle as close as possible to the outer edge, Cut out the 1/2" inner circle (Fine Blade Hobby Knife) cut out the small part circle key on the left, cut the top piece out and make a "V" in it for location purposes. See Illustration Below:

Dash Drilling Template, Cut To Size and Stock Tachometer Dial

Dash Drilling Template, Cut To Size Aligned on Stock Instrument Cluster

• Overlay the cut template over the OEM dial and secure with tape to ensure absolutely no movement what so ever. Note the location of the key and the 4000 RPM Marker, this marker is exactly in the center of the dial as far as we could measure. We used a 1/2" diameter socket in the center hole to align them exactly.

Dash Drilling Template, With Notch & Key Detail, Overlaid onto the Stock Tachometer Dial

• Place the overlaid dial and template on a hard wood surface and using a small center punch lightly punch the holes that you wish to drill. you only want to mark the dial not punch right through it.

As you will see in the photos that follow later in this article, it was a tight squeeze.

Step 4 - Drilling the Tachometer and Applying the Lettering

As we only had one chance to get this right, we checked and double checked our measurements, and then we checked them again. The drill we chose was a 0.070". of an inch. (a tick smaller than needed for 1.8mm LEDs) It was the Brad type that made a very small pilot hole prior to the actual drill size. We used a drill press at 2340 rpm.

• Using a .070" drill and a speed of over 1750rpm, carefully and SLOWLY drill out the holes. We sandwiched the dial in between 2 pieces of 1/16" plastic sheet to give it some support and placed another template on top of the whole assembly.
• Remove the templates and supports, you should have something that looks pretty good.
• Stand back, admire your work, and congratulate yourself on the fine drilling job you just did...... You deserve it. In fact, go get a beer.

As you will see in the photos that follow later in this article, it was a tight squeeze.

Tachometer Dial, Drilled for Integrated Boost Gauge (180* & 206* Styles)

The above photo shows all 30 1.5mm LED holes, 20 in a semi-circle for the Boost Gauge.  As you can see it is a very tight fit. But we were really pleased with the out come.  Now onto installing the LEDs and the electronics.

Now you will want to apply the Dry transfer lettering to your newly drilled tachometer in order to get that professional look and feel. This is not too hard to do providing you take your time and do the job properly. Make sure your hands are clean and free from grease and dirt.

Here are the steps we took to do our example:

• First take the dial and stick it to a sheet of clean paper with painters masking tape. Use the not too tacky kind (the cheap stuff) so as not to leave an adhesive residue on the dial.  Test it first in an inconspicuous area of the dial.
• Select the area where you want to place the lettering and use a strip of masking tape to get a straight line to use as a guide.

Tachometer Dial Masked for the "Boost" Title Lettering

• Carefully align your lettering on the masking tape straight edge and gently apply the lettering using a ball point pen or a special dry transfer lettering burnishing tool.

Tachometer Dial Masked, with "Boost" Title Lettering

• Repeat the above process for all the lettering you have to apply, being very careful not to mask the lettering. Otherwise it WILL come off.

Tachometer Dial Masked, with the Numerals

Step 5 - Installing the LEDs

This is the most time consuming of all the steps.  It is not hard to do, just slow. As you can see in the above photo of the dial, we applied 12 point white dry transfers to the dial. You can choose your own numbers and letters, just be careful not to obscure the warning decals on the dial.  The decals are clearly visible when you hold it up to the light.

You will need the following items:

• A tube of colored adhesive household caulk, we used a non silicone based white tub and tile type that you can get from your local hardware store. You want a color as opposed to a clear so that when the dash backlights are on, you do not get light through the LEDs from the back when they are off. Any color will do.  Black would block the light, so it is a good choice also.
• A small round needle file so you can gently file the holes for any slight discrepancies in the LED diameters.
• A clean lint free cloth.
• Three small (approximately 1" x  1/4") pieces of 1/16" thick plastic or wood. We used 1/16" plastic sheet.  But you can use thin modeler's type ply wood, just make sure they are clean and have no sharp edges.
• Some white lettering 12 point Dry Transfers, these can be pickup up from a local hobby shop.
• A lot of patience.

Tachometer Dial, with Indicators Installed Integrated Boost Gauge with Lettering Applied (B5 & B6)

   Here is how we installed the LEDs.

• Make sure you hands are clean and dry. And when handling the face of the dial us a clean cloth or lint free tissue paper.
• Place the dial face down on a flat surface, use the pieces of 1/16" plastic or wood to raise it off the surface. Make sure the LED holes are not obscured.
• Line you 20 LEDs in the color order you wish to have them, so they are close at hand.
• Offer each one up to their respective position and check to see if they can be set to protrude 1/16", they should just touch the flat surface and should only be tight enough to stop them from falling over.  You may need to carefully file each hole using the needle file for the correct fit. Do one at a time, remembering which LED goes in which hole. Sometimes the different color LEDs vary very slightly in diameter.
• Now, one at at time position each LED and apply a small bead of the caulking round it so it stands up on it's own. (We used a small toothpick to spread the caulk evenly) Make sure you get all the cathodes and anodes (the anode is normally a little longer than the cathode) on the same side because you will have to join them together later. We found that setting 3 or 4 at a time then letting the caulk dry was the best way to go. Once they are all set, leave for 24 hours to dry. you can cut off excess caulk with a small razor blade or a hobby knife.
• We painted the caulking black once it had dried, just to be 100% sure that no light got through. The photo on the right below shows the rear of the dial. The Boost Gauge LED caulk s has been painted and the LED pins snipped ready for wiring.  The RPM numbers look as if they are obscured in the photo but it is only the camera angle.
• Stand back, drink another beer and admire you results again. Now doesn't that look sharp?

Tachometer Dial, with LEDs Installed for Integrated Boost Gauge

Step 6 - Wiring Up The LEDs

You can do this any way you wish. We used 28 swg stranded wire with a very thin insulation.  You need as thin wire as you can get.  28swg is the thinnest we wanted to go and stranded wire is more flexible than solid, although there is no real reason that solid wire could not be used. We wired 20 LEDs one by one , starting with the closest to the exit point.

• Cut all the Anode Leads (they are usually longer than the Cathode) on the LEDs to about 1/4" in length.
• Bend them at right angles to each other, overlapping them slightly, so you can solder them together. The last one will have to be bent in reverse of all the others so as to be soldered to them.
• Solder them all together so they are in a nice straight line as close to the caulk surface as you can get. Ours were about 1/4" from the rear face of the dial to the top of the leads when bent.
• Bend all the cathodes at right to each other, the last one will stick out a little. Once bent there should be only about 1/8" of lead after the bend. Line them all up so there is no contact between them.
• Then we wired each of the leads to each LED one by one. Make sure they originate from the right of the dial as you are looking at it from the rear. All LEDs were wired from left to right. So all the wires ended up on the right hand side of the tack dial when facing DOWN. This will actually come out of the left hand side of the cluster when assembled.
• There are actually 21 wires as one is for the common anode wire. Make sure you remember which is LED 1 - 10, and 11 - 20. If not, mark wire number 1 and the common anode wire.
• You may want to shape the wires into a loom and tie them off with a very small tie wrap or tie wire. (The type that you get on power cords of new appliances) You will find that most 0.1" ribbon cable will roll up to about a 1/4" in diameter.  This is tricky and should be done so that the wires do not foul on any parts of the instrument cluster.
• Bend the loom down so it runs parallel to the Clock Cut Out about 3/8"towards the edge of the tachometer dial right side when facing down.

Tachometer Dial, with LEDs Installed and Wired for Integrated Boost Gauge

• Getting the Dial back in the cluster is quite tricky. You cannot rotate it as the LEDs are in the way. What we did is place it done flat, and gently pressed it onto the keyway, being very careful not to damage our lettering.  Actually we pressed a little too hard on the letters and had to redo a couple of them. You will find it will eventually clip down into place. This is the only really tricky part of the installation.

Step 7 - Rebuilding and Testing the Instrument Cluster

Depending on what wires you used for the LEDs you may need to cut some plastic out of the White Tachometer backing. We ended up cutting a little as shown in the photo below. It was just easier as all our wires for both the Boost Gauge and the Air Fuel Meter had to fit through the gaps. If you are only doing one gauge it will should not be required. The black instrument cover will however need to be cut so the wires can get through.

Tachometer Cluster Assembly, Showing Cuts for Wire Routing for Integrated Boost Gauge

This is what we did:

• Located the wires evenly down the left hand side of the Clock Assembly under the tachometer dial. (Looking down at the cluster.)
• Offered the cover onto the cluster and marked where we wanted to have the wires come out. It happened to be located right below where the wires came out of the tach to the left of the clock assembly.
• A hole/slot was then cut out of the cover at the marked location. It turned out to be about 3/8" in diameter open at the bottom (see picture below) It was easily cut out with a small  hack saw, then shaped with a file.
• The cables, now rolled up at this point were routed around the clock assembly and through the hole/slot. This worked out fine for us.

Cluster Fasia Assembly, Showing Cuts for Wire Routing for Integrated Boost Gauge

Here is a nice photo of the finished tachometer mounted in the car's cluster ready to go. If you click on the photo you will see it in action.

Tachometer Installed in the Cluster and Powered up, Click on photo for In-Action Video

Step 8 - Testing & Wiring Up The Boost Gauge Control Board

This part is pretty straight forward, if you are electrically inclined that is. The LEDs were wired to the Printed Circuit Boards (PCBs) using the wires that we wired in Step 6. It is best to keep the wires as SHORT as possible to reduce any interference or noise, although we never experienced any problems and the circuit is compensated for longer LED leads. 12" should do the trick. This ends up being 5" outside the cluster, this is more than enough so the control box can be tucked away below and to the left of the cluster once installed in the car. It is a good idea to check each LED before you finally solder them to the PCB. If you use a 2k or 3k Ohm resistor with a 12v power supply or a 9v battery will also work), you can easily check each wire and establish which LED it represents, then wire it to the appropriate pin on the PCB. Simply connect the +12v lead (or the +9v from the battery) to the common anode wire you marked earlier, then connect the -ve lead to the resistor and touch each LED wire with the other end of the resistor. The appropriate LED will light up dimly in the cluster to let you know all is OK. If you get problems with multiple indicators lighting or an indicator does not light, now is a good time to diagnose and fix it.

Boost Gauge Main Control Unit, Interconnection & Wiring Layout, Not To Scale

It is best to test the whole setup before you put the cluster back in the car just to make sure you have everything all in order. It is far easier to troubleshoot a problem when the cluster is still on the workbench as opposed to being in the vehicle.

Perform the following:

• Turn the Zero Adjustment Control (P1) fully Clockwise
• Connect  +12v to the RED main unit Power wire (Pin 5 of the I/O array) and Ground or -ve to the Black wire (Pin 1 of the I/O array)
• Take pipe from the car's vacuum line and connect it to the computer grade pressure sensor nipple, it is 1/8" NPT.
• If the some lights are on this is OK as the unit will need to Zero'd out.

Final adjustments can then be made with minimum of contortion on behalf of the owner. Referring to the Circuit diagram, wire the 20 LED wires and the +12v LED power to the Boost Gauge Control Unit. Make sure you get the correct bank on the correct driver chip. Then the PCB was placed in an enclosure and tucked behind the dash. There are three other wires that need to be connected to the car in order for the Boost Gauge to function correctly. The + 12v in from the battery (Red Wire).  Make sure this is wired so it comes on only when the ignition is on. Ground (Black Wire), this can go to any solid car ground. Try to use a common ground used by other devices on the car so you know it is good. Then the Signal Wire (White Wire) goes to the MAP sensor on the car.

The "Dimmer" wire needs to have +12v when in Dim Mode, and ground or floating when in Daytime or Bright Mode. This can be done in many ways. There is a Fuse in the fuse box that is marked "Headlight", you can tap +12v from here. Or, from under the dash you can take the dimmer wire from the power connector harness and splice it into the GREEN/RED or GREY wire on your headlight switch harness. This is the wire that activates the dimming circuit on the control unit when it receives a +12V signal from the parking or main light circuit.

 Boost Gauge Control Unit and Enclosure 3.1" x 1.57" x .7" Wired to the Cluster

 Boost Gauge in Final Enclosure, Note Calibration Adjustment at Bottom Left

In our final installation we installed both the Boost Gauge and the Air Fuel Meter at the same time. We mounted them in one enclosure 4" x 2.25" x .8". For those who only want the Boost Gauge, the smaller enclosure is also shown above. You will also notice we installed a plug so we could easily remove the dash in future.

Step 9 - Setup & Calibration

Calibrating the Control Board after Installation.

Calibration is really simple. You custom gauge has been pre-calibrated prior to shipping, but we recommend you perform the Zero Calibration at your altitude. When you first power the unit on, but do not have the engine running, you may or may not have indicator lights on.

·        Turn the Zero Calibration Adjustment CLOCKWISE until the first few indicators are on. If there are already lights on, then proceed to the next step. Remember ANTI-CLOCKWISE Turns the lights off and CLOCKWISE turns them on. If you are in doubt, simply turn P1 CLOCKWISE for a few turns until the lights come on.

·        Then turn the adjustment trim pot ANTI-CLOCKWISE until LED 1 just goes out + about 1/2” of a turn. That’s it, you’re done! Some sensors vary slightly with temperature, you may need to re-do this zero adjustment depending where you locate the sensor. 

Setting up the Dimmer for Night Time

This adjustment is also really simple. You may need to wait until dark to do this.

·        Apply +12v to the GREEN dimmer signal wire, or turn your main light switch on if you have wired it up. You may hear a small click as the relay inside the control unit activates.

·        When in operation the lamps will be noticeably dimmer. You may want to adjust Zero Calibration (P1), with the ignition on but with the engine off, CLOCKWISE to turn some lights on, (only a few of the lower LEDS will be able to be turned on) so a few indicators are on.

·        Then adjust the dimmer’s night setting adjustment (P2) CLOCKWISE to set them dimmer or ANTI-CLOCKWISE to set them brighter, until you get the desired brightness. Once this is complete you may have to re-do the Zero Calibration of P1 as mentioned earlier.

Setting up the Programmable Peak Hold Option

The Programmable Peak Hold ^tm Option is available on request. It includes user selectable modes configured with an external switch, potentiometer and/or a fixes resistor. A single indicator remains on for a programmed amount of time indicating the Peak Boost Pressure reached. If a higher boost is recorded before it is reset it moves to the higher reading. Spikes can easily be monitored and boost peaks tracked efficiently.

Programmable Peak Hold Modes Available:

• Indefinite Delay/Decay (Peak hold remains on for a long time before slowly decaying)

• Pre-Set Delay/Decay (Peak hold remains on for a fixed time before decaying)

• Variable Delay/Decay (Peak hold is adjustable via an external potentiometer)

• Reset (Peak hold is reset to zero by an external switch)

• No Peak Hold (Peak hold feature is switched off by an external switch)

You will need some additional components in order to get the full functionality as mentioned above they are as follows:

• 1 x SPDT  ON-OFF-(MOM) Toggle Switch

• 1 x 1Meg Ohm Pot for Variable Delay/Decay

• 1 x Selected Fixed Resistor for Fixed Delay/Decay

Here is a diagram of a sample setup:

Step 10 - Admiring the Finished Product

Tachometer Mounted in the Car, Showing Optional Air Fuel Meter Installed

We hope you find this article interesting. If you would like to receive your own completely assembled Boost Control Unit, all ready and tested for you to install this great mod, please go to our Gauges For Sale pages.

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