1. The car didn’t want to run when it was cold, it would chug and stall unless I kept my foot on the gas. This was for the first few minutes until it warmed up. This was due to having absolutely no idle speed control.
2. The car ran like absolute garbage during this time.
3. It started stalling at traffic lights.
4. I was still only getting 1.2 bar on the boost gauge

I guess that’s four. We’ll address these issues in order.

For starters it was clear that I had an issue with the idle speed control circuit, as in, it just wasn’t doing a darn thing. The base Idle Speed in this car has a mechanical setpoint. The idle speed is then adjusted by an air bypass valve controlled by a module outside the realm and knowledge of the ECU. This system increases airflow into the system ( and hence raises the idle speed) under certain conditions.

The inputs to the control module are:

1. +12v Power (duh)
2. Coolant Temperature
3. AC Compressor active signal
4. Instrument cluster speed signal
5. Radiator Cooling fan active signal
6. Idle mode signal
7. and probably some others I’m forgetting.

The system will adjust the idle speed a little or a lot depending on those various inputs. You’ll note it doesn’t take an RPM reference speed to try to achieve a target or anything. It assumes a known baseline and simple adds some number of volts to the air bypass valve according to some calculatio ( x number of volts = y rpms). Reading on forums, folks say the first thing to check are the resistors in the unit, as there’s one in particular that goes bad and messes up the entire works.

So, I opened it up and, yeah that looks pretty toasted. (Don’t mind the torn ribbon cable, that disintegrated on me pretty quickly, but was intact when I started.)

I took an inventory of all the capacitors inside as well, figuring I might was well replace them too.

And here you can see a nice new resistor with some pin headers to keep things simple. Also note the new red capacitor in the lower part of the board. One thing I did learn when recapping this board is that, the traces on this thing are super duper fragile, and I ended up completely nerfing a couple pads and traces along the way. So there are currently about 4 different bodge wires and solder bridges. Not my best work, and when I lifted my 3rd pad I was starting to have a serious bout of “Did I forget how to solder?”

Anyway, excited about my handy work, I plugged it, crossed my fingers…and NOTHING. It still wasn’t doing anything. I was devastated. I kept driving the car as it had been pressed into service as my Daily Driver at this point. After some number of months of chugging and sputtering through the neighborhood first thing in the morning, I found another of the exact part number: 443.907.393P on ebay about $100. I ordered it, because I was 100% convinced I screwed up the rebuild with all the lifted traces and what not.

A note on Audi Part Numbers. That little P suffix on that part number is critical. These modules were used across the Audi / VW line in anything with the Bosch CIS fuel injection system, but they were VERY application specific, because different systems had different accessories and different loads which needed different amounts of voltage supplied to the valve under different conditions — all denoted by the suffix on the part number.

Anyway, do you want to know what happened when I plugged the new one it? Yep you guessed it. Nothing. This meant I’d been chasing my tail for months on this and the problem was elsewhere. So, I went back to the drawing board and started testing all the inputs to the control module AGAIN.

I found that the throttle position switch wasn’t actually working. This switch comes in the form of a little black box. The armature from the throttle plate goes in one side. When you’re foot is off the gas, it closes one set of contacts with tells the computer “We’re in idle mode!”, when you’re flooring it to get out onto the highway, it closes another set of contacts which says “We’re going for it Hoss!”. So basically, the computer has three operating environments. Idle, driving, and YAHOO!

Well, I found it was never sending the idle signal. I decided to open up the box to check it out, figuring it would be a bad solder joint or dirty contacts or something. It’s a very simple device, so shouldn’t have been too hard to diagnose and repair.

Friends, when I opened the box and found there was no circuitry inside for the idle circuit I may have lost my cool a little. Doing some number of hours of research on the finer points of Bosch part numbers and their automotive applications, I found that the part number on my little box actually pointed to a ’77-’78 Porsche 911 — which didn’t have the idle circuit.

I ALSO learned that these haven’t been available for a decade or more. I eventually found a used one, with the correct Bosch part number, listed for an ’85 Porsche 944 — Like I said, they used this system on everything for decades.

After installing it, I had to adjust my idle baseline. In Idle mode, the computer advances the ignition timing and adds a ton of air (via the Idle Control) system to keep the idle speed up. So *that* part was working at least — but that’s all controlled by the ECU. Idle speed control, however STILL wasn’t working. Okay. Got to keep going.

It was at this point I found this guy:

A cruddy looking temp sensor, but like the end is soldered on, so it must be fine right? No. No it was not. It turns out the core of the sensor was loose and when I would press on it with the multimeter to read the value, it made contact and was fine. Remove the probe and it would make no contact internally. Well, measuring this with the engine running, and pressing on it in just the right way, the Idle control circuit sprung to life! I jiggled the sensor, idle speed contol stopped. I pressed on the sensor, Idle speed control resumed….

$8 later and little wiring work later things looked and worked a lot better!

Now, please dear reader, forgive me at this point for not checking the temp sensor more closely. The fact of the matter is, this car has more temperature switches and sensors than I’ve ever seen on a car, and it’s not always clear which one goes to which. After two years however, I now have a decent understanding of all the ways this car cares about temperature.

1. A temp sensor for the ECU
2. A temp sensor for the Idle Circuit
3. A temp sensor for the gauge on the dash
4. A temp sensor for the warning light on the dash.
5. A dual stage temp switch in the radiator for the fan
6. A temp switch in the thermostat housing for the after-run fan
7. A temp switch under the fuel injectors for the injector cooling fan.

Anyway, at this point, I’ve verified they’re all working! AND I HAVE IDLE SPEED CONTROL. I had to readjust my idle baseline again, but that’s fine because, it idles when it’s cold. And the idle goes up when the AC compressor kicks on. And the idle stays high when coasting to stop (kinda, more on that later). And the idle kicks up when the 500W cooling fan kicks on. Amazing.

At this point, it still doesn’t run great when cold, and it started stalling at stop lights, and I’m still not getting ECU modulated boost, only mechanical waste gate level boost. I’ll cover these in Part 2!

The procedure to pull the cluster couldn’t be more straight forward.
1) Remove the horn pad (just pull)
2) Remove the nut holding the wheel to the column, and pull the wheel off.
3) Loosen the clamp which hold the Multi-function switch to the column, and slide off.
4) remove two small screws from the cluster, pull it out and unplug 3 electrical connectors.

I knew I was in trouble when, as I completed Step 2 , I was showered with a dozen tiny ball bearings. You’ll note, that “pick up ball bearings and try to insert them into their mangled nylon carrier” is NOT in listed in the procedure above.

Having a suddenly acquired a new side-quest, I started searching the floor and by some miracle, was actually able to find all but 1 of them. Turning my attention to the mangled carrier, I was able to kind of bend it mostly back into shape, and pop the bearings into their spots. This bearing, sits just inside the column housing around the steering shaft, behind the steering wheel, offering support and smooth operation. Without it, there is a disconcerting amount of up/down left/right play in the wheel, along with unhappy metal scraping sounds when the wheel is turned. Thankfully, the bearing carrier seated well in the column. It seated so well, I was unable to remove it again, which make me wonder what kinds of abuse this poor car suffered before it came to me. Regardless, with this intermediate crisis out of the way, I proceeded down the list.

Once the cluster was removed, It was immediately obvious that I’d acquired another side quest. The contacts between the various bulbs and needles and the electrical connectors, consist of very very thin strips of copper, embedded in a fragile plastic sheet. The plastic contact mat was torn in several places, preventing signals from getting to where they needed to be.

Repairing a broken circuit trace is generally a straightforward operation. You can either bridge the trace with solder if it’s a small crack, or run a bodge wire to give a new electrical path. This is a not-uncommon occurrence when working with old computers in general. Unfortunately, given the light plastic nature of the “board”, it would simply melt and shrink when the soldering iron would come near it, so it was VERY challenging to try and bridge the broken the traces, but I persevered and managed to make some repairs which showed that I had restored electrical continuity.

So, with repaired traces and all new light bulbs, I plugged it in, turned the ignition switch to “on” and learned that while the fuel gauge came back to life, the center LCD did not. Bummer. I clicked the lights on to be sure all the new bulbs worked and several bulbs in the cluster EXPLODED.

Sigh. So, I took it apart, cleaned out all the broken glass, and replaced the exploded bulbs and the various newly burnt out ones, with old working ones. After getting everything back together I tried again. Success, illumination and no explosions!

All of the above happened while I was working on the breaks and the power steering pump. Once I started driving the car, a few things became apparent. First I really wanted a boost gauge in order to know what the turbo was doing. The boost gauge is a digital readout, provided by the non-functional center LCD. Second, the check engine light worked, I know because it would light up every time I accelerated. Third, by some miracle, the odometer still worked (this is a known weak spot for gauges made by VDO from this era) — at least for 36 miles, when it stopped counting miles.

That the odometer broke wasn’t too surprising or upsetting. There is a set of nylon gears which interfaces between a stepper motor and the gears behind the numbers. The gears simply crack and fall apart due to age. This is such a big problem for every 80’s and 90’s European car with a VDO speedometer, that new gear sets are readily available for a few bucks. I removed the cluster (again) and fully disassembled it. I found no cracks in the gears, which perplexed me. As I opened things up, I was hit the face with a vaguely fishy smell, which is indicative of leaking electrolytic capacitors. Dead/leaking caps can absolutely cause circuitry to just not work, so I took inventory and placed a Digi-Key order. I also ordered a new gear set. I put everything back together and reinstalled it, so I could continue to drive the car.

Once all the parts came, I made quick work of the capacitors and the gear replacement. I did find a missing tooth on the old gear, which helped explain the stoppage.

I put it all back together and turned the key on, but I still had no center LCD – at this point I figured it was just a bad LCD panel, and I’d have to source a new one. I started the car and drive down the street. It through clicked 2/10 of a mile and stopped counting again.

So, I took everything apart again and found a small bit of debris in the gear track. With that cleaned out, I figured I’d solved it and put it all back together and reinstall it into the car. 1/10 of mile and it stopped. At this point, I was beyond frustrated and was questioning if an oily rag and a match might be a better solution to this issue. I took everything apart AGAIN and decided to test the circuitry itself on the control board for the speedo/odo. I knew it was getting signal from the speed sensor, as the speedemeter worked without fail.

So, on the bench, I simulated input to the board. I measured voltages coming out of the IC chip and everything checked out, the motor was simply not turning when it received voltage. It’s at this point I noted, that the motor, wasn’t sitting flush to the board, a very small amount of wiggling and it popped off the board all together — except the pins which are used to solder the motor to the board, which were still soldered in place on the board.

It turns out, this is possibly one of the worst designs I’ve ever seen. The windings of the motor, which are about as thin as a hair, are soldered to a metal pin, which is held in place with nothing but friction to the plastic housing. The other end of the pin is soldered to the circuit board. That’s it, there is no other mechanical attachment of the motor to the board, so with any movement or manipulation of the board, you risk damaging the connection from the windings to the pins, irreparably damaging it.

I mentioned above that tons of cars with VDO gauges have an issue with the nylon gears. Unfortunately, almost no other car uses this particular motor and control board. It seems only 86-88 Audi 5000 Quattros and similar year Volvo 740s used it. I’m guessing that VDO realized they had a turkey of a design on their hands as all other cars from this era that use VDO gauges seem to have a far more sane motor/board interface.

This also means that it’s incredibly hard to find a replacement. There are no markings on the motor itself, so I needed a donor cluster from one of the above cars. The only thing I was finding on E-bay was a cluster for $450 out of Latvia. Considering the dead center LCD, I needed a donor cluster anyway. But $450 seemed a little too steep, so I kept periodically checking E-bay, hoping for a cheaper option.