Louie928

My neighbor has an '89 S4 5 spd. It has a problem that the low oil pressure warning is on continually. The alarm can be cancelled in the normal manner, but it should be fixed before he sells it. The oil pressure is normal and the oil pressure sender has been replaced. The wiring and plugs all checked good. I swapped the digital dash unit with a spare and that cured the alarm problem. I'd rather keep the original dash unit so the odometer reading is correct. Does anyone know if these digital dash units can be repaired? Who does it? I checked over the solder joints on the circuit boards and they look ok, but I didn't try re-soldering. Some of the components are surface mount and there is a good chance of causing more trouble than may initially exist.

Thanks.

the flyin' scotsman

I haven't fully read the tech book yet but the answer may lay in Technik WDK 495 821 Diagnosis of the Instrument Cluster.

Tails

The 1989 MY and up, you can carry out a diagnostic process as per WDK 495 821 "Diagnostic of the Instrument Cluster" (digital dash) by using tool No 9293, however, you don't have to use this tool all you need is to bridge terminal 5 to 13 on the diagnostic socket adjacent to the passenger seat located under the cover.

There are fault codes for low oil pressure and comparison of reading from instrument read out and sensor values.

This Diagnostic Booklet is contained in Jim Morehouse's Technical CDs.

Tails 1990 928S4 Auto

ROG100

Most of the faults I have encountered with the digital dash have been fixed by resoldering the joints of the various gauges. I did have an intermitent fault on my GTS and had to replace the CB - $1000 OUCH.
Rich Antrade repaired the CB so he may be a good starting point.

Louie928

Thanks for the info. I do have Jim's CD and I'll check the technical manual there. I often forget how much useful information is on that CD.

the flyin' scotsman

The manual is in the file 'other' of CD1 of Jims Tech publications.

I did read more of it last night and did see where Tails gets the 'bridge' of the 2 pins, there's data supporting the OP sensor and reset thereof.

Louie928

I went through the dash diagnostics according to the service manual. As expected the oil pressure switch (#22) showed that the oil pressure was <.25 bar although the oil pressure is normal. The oil pressure switch input to the dash, plug 4 pin 10, shows normal. That is, low with low oil pressure, and high with normal oil pressure. However the dash always reads it as low oil pressure, and alarms. This confirms that the dash has a problem, and further confirmed that with a different dash connected there is no oil pressure alarm .

If I had a schematic diagram of the digital dash circuit board I would have a chance of fixing it.

the flyin' scotsman

I believe the cct diagram is in the '89 Tech manual as it was the intro to the digi dash...........BRB.

Nope not there...........need the other CDs, BRB again.

Louie928

The dash circuit diagram would be quite complex. Lots of components, two muli layer CBs.

no doubt

If, big "if," you think that the PCB is toast such that you are considering going with a new board, there is a trick to first test the "dead" or malfunctioning PCB. Remove it from the dash, wrap it inside a large zip lock bag (perhaps the footlong bags) and seal it.

Now put the bag in your freezer overnight. Remove the PCB, install it back into your dash, and test it while it is still frozen. You should have about 2 hours in a normal garage.

If it works while frozen, you have a bad chip.

If it still has the same malfunction, then the problem is in a PCB trace, or solder, or solid component like a resistor, diode, capacitor, inductor, etc.

So the test will greatly narrow down your trouble shooting to either your chips or board/solid components.

Just a thought.

the flyin' scotsman

Louie...........check the 2nd half of page 4 of the '89 Wiring diagrams.

While for there day these PCBs may have been considered complicated my days in the Telecom world dealt with +20 layers, imbeded components, differential impedance etc......my head hurts just thinking about it.

Louie928

Malcolm,
On my '89 wiring diagram, pg 4, (pg 97-317), I have a "box" on the left side that represents the instrument cluster with the inputs/outputs. The right side has the various sensors and the central electronics interconnects. I have nothing showing the instrument cluster internals.

While the CBs would be considered primitive by today's standards they are too complex, and small, to eyeball the circuit path to check components on the way to the active circuits.

If it weren't that this car will be sold and that the spare dash I have has 17k more miles on it than this car has, I'd swap in the spare dash and not fuss over it. The car will appear to have more miles than it really does have. Not a good selling point.

Louie928

Thanks for that tip. I'll try it. I'm tired of working on this thing today anyway and it'll give me an excuse to stop for a while.

the flyin' scotsman

I did another search and cannot find the specific internal diagram either......its well buried or non existant.........I may have to look at my hard copy collection.

I've used pin test beds then functional test to confirm and they get very complicated very quickly.



Very true or the buyer would have to be very understanding or the speedo would be changed to reflect another measure of distance.

ROG100

Louis,
I put my CB in the freezer to confirm the hot & cold function/not function. All this told me was that there was a circuit problem.
There are two main solder points on each gauge and these were cracked and easy to see. Resoldered and they worked.
Shocki had a thread on the fix. Worked on his oil pressure gauge and also mine.
My original CB was toast so I renewed it. They were NLA but I checked recently and they have some in Germany.
Roger