As some of you may know I have had the oil light flash at low speeds and beep at me.

After realizing it never happens above a certain speed I just drove it 650 miles for thanksgiving with no problems on the highway, so obviously the oil is flowing.

When I got back I did some testing as I have noticed some things.

I found that my tachometer is faulty and bounces around 2-3 RPM randomly while driving. It doesn't seem accurate at all. It also goes to 0 while running if I apply the brake, but instantly goes back up to 1k RPM when brake its not applied (doesn't matter if stopped or moving at the time). I have found that at the point when the tach is showing 0RPM and engine is still running is when the oil light is coming on and flashing. It obviously is somehow related to the tach hitting 0 while running because thats the only time the oil light flashes and beeps. Once the RPM is back up to 1K or higher the oil light never beeps at me.

So... I am thinking if I fix whatever the issue with the tach is then the oil light problem will go away.

Could someone explain why this may be happening ? I am guessing the tach gets data from some sensor, and somehow that sensor is also throwing bad readings to the oil system which maybe makes it think there is no oil flowing- or its low? .. only thing I could think of.

Note it doesnt matter what speed I am - as long as the RPM goes to 0 if brake is applied it will oil flash randomly. I can be going 40mph and hit brake- slow a bit- RPM goes to 0 - flashes- let go of brake- all while on gas and the oil light goes away bc RPM went back up.

I can even just let it idle, hit brake- rpm 0 let go of brake- rpm 1k hit brake -0 let go - 1 ect ect over and over and shown in the video.

In Video: If you see tach going from 0 to 1k over and over thats me hitting brake and letting go of it. Also notice the oil light going away pretty quickly once its back to its normal idle 1k RPM.....

Tach bones connected to the oil light bone...?
sry for crappy quality and such- did it in the parking garage leaving campus today....

Any help would be appreciated.

 

With that kind of screwy **** going on, unless the ECU correlates indicated engine speed with expected oil pressure, I think you have some shorted wires somewhere. Seems strange that brake application ties into it unless that's something else that is shorting but man, it seems pretty unrelated and I doubt that that correlation is looked for - other variables (such as load and throttle angle) would also come into play and I doubt Audi saw a reason for that much complexity way back when our cars were built.

That being said, it seems that the sensor that drives the tach signal is the starting point since that's a failure you can readily see on the gauge. Ohm out the wiring from the sensor connector to the ECU, and if the wiring is intact, replace the sensor and go from there.

 

Yeah I thought it was odd, was just what I noticed and it is correlated on my car for whatever reason.

I will focus on getting the tach to work right and see if that fixes it.

Where is the sensor @ needing replacing? Behind the gauge cluster or? (headed to my hanes manual now to find it too... but just in case..)

Thx for response and any more helpful comments welcome

 

I cant help you with the screwy tachometer signal (it sounds like a short of some kind or bad ground, and sounds pretty hard to find), but the reason the oil pressure light comes on is this: the cluster expects the oil pressure switch to show low or no pressure when the engine is off (0 rpms, open switch), so when it sees the oil pressure is above that when the engine isnt running (switch closed), it thinks there is a problem with the oil pressure switch and turns on the oil pressure light. On a side note, the cluster will not illuminate the low oil pressure light while the engine is running unless the switch shows low pressure (open) AND the engine is above 2k rpms, this is because the idle oil pressure is not enough to make the switch close, generally.

Edit: Im curious where the brake signal wire and the instrument cluster tach signal come close together. Does anyone know if for some reason the cluster is told the status of the brake pedal, if so I would pull your cluster out and just see if you see anything wrong.

 

Thanks for the great info. Basically the oil pressure is probably good it seems. Any idea where the sensor is that controls the tach (sends the signal?) Would it be behind the cluster? I am very curious why this is happening, I think I will call the dealership tomorrow and see what they say. Otherwise I think I will just pull the cluster out and look at the wires /check their condition ect.

Thanks all.

 

Hey you know now that I think of it, I have noticed something else strange about the brake pedal:

Usually when you press ANY brake pressure with your foot I would guess the rear brake light should illuminate, like my previous cars, however with my car it takes more pressure then it takes to slow down to illuminate the light. I noticed this while driving at night and doing small braking motions while seeing cars were getting awfully close behind me because they didnt notice I was breaking as the light wasnt going on until I did a bit more pressure.

maybe it means something related, idk.

 

The tach signal should come from the crank position sensor as far as I know. It is sent to the ECU and then out to the cluster. You would have two sets of wires to ohm out and inspect - sensor to ECU and ECU to gauge cluster harness.

 

This is for Pre-99's (im not sure about post 99, i found a diagram for pre 99, and post mid-year 2000)