PDAS and Brake Lights on After 20 Seconds

ma702tt · 01-26-2008, 09:06 AM

91 C4 Cab
50K
Austin, Tx

Issue:

After 20 seconds (exactly) both the PDAS warning light and the BRAKE light come on as does the Master Warning Light.

For a little while now these lights have been coming on but not at 20 seconds and not all of the time but most of the time.

This morning I ran the car to operating temperature and there was no effect on the 20 seconds.

Not too long ago the warning horn was coming on and the shop determined it was a booster pump which they replaced. I do not believe the current problem began immediately but has seemed to get worse over time.

There is no noticeable effect on the performance.

If I shut the engine off and restart- it repeats itself... lights on in 20 seconds.

Here are photos of the lights before starting and after 20 seconds.

I read in Adrian Streather's book in an attempt to understand and diagnose the problem but nothing seemed to match.

Doe anyone have an idea of what could be causing this?

Thanks,

Matt

john walker's workshop · 01-26-2008, 10:20 AM

my '89 C4 does that exactly. the hammer shows nothing. my voltage is on the low side, 13.2V, so i was going to replace the alt/reg unit anyway, but it's possibly a voltage issue. be interesting to find out.

ma702tt · 01-26-2008, 12:48 PM

John,

I do not have my voltmeter here or I would check it now. I may go out and get one as it will be a couple of weeks before i can get mine back.

Do your lights come on at 20 seconds after start also or it is just the same light pattern.

Also, i had the shop check it with a hammer a while back when it was not so regular and they nothing was stored.

Thanks,

Matt

911quest · 01-26-2008, 01:29 PM

I found this on-line check out page 7 for light symptoms

http://www.adrianstreather.com/docs/PDAS%20Technical%20overviewR2.pdf

john walker's workshop · 01-26-2008, 01:31 PM

mine is the same as yours, 20 seconds or thereabouts. it's a car i'm fixing up to sell, and i haven't had a lot of time to work on it, so maybe this thread will help us both.

ma702tt · 01-26-2008, 02:01 PM

Thanks Tony. I had found that and started reading it but really need to study it not just read it. It is complicated.

John, I believe that the brake pedal is a little soft though I don't really have anything to compare it to. If i pump the brakes for a while (maybe 15 to 25 times) the lights, though already on, will get slighly brighter and the ! light will come one (if i had reset it) But once the pump has caught up the lights would get a little dimmer and the ! light would go off.

When i hold down on the brake pedal actually push pretty hard it will slowly give way. It is not super hard.

I don't know if any of this makes sense it does not to me but having worked on cars without presurized brakes the pedal should be a little harder.

Thanks,

Matt

911quest · 01-26-2008, 02:22 PM

Have you checked the brake fluid...why I say that if they did put a pump on there the fluid may be a little low. The fluid level has to be correct on these first generation ABS systems

ma702tt · 01-26-2008, 02:45 PM

I have checked it and it seems that the range is very small ... like 1/2 inch of so. Is it really that critical. What if it is a little over?

Matt

john walker's workshop · 01-27-2008, 08:29 AM

bump

ma702tt · 01-31-2008, 03:44 PM

I found that the lights will not come on if driving (distances) but upon stopping they will.

Today I drove out 30 miles on the highway no lights. Then drove back no lights. I pulled over sat at idle (with no brake pressure) and in a minute or so they came on.

Any ideas. I don't understand the system.

Thanks,

Matt