Just found this thread with great info about PDAS/ABS problems on 89 C4, for posterity:

https://rennlist.com/forums/964-foru...-on-89-c4.html

 

I had the same problem on my 1990 c4, had it in storage for about 3 months, took it for the weekend and as soon as I got on the highway , same light and alarm, got home added fluid and took it for a HARD drive and problem solved

 

This link was posted in another thread on "switching off the PDAS system".

It is an article from Adrian Streather on the PDAS and ABS systems in the 964 C4.

Go to page 7 and you will see all of the warning system issues described in this thread.

The common issue is low brake hydraulic boost pressure.

The whole article is great reading.

http://www.adrianstreather.com/docs/...overviewR2.pdf

 

Dead link?

I have the download if anyone wants it; pm me your email.

 

Let's try this again.

http://www.adrianstreather.com/docs/...overviewR2.pdf

 

I have read previous posts. I have had my 964 C4 for about 6 months. On start up the 4WD warning light illuminates and some 10 seconds later the alarm sounds for about 15 seconds before stopping and the light going out. This doesn't occur if the car has been recently driven and started up when still relatively warm. It only seems to occur during a cold start up. Shall I start with checking & filling the brake fluid reservoir if necessary? The fact that the alarm stops after 20-30 seconds of starting car suggests to me (as with the light going out) that the issue maybe pressure related? So 30 seconds in there is sufficient pressure to then de-activate the alarm. I'm no mechanic! but just wanting to know if safe in meantime to drive car in this state?

 

Yes, it's safe to drive when the light is out.

 

Mine behaves similarly on occaision; it's taking a little too long to reach cutoff pressure is all. Is it an issue? Dunno, but those parts are spendy so I'm not up for replacing them until it's required.

 

If I haven't driven my car in the past couple of days, I get the buzzer and '!' for about 3 seconds as the pressure builds. No issues if I start the car a day or two later. Wait more than 3 days and the pressure drops to a point where it can't build pressure in the allotted time and the buzzer sounds until the last few PSIs are accumulated.

 

IMHO 20-30 seconds is too long. I would bleed the C4 system (accumulator (the bomb), solenoid block etc per Adrian's book.

 

It's a simple system. The pump builds pressure to the required level as regulated by the pressure switch. The accumulator is a store of that pressure if you use a lot at once e.g. Track work.

My car had multiple problems. The pressure switch had the pump going more than needed and the accumulator was not holding full charge. It took a long time to build pressure then use of the system on the track expended whatever the store was and it alarmed. I had to replace both $$$.

As long as the light is out you are safe to drive. However recognise that your system isn't at peak and don't push it. I would never go to the track with a sub par braking system.

 

Hi guys ,
+ 1 to Laker

I suggest as well a bleeding of the accumulator & solenoid valve ,
I Just did a bleed and the slow start and beep warning light is gone ,
If you do a search, a rennlister named RicardoD wrote a tutorial for this procedure, the only thing is you need a power bleeder even a simple one pressured from a tire...

 

And at that point I would bleed the whole system

 

Really appreciate comments above fellow Rennlisters. I'll take it easy short term and have my guy look into the system in due course to get it back to more optimal performance.