Bleeding diff locks on an '89 C4
 

My '89 C4 is currently in the shop for an ABS problem (i.e. it doesn't work) to be investigated. As part of the diagnostic work a full hydraulic bleed was undertaken but there seemed to be a problem in bleeding the hydraulic diff locks. The shop have (I beleive) a PST2 which acknowledges the PDAS system but the "Bleed" button remains greyed out preventing the necessary cycling of the locks. The bleed to the primary hydraulic system has improved the "pump time" from more than a minute and a half to less than 30 seconds.

Is there a known issue with the PST2 and the '89 C4 or can anyone suggest why the PST2 would grey out the "Bleed" button. Could it be related to the non-operational ABS?? The PDAS system appears to work as expected - the light flashes when driving over some bumps and on enthusiastic cornering. I've installed the PDAS bulb in the oil gauge just to make sure I can notice it!!

All suggestions appreciated.

Regards

Dave

Could always get them to do it by using the C4 switch on the lower dash - that's what I did last weekend when I did mine. Turn it to the left, crack bleed valve, when fluid stops close valve. Cycle the switch to the right then back left again and repeat. Takes about 20 turns to do each of the locks, but works.

Ian,

Thanks for that. I knew it could be done with the switch but I didn't know the procedure. Quite apart from the practicality of doing the bleed I was also concerned why the PST2 wouldn't do it.

I've seen mention on the PCGB forum that somebody else had the same non-working ABS and that turned out to be a faulty ABS/PDAS ECU. We had tried another ECU from another C4 although admittedly didn't test the ABS but the PDAS bleed issue was still the same with the PST2.

Regards

Dave

I must admit that my ABS is now working better than ever since doing all of this - before I never locked wheels up when braking hard - now you can feel it pulsing as the tyres start to lockup and overall braking force is much higher.

Dave,

Do you have an after-market chip? My car had one when I bought it and the Hammer would report dtc 41 (control unit faulty) due to a missing checksum. I could still bleed the locks, however.

Still, perhaps the PST2 is a little "smarter" and is stumbling on that??

Dave,

I DO have a ProMax chip in the DME, yes. I wonder if that could be the issue?? Thanks for the suggestion.

Dave

Don't you mean turn it to the Right:confused: (to engage the PDAS) first, crack the bleeder, close, cycle the switch to the left (disengage {PDAS), back to the right, repeat about 20X. That is how I bled mine. (but would not be the first time I went astray)

Well, I had the wife inside the car turning the switch one way then the other...the fluid seemed to come out fine. In fact it was your write up that I followed!

Hang on, now realised we were both doing it the same way - when I said to the left, I meant that the bottom of the switch went to the left, which means turning it to the right...err...easier to say we both turned it clockwise first, then opened and shut drain valve, then anticlockwise again. Then repeat! That should make it clearer rather than left and right - depends on whether you are looking at the top or bottom of the circular switch!

It is interesting how various aftermarket DME chips can screw with the Hammer readouts... with my Vitesse MAF sometimes the PDAS doesn't show up, or sometimes the airbag doesn't show...

Is this done with the ignition on, alone, or did you have the car running?

Just with the ignition on - it will pressurise the high pressure system, and as you bleed it down the pump will come on to put it back up to full pressure again.

Just to close this thread I found out today that the shop managed to bleed the diff locks using a KTS500 without any problems. The PST2 tester just didn't want to do it.

I wonder if this is a common problem??

Dave

I know with my scantool it won't acknowledge the ABS system if the engine is running. It will only see it if the key is in the on position but with engine off. Maybe that's the problem with the PST2?

I think I need to do a full bleed as well as my ABS/PDAS lights stay lit for a while on first startup - although I can't hear the PDAS pump running excessively. Sometimes the big red "!" lights up at startup with the abs lights but goes out within a couple of seconds (ABS lights will stay lit). No codes on scantool either.

I had an issue using the Durametric software not seeing the ABS and Durham mentioned that is a common problem on our C4's, especially when the battery has been disconnected for sometime. Turning the PDAS switch on fixed the issue. Perhaps give Duram a call as he seemed to know the issues pretty well.

tom

Could someone simplify this for me...:confused: I think I know what it means to bleed brakes - all fluid, no air bubbles.

Are we talking about cycling the diff-lock knob ahead of the stick-shift counsel? Or, what I just said, but with fresh fluid? :typing: