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PDAS or ABS System Failure C4 1990

Old 01-05-2013, 02:24 PM
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Originally Posted by bw-porsche View Post
Hi there, this may help - check the level of your brake fluid and make sure it's topped up to the limit. I had those same lights coming on after I had to replace the switch on the high pressure brake/clutch system. Another tip with the C4 - make sure your battery is in top condition. When my battery was below par, the central locking would suddenly unlock the car as a safety measure. Good luck anyway!!
+1 on the brake fluid level. Had this issue come up three year's ago and a top up on the brake fluid solved it for me.
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Old 01-15-2013, 12:36 PM
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Originally Posted by glyndrea View Post
I have a 1989 C4 with this problem. I have unplugged the relay 34 and when I lightly shake it, it sounds as if something is loose inside. Has anybody had this before? Would it mean that the relay 34 must be replaced as it would appear?
Hey welcome to another ZAer on RL!

also found my r34 (pn 928-615-124-01) to be the cause of my abs fault.
bridge your 30-87 in the fuse box (where the relay would go) with a small piece of wire and see if the light goes off/abs works. Worked for me.
Don't leave it bridged tho as will drain battery.

Then put a 12 v battery onto those same posts on the relay (see numbering on the bottom next to each post)
and if you can hear it click, its working. If not - like me - get a new one. R1k from PC.

I have opened mine up - just pry the ali case off gently around bottom and slide off - and diode is fine, as is resistor (both 687 ohms, spec is 680 as printed inside relay)
so its likely the coil that isn't working and hard to replace.

You can even put it in the r34 slot with case off and with a small plstic pic (toothpick) push the switch (pinkish color, small, near bottom) closed and you'll hear the other 2 abs relays (those ontop of the abs pump in frunk on left side behind battery) clicking ie working. Let me know if you find the VW or AUdi partn number - their relays ar 20% of the price
and do the same thing ie 12v 10amps 680 ohm resistance - but not sure where to find them .... yet

(ps - that was my day today lol)

Last edited by HiWind; 01-15-2013 at 01:38 PM.
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Old 01-24-2013, 12:20 PM
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Location: Johannesburg, South Africa
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Thanks Hiwind, I did check by bridging 30-87 where the r34 goes. Everything has checked out. I have done other checks that was suggested on the internet to try to solve the ABS/PDAS problem.

The happy thing is that I took the car for a drive and the usual thing happened with the ABS and PDAS light coming on with the warning light and buzzer. I left it like that until I hit a bit of open road out of town and thrashed it a bit. after about 30kms I stopped turned the car off, waited for about 3 minutes, started up and drove about 40kms back home. The problem did not come back. I have since taken the car for another 3 drives of about 50 kms each and the problem has not come back. At this atage I will see if some how I have cured the problem.

I am now going to drive the car at least once a week as I think that was part of the problem. I did go throught a period of 3 months without driving the car. That seemed to aggravate the problem.
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Old 01-24-2013, 01:05 PM
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nice Glyn - parade in Cape Town this year - hope you can make it

ps - if your abs worked (ie light off and abs actually functions when tested - empty road!)
when you bridged the 30-87 then it should mean the r34 relay is the problem.
I got a spare from a porsche scrapper for R150 - shd be easy to find up in Jozi as there's far more 928/944s there and the part number is the same.
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Old 01-03-2019, 11:11 AM
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Join Date: Jun 2018
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When your lights come on, does your brake pedal become spongy? I am having similar problems with the lights coming on after a short distance and staying on. I have tried new abs pump, bomb, longitudinal valve, bled brakes and have solid state accelerometers. Pedal feels like air in the system when the lights come on as if there isn't enough pressure in the system.
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