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Background: I've had my 964 for about 6 months, and I've been getting VERY intermittent ABS/PDAS warning lights. They pop on seemingly at random, then tend to go way either overnight or if I stop/re-start the engine.
This week, they've been coming on at start-up and do NOT go away.
• Battery is fine.
• Brake fluid is topped up.
The question: I read up on the R34 relay today. So I pulled the R34 relay and bridged the 30 and 87 pins as discussed in several threads. Doing this eliminated the ABS/PDAS warning lights at start-up. Put the relay back in and the warning lights came back. I guess it's an obvious question, but does that result prove definitively that the R34 relay is the culprit? Anything electrical is foreign to me, so I guess I just want a second or third opinion here.
Without looking at the wiring diagram details of the relay, I can say that there are many reasons for the lights to pop on. I discovered recently that low-ish starting voltage will trigger them (always accompanied by the chime shortly after). Also: slightly low fluid after the pump is pressurized, slow pressurizing due to the accumulator diaphragm going bad, a flaky wheel sensor ground, corrosion in the pdas ecu harness pins, leaking pdas slave cylinders that cause a pressure drop once activated. The list goes on...!
Have you done a full bleed? I would start there. Full bleed on the entire system. Then do a pressure test of the bomb. The bomb should take approximately 60 seconds from zero to full pressure. If you get over a minute and a half, replace the bomb. If the bomb and air in the system isn't an issue, then start looking into electrics/ cleaning sensors etc.
If you are near lower Connecticut, pm me, happy to help.
Without looking at the wiring diagram details of the relay, I can say that there are many reasons for the lights to pop on. I discovered recently that low-ish starting voltage will trigger them (always accompanied by the chime shortly after). Also: slightly low fluid after the pump is pressurized, slow pressurizing due to the accumulator diaphragm going bad, a flaky wheel sensor ground, corrosion in the pdas ecu harness pins, leaking pdas slave cylinders that cause a pressure drop once activated. The list goes on...!
Yup, been through most these in tracking similar (non-issue) lights remaining on on start-up - untill 2k rpms is exceeded. Nailed this down to low voltage (i.e. <8v) detected on start up. Once passed 2k revs lights stay out.
Other PDAS light issue early last year (random lighting - sometimes with buzzer) down to corroded/furred PDAS/ABS ECU pins - design fault of having ECU connector mounted upside allowing water to settle in connecter recess. Removed ECU, dried area out fully, cleaned up all pins and female opposites in connector. All good. Fully bled entire brake/PDAS/ABS afterwards for peace of mind.
My friends 964 had the ABS/PDAS warning lights that came on after 1 minute of idle then stayed on. I had the car in my garage for the week so I tried a few things, including no faults shown on the PDT999, cleaning both accelerometers which were both gunked up with filings, swapping my PDAS controller to eliminate the big ticket item. Checked the earths, connectors on the system, & relays.
Then finally I went backed to basics, I looked long and hard at the brake fluid level. It was just below max. Topped it up. Started the car, no ABS/PDAS lights, FIXED! Sometimes you need to check the fuse first!
Good catch. The reservoir level is supposed to be checked after the accumulator bomb has fully pressurized. What seems a full reservoir can actually be just below full after the bomb is filled with fluid, and as anyone with a C4 has discovered, it has to be FULL to not trigger a PDAS warning light.
02-15-2018, 05:58 PM
Yes, another ABS/PDAS warning light question
