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964 1991 C4 "Brake" & "Diff" light on

Hi,

I got a 1991 C4 5SP with 150k miles and I have been having an issue with the brake light and the diff light turning on at the same time.

Say in the morning I start the car and let it idle for 30 seconds or so sometimes the diff and brake light will turn on and go away sometimes it would stay on. If it does turn off sometimes it stays off and sometimes it come back on while driving however the car feels the same.

Not sure what is going on...please help!



Also if sometimes if I pump the brakes the siren/horn will go off but still feel the same...
Old 10-24-2013, 02:18 PM
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Check your brake fluid level.
Old 10-24-2013, 10:09 PM
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Been out of town; but I have checked the brake fluid level and seems all good. No issues with braking too.
Old 10-30-2013, 10:55 PM
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I have found that it is very sensitive to the brake fluid level. It must be at the same or slightly higher than the max level or else I start to get differential warning lights.

Maybe the 4 wheel drive system utilizes the brake fluid .... not sure.
Old 10-31-2013, 09:20 AM
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Try cleaning the accelerometers that are located in the center console - that fixed this issue with my car.
Old 10-31-2013, 11:05 AM
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Hmm, okay I will check the fluids again and try the level idea.

Will try to clean the accelerometers also. I have been trying to find a post about this but no luck thought I was the only one with this issue.
Old 10-31-2013, 10:08 PM
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964 991 c4 "brake and diff' light on

the awd does use brake fluid. i'm in the process of replacing awd differential cylinders that have a leak. so if brake fluid's down, you might want to check for a similar condition.

good luck
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Old 11-01-2013, 06:08 AM
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Got the two accelerometers out cracked them open and did not find anything out of the ordinary besides the two burn marks on the right screw. Did not find any dust dirt metal particles etc. if I shake it pendulum rotates.

Any ideas on what to do next?

Will try hitting it with air and put it back together to give it a try.

Porsche 968 928 993 Longitudinal Acceleration Sensor Accelerometer 0265005100 | eBay

Most likely going to go for two of these.

Thanks again!
Old 11-01-2013, 08:01 PM
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First, throwing used parts at a problem isn't the most efficient way of getting it sorted.

Second, if you DO need new accelerometers, there's a solid state replacement available (search on the RennList forum I think).

Third, a real Porsche shop should be able to use one of the Bosch tools to get a code (thereby avoiding item 1),

...and finally, are you sure you have enough brake fluid? It needs to be at Max with the car running on a level surface.
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Old 11-06-2013, 08:29 AM
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True, I will double check fluid levels again, clean the accelerometers and put them back in there to see how it goes.

I have contacted the rennlist member for the solid state fix $765 for two.

I just wanted some input on the brown/tan burn mark seeing if it shorted somewhere or not.

Thanks
Old 11-06-2013, 01:48 PM
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Quote:
Originally Posted by freestreamt1 View Post

I just wanted some input on the brown/tan burn mark seeing if it shorted somewhere or not.

Thanks
Haven't had mine open, but they're low voltage and the likelihood that both of yours would function intermittantantly with the exact same (and so far as I know, unknown) failure mode is vanishingly small. I don't think those marks are significant.
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Old 11-06-2013, 02:22 PM
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Also, check the connection at the PDAS/ABS computer: they are somewhat prone to corrosion.
Old 11-06-2013, 04:34 PM
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Had same lights and it ended up being connector to ABS module.
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Old 11-06-2013, 05:59 PM
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964 1991 C4 "Brake" & "Diff" light on

we found my leak when the underside 'pan' was removed to do some other diagnostics...otherwise we'd not have noticed it

really suggest u do the same to see if there's a leak....the mechanics told me that the loss of a tablespoon of brake fluid is a big deal

just had my repaired and was lucky; one was shot and about to go

good luck
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Old 11-07-2013, 10:53 AM
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Could you provide the link to the Solid State Accelerometer;s.
Thanks
Old 11-08-2013, 03:29 AM
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Lateral Acceleration sensor - JDS Porsche
Old 11-09-2013, 08:38 AM
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did you ever fix your problem?
Old 05-07-2014, 05:19 AM
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I had the same problem last week. I replaced R34- both light come on when the power is not supplied to ABS computer.

Google Porsche 964 r34 bypass

Andrew
Old 05-08-2014, 04:44 PM
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Quote:
Originally Posted by adeak1 View Post
I had the same problem last week. I replaced R34- both light come on when the power is not supplied to ABS computer.

Google Porsche 964 r34 bypass

Andrew

I'm having a similar problem (I think). Two fault codes came up: #136, "unknown DTC" and #24, "left rear speed sensor". The speed sensor has been replaced, but the problem reappeared.
Did replacing the R34 relay fixed the problem?

Last edited by 2000m2; 09-27-2016 at 12:40 PM..
Old 09-27-2016, 12:30 PM
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