Intermittant abs light/buzzer on & speedo off - 1993 964 3.6 Turbo

[arjay]

Hi all.

I have a 1993 964 3.6T. I have an intermittent ABS light/buzzer warning that comes on once or twice in a 30 mile trip and perhaps five times in a 100 mile trip but then not again for a dozen trips. I have read dozens of posts on several forums but still not sure exactly what I might have. Nearly all posts relate to C2 and C4 rather than any turbos

I have read about and am contemplating the clock repair suggestion - see this link: 964 clock repair. Before that, can anyone help troubleshoot this set of symptoms - and advise anything less drastic to check before a "clock job". I could do the clock repairs but it is at the top end of my skills. OK:

1. Occasionally, when starting a journey first thing in the morning, the ABS light will come on after 100 yards and the buzzer will sound. Light goes off 10 seconds later - presumably when system is pressurised?

2. On a trip from Spain to Portugal (800 km) the light/ buzzer combo activated. I cancelled the buzzer but the light remained on. The buzzer then activated another 5-6 times during the trip. The light only went out after a restart at a coffee stop. Coming back on the same route I had no problems at all.

3. I was about to try the clock repair but, with no further probs for 3 months, I got cold feet. Now the problem has started again. But now, the light comes on and the speedo goes off simultaneously on about half the occasions. The computer trip also stops functioning. On the other half of faults, only the light illuminates - speedo goes on working

4. For no apparent reason, the light will also sometimes go off after another 10-15 minutes of driving and the speedo start working again - only to "die" again 30 minutes later! It is not apparently activated by hitting bumps (of which there are many in this part of Spain). I understand that the speedo gets its signal from the ABS system

5. I have an almost new battery and the alternator was replaced relatively recently (well before the ABS light problem). I am pretty sure that the brake fluid reservoir is up to the max level but will check again tomorrow as it seems to be a key factor.

Thanks for any help you guys can offer

Edited June 25, 2011 by arjay

[wvicary]

You need to have the fault code read out for the ABS system. I think you have an intermittent wheel speed sensor but without fault codes it's all guess work.

[arjay]

You need to have the fault code read out for the ABS system. I think you have an intermittent wheel speed sensor but without fault codes it's all guess work.

Many thanks for replying. Sorry I was late getting back to you - not notified of a reply.

I live in rural Spain and it is a very long way to any porsche garage who'd have a hammer or equivalent. Is there anything I could do to fault find by trial and error - eg, find the wheel sensors, clean them up and any grounds etc or trace with a multimeter or something?. Or is that just a needle in a haystack job?

RJ

[wvicary]

You can check the resistance of each sensor but as the spec will change due to temperature just look for major differences (greater then 5%) between each wheel speed sensor. Also be sure each speed sensor is fully seated into the wheel hub and that there isn't a damaged trigger wheel or a build up metal on the tip of the sensors. The odd time the electrical connector for the sensor can be at fault and just the act of unplugging it and plugging it in again is enough to clean the contact and secure the connection.

[arjay]

You can check the resistance of each sensor but as the spec will change due to temperature just look for major differences (greater then 5%) between each wheel speed sensor. Also be sure each speed sensor is fully seated into the wheel hub and that there isn't a damaged trigger wheel or a build up metal on the tip of the sensors. The odd time the electrical connector for the sensor can be at fault and just the act of unplugging it and plugging it in again is enough to clean the contact and secure the connection.

Many thanks for your helpful reply. I can't do much until Saturday but I am going to give it a go then. I'll report back what I find.

Cheers

[arjay]

You can check the resistance of each sensor but as the spec will change due to temperature just look for major differences (greater then 5%) between each wheel speed sensor. Also be sure each speed sensor is fully seated into the wheel hub and that there isn't a damaged trigger wheel or a build up metal on the tip of the sensors. The odd time the electrical connector for the sensor can be at fault and just the act of unplugging it and plugging it in again is enough to clean the contact and secure the connection.

Many thanks for your helpful reply. I can't do much until Saturday but I am going to give it a go then. I'll report back what I find.

Cheers

Progress report and a possibly significant finding

Since this is an intermittent fault, and as the light is currently not coming on (typical!), I decided to follow up troubleshooting leads from many posts on the subject. Hopefully I would find something significant. Here is what I have done so far:

Brake Fluid

1) Checked level and it is exactly at MAX with engine running and a few pumps of pedal.

2) Checked and cleaned float under filler cap - moves and clicks smoothly.

3) Removed, cleaned and reseated connectors on top of filler cap.

Battery

Checked voltage of battery. Ignition off - 12.34V. Engine idling - 13.96V. Battery warning light is on when key is turned but goes out immediately engine starts. Does not come on when idling. Figure that's OK.

ABS Power Supply

The ABS system gets its supply from a second lead attached to the +ve battery terminal. Some say this can come loose, fray and/or corrode. I removed the terminal connector, cleaned the connection, checked lead was in good condition and replaced - tightening firmly.

R34 Anti-lock Relay

Started car up. ABS light comes on when ignition is turned but goes out a few seconds after engine starts. I removed the relay and re-started engine. ABS light stayed on and alarm sounded. Put the relay back in. Again light stayed off/no alarm when engine started.

To duplicate hitting a bump: with the engine running I wiggled, lifted, pressed down on, tapped sharply, and pushed relay from side to side. It rocked quite easily but I was unable to make the light come on - i.e. connectors seem well seated. To be on the safe side I removed relay, cleaned connectors and reseated it firmly.

Front wheel ABS sensors

1) Uncoupled the white front sensor connectors from the black cylinder. Tested the resistance across the terminals in each front sensor. Results: 1.044 KOhms on right front and 1.041 KOhms on left front. Unless anyone says different - I take it that the two sensors are working correctly as these figures are all but identical to each other and in the range reported elsewhere. Reports of faulty sensors state that resistance shoots up to figures like 27 KOhms or 10-30 times normal.

2) While here, I removed, cleaned and reseated the connectors for the brake wear sensors on each side.

3) Unable to remove and clean the sensor grounds on either hub. The hex nut, which holds the sensor on, and which also bolts the ground to the hub (brown lead), was well rusted and could not be moved. (same on both wheels). I did not want to end up breaking it off as has happened to OPs with same problem. If I have to change the sensors then I'll have to break them.

4) When I came to reconnect the right sensor plug I found what maybe a significant development. One of the metal tubes which forms the female side of one of the connectors (where the male plugs in) had sheared off and was lying loose in the plastic plug. It fell out when I was measuring the resistance. This "tube" should connect to a piece of metal at the back of the plastic plug but it is possible that the connection was not being made reliably - hence an intermittent fault.

The first photo shows the two plugs (wear-sensor and ABS sensor). The female parts of the connectors can just be seen in the left-hand plug. The second photo shows the broken piece of the metal connector.

This plug is welded to the short piece of cable and presumably is part of the ABS sensor assembly and could not be bought separately. I thought of trying to get one second-hand or otherwise repair this one but have not got much further than that yet.

I jammed a bit of silver paper down the hole and forced the broken piece of tube back in - then pushed the white plug back into the male as firmly as it would go.

I just can't get the abs light/alarm to come on today on a short drive. The task now I suppose is to drive the car until either the light comes back on or I presume it is temporarily fixed. (And in the meantime investigate a more permanent repair for the broken connector).

Other things not yet tried:

ABS Valve Unit

The whole of the boot (trunk) area is immaculate on my car - not even a scrap of dust. The valve unit looks brand new and I can't spot loose or dirty connectors inside it. OPs have said that this unit does not normally cause any problems so I have left it alone.

ABS Central Unit.

I have yet to identify this but am going to look again and check for dirty/corroded connectors here..

Thanks for all the help people have offered.

[wvicary]

That could very well be the fault you've been chasing. I would replace the affected sensor.

[Max964t36]

Hi. I had similar although less annoying warnings ... in my case the problem was the low tension in the battery. I have seen in your list that you checked it and it's ok, so I guess that what you discovered might be the cause. Hope you'll get it solved soon, I understand that discovering the hidden reasons for the alerts can be very frustrating ...