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As in the previous post about master cylinder, the brake problem isn't gone (bleeding didn't help, but of course firmed up the brakes).
The ! warning light pops on, along with the red brake warning on the right of instrument panel. It comes on randomly, just driving down the freeway, stays on for just a second, it might happen about 5 times in half and hour, never stays on for more than a second. I can't correlate it with anything like speed or driving behavior. Fluid level is fine, and with the bleed it was all inspected, no leaks. One odd thing, the light has come on a few times when I was actually braking, and the pedal looses pressure and goes down about 1/2 an inch -- yeah, this experience certainly is confirms that your body still secretes adrenalin. But then when let off the pedal, and it is all back to normal.
So there is something that is "playing" with the pressure, I almost suspect the ABS control unit, because I can't think of anything else that would adjust the pressure. The shop says that essentially always when the light is coming on, without a known problem, it is the level sensor in the reservoir or the wear sensors. I was planning to "short" them all out to try to diagnose this thing, but then those causes wouldn't create the condition mentioned above where the pedal slips down when the light comes one. The shop simply recommends to replace the master cylinder, but there is no evidence that is that problem (only experienced guess work). I can't think of a caliper condition that might cause this.
So very odd, why does the light come on randomly while not eve braking, why would the pedal "loose pressure" when combination of braking and the light comes on.
Any ideas or suggestions would be much appreciated. Thank you.
Roland
Roland
'93 Coupe Tip Silver on Grey, '02 911 C4S, '89 Vanagon Syncro -- (RIP: 944, 911SC, 931, MGB, VW Bug, GTO, Sprite.)
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Thanks Flash,
"start by flushing the system and see if it changes." is what I had done last Tuesday, didn't help.
"pad thickness is one. ", shop said plenty of pad on all of them, that is what I expected.
"the ABS system could be acting up. if contamination gets in there, the system could be partially kicking in, which would affect the pedal.". Yeah, that is what worries me, I have no idea how to service it let alone where it is? Is there a way to disable it, maybe a fuse, and see if that changes things we can use as additional root-cause analysis?
"i'd take a look at the booster and see if there is a vacuum leak. ", yeah I wondering about this one, since the light comes on randomly without any active braking -- what if while driving along there is funny stuff about a vacuum leak
"the master is the more likely culprit though. " is what they said, but that is at least $800.00 (P&L) just to "try this".
Thank you!
Roland
'93 Coupe Tip Silver on Grey, '02 911 C4S, '89 Vanagon Syncro -- (RIP: 944, 911SC, 931, MGB, VW Bug, GTO, Sprite.)
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I pulled the ABS fuse, #26, and will drive that way, see we can learn anything from that.
Roland
'93 Coupe Tip Silver on Grey, '02 911 C4S, '89 Vanagon Syncro -- (RIP: 944, 911SC, 931, MGB, VW Bug, GTO, Sprite.)
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Thanks Dave, it sure gave me hope at work today, but then checking whenever the big red light comes on:
- the parking brake is in the lower left of the instrument panel, below temp and gas
- the detailed light coming on in this case is the "ABS" detail light, slightly above the oil pressure / voltage gauge.
Oh darn!
Removing the fuse for the ABS did nothing I can detect, other than both the [!] and detailed ABS lights stay on the whole time.
It does however suggest, I wonder if my ABS unit has a power supply problem, an intermittent, perhaps a bad ground that needs to be cleaned up. Sure would explain why it just flashes on for a second in middle of driving.
Roland
'93 Coupe Tip Silver on Grey, '02 911 C4S, '89 Vanagon Syncro -- (RIP: 944, 911SC, 931, MGB, VW Bug, GTO, Sprite.)
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I had a failure of the abs relay a couple of years ago. the same symtoms........
those relays wear too.......
Ritchie owner of a '92 Coupe Tiptronic with the following standardoptions: C00, 030, 139, 249, 258, 340, 383, 387, 403, 418, 454, 490, 494, 567, 573, 650, 690, 14951
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Reviving this thread since Kiwichris wrote with the same symptoms:
- the ABS warning light comes on intermittently both while simply driving and also when on the brakes (completely random, can't correlate to anything)
- speedo falls to zero when the light comes on
- I can hear some kind of "hydraulic" sound, almost a whooshing, when it happens -- suspect the hydraulic control unit is powering on/off.
- if the light comes on during braking, the pedal goes down 1/4 inch or so (wonderful feeling!).
- the frequency increases with time. When cold, the light may not come on for the first 1-2 minutes of driving, after 4-5 minutes it will come on more often, after 10 minutes it comes on more often and stays on longer.
Status:
- bench tested the ABS relay in the fuse box, it seems fine. But this relay seems to pass on the "depressed brake" signal to the ABS controller (can see this in the wiring diagrams). And so I am discounting this a potential problem anyway, since the ABS warning light comes on without the pedal depressed.
- warning light frequency is becoming more frequent, happens more often now.
- there is an ABS tester available: called the "KDAS 0003", or see here: http://www.w124perfo..._ABS_tester.pdf (I don't have one!)
- there may be another relay, see drawing 605-00 in the PET, item 10 is a relay which is a different part number than the ABS relay in the fuse box.
Next steps:
- (I feel silly) does anyone know where the ABS hydraulic controller and the ABS computer are? Suspect they are behind the right front fender well liner. Do I have to remove the liner to get access? I'd like to check if there is another relay there, or if anyone knows where this other relay is on drawing 605-00 then please let me know.
- Got advice that most often the problem is "the relay" or the wheel speed sensors. Evidently the wheel sensors can get dirty, or the wiring becomes unreliable. I think this is probably the next step, but I have no way of know which wheel!
Thank you!
Roland
'93 Coupe Tip Silver on Grey, '02 911 C4S, '89 Vanagon Syncro -- (RIP: 944, 911SC, 931, MGB, VW Bug, GTO, Sprite.)
(This post was last modified: 11-19-2013, 08:11 PM by
968Syncro.)
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Flash, that gives me some hope, simply blowing the dust out? That sounds great, I imagined need to take things apart, or unscrewing sensors, get a rag and brake cleaner, inspect / test wires (of course break something else along the way), etc. I have a compressor, no big deal to just remove some wheels and assert some air. Even better, the porterfields I got from the group buy only went on the front -- there was plenty of pad left on the rears. So even better this suggests I start with the rear wheels. Thank you!
Roland
'93 Coupe Tip Silver on Grey, '02 911 C4S, '89 Vanagon Syncro -- (RIP: 944, 911SC, 931, MGB, VW Bug, GTO, Sprite.)
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Actually, I was suspecting the wheel sensors that feed the ABS.
Roland
'93 Coupe Tip Silver on Grey, '02 911 C4S, '89 Vanagon Syncro -- (RIP: 944, 911SC, 931, MGB, VW Bug, GTO, Sprite.)
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