ABS light, limp mode - INTERMITTENT

[Chris.Hawkes]

First, apologies if I have made any mistakes, this is my first time posting on ANY Forum! (Extreme Newbie!). I have scanned all the relevant looking threads on this site and although I can see some similar, which give me some clues, I cannot see anything QUITE like my problem.
I live in Hampshire, United Kingdom, and although I used to do simple servicing on my MGB, many years ago, I don't claim any level of technical knowledge - particularly on anything as sophisticated as a Mercedes.

Car: 1997 C200 Elegance (owned by me since 1999).
Mileage: 146,000+ (Regularly serviced. Trouble free** - 'til now )

Symptoms:
Starting from cold: ABS light stays lit when the rest go out. Gearbox stuck in limp mode, and "thumps" when engaging drive. I drive 100 yds or so down the road, stop the car, gear lever to Park, turn off the ignition and restart. ABS light stays on, but this time there is no thump when engaging drive. I pull away and when the box shifts (smoothly) to 2nd gear, the ABS light goes out.
I can then drive for many milies (up to 70 so far) without any problems EXCEPT, (1) if I knock the gear stick OUT of drive (e.g. when waiting at a junction) the ABS light comes on again and I have to go through the stop/start ritual. (Not fun at a busy junction!). (2) On SOME occasions, the car will NOT switch into Cruise Control mode.

Background:
Over the 8 years I have had the car it has occasionally got itself into a "thumps when changing gear" mode, but a stop and restart has always cleared the problem. Today's problem only surfaced last weekend, during a spell of unusually cold weather (i.e. salted roads cold).

Reason for posting:
My dealer cannot look at the problem until after Christmas, and I really need to use the car before then. I need to understand what risk there is of it sticking "permanently" in limp mode (or worse) and effectively stranding me.

Please can any of you experts advise?


** "Trouble free" apart from one curious incident during the floods which demonstrated to me that the car can actually see into the future to protect its owner! I will post this elsewhere if anyone is interested in a lightweight, curious story.

 

[star]

get it diagnosed on a star machine, but ur leaving it a bit late for xmas. our workshop is booked solid.

 

[television]

You could try BlackC55 in Portsmouth he is very good

 

[jibcl500]

Just a suggestion, and it could be completely wrong to suggest this but get your battery checked. Try ATS or somewhere that can test it.

jib

 

[star]

You could try BlackC55 in Portsmouth he is very good

its a matter of time, 3 days to xmas, and fridays always a wash out. parts dept seem to shut early etc etc. That reminds me, i better get to work instead of dicking around here.

 

[television]

Just a suggestion, and it could be completely wrong to suggest this but get your battery checked. Try ATS or somewhere that can test it.

jib

A very good point, a low voltage at the start could cause your faults, though the car starts the battery could have a low voltage due to a short circuit cell

The voltage should be over 12v before the start and 14v ish when running.

I did not tune in to this first thing this morning

 

[television]

its a matter of time, 3 days to xmas, and fridays always a wash out. parts dept seem to shut early etc etc. That reminds me, i better get to work instead of dicking around here.

Have a good day

 

[BlackC55]

Tell me about it.

So much to do......

 

[Chris.Hawkes]

Thanks for all your suggestions so far. The battery is nearly new, supplied and fitted by a dealer earlier this year. The old one lasted 9 years - true! - so I don't really believe that my useage pattern would have depleted it by now. The symptoms I had when the old battery was dying were nothing like this. But I will do the voltage check anyway, it is simple enough.
I take it, then, that this DOES look like a new symptom, never seen before on the Forum?

 

[television]

Hard question Chris, you do have a speed signal that goes missing causing all the above problem, putting it on STAR would tell you what it was

 

[Chris.Hawkes]

Dear Malcolm/Television.
Yes, I'm afraid it IS a hard question (for me). I am afraid that a "STAR" in my experience is a little light seen high in the sky on clear nights. I guess you are referring to some sort of diagnostic tool (expensive, sophisticated and normally only available to a dealer, presumably)? As I explained in my first post, my dealer is, understandably, unable to investigate until the 27th, so I am afraid I cannot give you a sensible answer.

Let us assume, for the sake of argument, that there IS such a signal missing - what would be the implications? Is it something that would degrade further over the next week or so, or having learned how to live with it, (by restarting and sticking in drive) could I risk continuing to drive the car in this state for a few hundred miles until I can get it sorted.

 

[television]

A missing speed signal will not harm the car, though it may make it a little uncomfortable when driving, this could be just a simple sensor.

Yes STAR is the MB diagnostic tester that can read out stored faults.

The ABS part of it is where the speed of all 4 wheels is known, then ABS knows what to do when its needed. The same signal is required by the gearbox to know what gear to be in, when the signal is missing it goes into the limp home mode (2nd).

I hope that this makes it a little clearer

Regards

 

[Chris.Hawkes]

Thanks for the detailed explanation, Malcom/Television. This speed signal comes from a sensor on each of the wheels - yes? [This gives me confidence that the diagnostic approach that will be taken by my dealer - to check the wheel sensors (presumably with help from the STAR diagnostic system) is the right thing to do.]

Meanwhile, I assume that the faulty sensor must actually be normally working, but sometimes not (e.g on first cold start and on shift into neutral when halted at traffic lights), explaining why a stop/start "reset" will allow the car to run OK for a while. Does that fit your theory too? Since I have done a lot of miles on salty motorways over the years, I guess it is also possible that the fault is merely a bad contact, maybe in the plug/socket or loom, rather than a completely dud sensor?

 

[star]

Thanks for all your suggestions so far. The battery is nearly new, supplied and fitted by a dealer earlier this year. The old one lasted 9 years - true! - so I don't really believe that my useage pattern would have depleted it by now. The symptoms I had when the old battery was dying were nothing like this. But I will do the voltage check anyway, it is simple enough.
I take it, then, that this DOES look like a new symptom, never seen before on the Forum?

you might have a couple of problems but unless you get them read its diff to tell.

 

[Xtractorfan]

At this late stage I would certainly try some of the independant any make mechanics some of the equipment they have can read and reset many of the MB faults..tho they dont have the software to update..but many will reset to factory settings.. worth a try and may be a life saver over the holidays

 

[Chris.Hawkes]

As a closure for this one, my dealer has replaced one of the ABS speed sensors on the rear wheels. (He considered that the "controller" was another possible candidate for change, but eventually rejected this idea after "some more tests"). No chance for extended test yet, but it got home without any difficulties, so fingers crossed that is the answer...