'94 SL500 - Gearbox? Ecu? Here we go again...

[E55BOF]

Strikes me as rather odd, this one, but possibly someone on here has had the same problem. The car is a 1994 SL500 - M119 engine, old hydraulic four-speed auto box.

Engine and gearbox warm. The car started fine, but the throttle seemed rather flaccid - plenty of travel and nothing, then it came with a rush (quiet in the cheap seats there...). It was idling at just over 1000 rpm, whereas it's normally 600 rpm in N, just under 500 rpm stationary in drive.

Driveable, but odd, so set off for home - 10 miles motorway, a mile normal road. On the motorway, it would not change into fourth, however much I moved the lever from 3 to D and back, though otherwise the changes were pretty much as normal.

I soon tired of keeping the revs down by trundling along at 55-60 mph among the lorries, so pulled over on to the hard shoulder, switched everything off, then restarted the engine. Normal service resumed.

Stopped at Tesco half a mile from home to pick up a few groceries, and when I restarted, the problem had returned - no fourth gear, high idle, floppy throttle.

Ladies and Gentlemen, the floor is yours. Does anyone know/suspect what it might be?

 

[Doodle]

ASR light illuminated or not?

 

[E55BOF]

Not, I think; I'd have noticed if it was (probably...)

 

[Doodle]

It's pretty bright (if it's working).

The floppy "all or nothing" throttle sounds rather like the ASR system playing up, but that usually kicks the light on. Other culprits can be the brake light switch or neutral switch on the gearbox, but it'll probably need a code read for some direction.

 

[E55BOF]

Yes, the ASR light works. The problem is obviously electrical, and the car needs codes read anyway, but what I don't have is the specific knowledge of how the electrical problem could prevent the gearbox changing up into fourth.

 

[Doodle]

Directly, I don't believe it can.

But the hydraulic pack will be looking for a specific set of conditions to be met before changing up - if the engine isn't running right you may not be meeting it.

 

[Terry_]

If the throttle body is reporting incorrect information it could prevent changing into top gear

 

[brucemillar]

I had "almost"the same issue on my wife's C240 Petrol after an abortive attempt at resetting the gearbox ecu by following some link on this very forum. This involved switching the ignition on, then holding the throttle down, switch off etc. It completely screwed the gearbox shift and rev patterns in both manual and auto modes.

A trip to Andy at Wright Tech, revealed no errors on STAR. He reset the gearbox adaptations and all was fine again.

I later read a post by Olly (Black C55) where he stated that the "re-set" that I followed was not advisable, as it was not a true re-set and could cause issues.

So. It may be worth getting the car onto STAR and having the gearbox re-set properly.

 

[Doodle]

Bruce, I don't think there's anything to reset. It's a hydraulically operated box, doesn't have a transmission ECU like your C240 does.

 

[JCW1]

Apart from the ASR light not coming on, it sounds like you are describing 'limp mode'. Did you ever get your TB wiring harness replaced?

 

[Eric Alex]

I also suspect a limp mode. Any diagnosis and the solution???


Sent from my iPhone using Tapatalk

 

[E55BOF]

Terry, JCW, I've come to rather suspect the same thing. The main engine harness has been done, but not the TB. I reckon the TB harness lead is a bit iffy by now, but haven't got round to it yet. It's got to be done anyway, so maybe now is the time to put some more money Si-Lec's way...

 

[JCW1]

Take a careful slice along a couple of inches of the harness outer casing and peel back. You'll soon know if it needs doing. Mine was in an awful state.

 

[E55BOF]

Well, I already know that the wires in the thick loom that crosses the cam cover from the driver's side front inner wing and terminates at the MAF are somewhat crispy-crinkly above the cam cover. Does that same loom split and go on down to the TB as well? And more importantly, if it does, is it possible to get it off without removing the MAF (he asked [not very...] hopefully)?

I'd go and look, but it's cold and 'orrible, and I'm busy wrestling with a manky Moto Guzzi, the getting of which through an MoT and sold is my number one priority at the moment. I need the space for another bike...

 

[JCW1]

The cable that you are looking for is thick and comes from the drivers side wing but it doesn't go to the MAF, it goes down to the TB body, below the MAF.

Having done it, I would say that getting the TB out without removing the MAF is impossible as there are too many fiddly things to do that need the room to work. There is a video on you tube which I found very helpful to plan the job in advance.

 

[E55BOF]

I knew it would, really. Life's never simple with an old MB....

 

[The Dude]

...a truism wisely spoken...i find with both cars and diy no job is ever easy!

 

[E55BOF]

Holy thread back from the dead, Batman!

Well, I finally took Richard F's advice and made a blink code reader. Using the operating instructions from the US Mercedes Gen-In website, it doesn't do what it should, or at least not in the way I was expecting, and I'm not much the wiser (except that the alarm module is confirmed faulty, but I knew that empirically already...)

I have the 38-pin socket, but my car has a Base Module, not an OVP. The instructions state to connect the code reader up, ignition on, press the interrupt switch briefly, and count the flashes. It only flashes if I keep the interrupt switch pressed; if I release it, I just get a steady red. (Yes, it's wired correctly).

This is what I got:

Pin 4 - steady red, no flashes, no red when the switch is held open.
Pin 5 - there is no socket at Pin 5.
Pin 8 - same as Pin 4.
Pin 19 - there is no socket at Pin 19 (I had high hopes of this one; the list
states it is a 'Diagnostic Module').
Pin 23 - numerous fault codes, some of which are listed, some not. This is
the alarm control module, and I knew there was a problem with the
alarm/immobiliser already.

I was hoping to find something to give me an indication of whether or not the throttle body is duff, and if so in what respect, but I haven't.

Two questions come to mind: firstly, is there anything in the rudimentary (by today's standards) OBD setup on my car that will tell me if the throttle body is duff; and secondly, am I operating the blink code reader correctly?

G on, somebody; give me a clue...

Please...

 

[RichardF]

Okay not really sure, but.

Your diagnostic socket is behaving a bit like mine when the ignition is off, or I don't have a good connection.

Do you have 12v @ pin 3 ignition on? And a good ground on pin 1. If not, maybe a fuse, (4 on the base module) you could also try connecting to a known live instead of pin 3.

 

[E55BOF]

Thanks Richard. I'm not so sure the socket is the problem, though. I have 12v on pin 3, Earth on pin 1, and none of the base module fuses are blown. The fault code pulses I got were: 5 (once), 10 (once), 11 (four times), 12 (four times), 15 (once), 19 (twice) and 20 (four times). All those are valid fault codes except 11 and 15, and if I was just getting random responses, I would expect more invalid codes. Besides, I know the alarm module is duff.

I ran the engine up before I began work; it started in something like limp mode - 1000 rpm idle, limp throttle pedal (is that why they call it limp mode...) and with a dollop of throttle to hold 1500 rpm it ran up to 80 degrees, though getting increasingly rough once past 60 degrees (but not seriously rough, even so). Once off the throttle, it stalled. The last time, with no throttle, it cut dead just below 60 degrees.

The throttle body is off - simple, straightforward, but B****Y fiddly to do. It's off to Sileck in the morning, and by the time it gets back I'll have the new gasket and breather hose (serendipitously, if unexpectedly, the rubber joining piece to the MAF is reasonably pliable when warm, so I won't bother replacing it). I'll put it back together and see how it runs; I rather think/hope it will be fine.

A thought occurs to me, though; presumably the same unsatisfactory insulation is on all the wiring in the engine bay, but I can't recall anyone having had trouble with, or replacing, the MAF lead. Have I missed something?

More anon....