Cruise control fault diagnoses W126

Onne

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Hello all, I have cruise control (tempomatic) fitted but sadly it doesn't work. Is there a way of finding which part is at fault before I start replacing things at random?
It's on a 1990 W126 300SE

I do have a spare of actuator, ecu/booster and stalk but rather go the intelligent way...

Thanks in advance
 

television

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It is 99 times the ECU amp box
 
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Onne

Onne

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Is there any way of checking the ECU?
I was told the same that's the ecu 99/100 times but the replacement "known working" one didn't make any difference...
 
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Onne

Onne

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Thanks Malcolm, looks like a print out and look into job.

The above aside, I didn't remove the old ECU from the bracket, does that make a difference? All I did was unplug the one and plug in the other, which I just had floating under the dash, or does it need the outer case as earth?

Cheers
 

television

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No grounding on cases to avoid earth loops
 
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Onne

Onne

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Thank you, I'll try another. Thank you very much for your help, I'll report back once I've tried another.

Cheers
 

Alex Crow

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regarding wheel sizes etc, it does not matter. the only speed signal comes from a pick up on the back of the speedo head, a reed switch from memory.
 

ianrandom

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1986 420SE, 1990 420SEC, 1989 2cv6.
Sorry to slightly hijack this thread, but its sort of related. My cruise doesnt work. I've had the box resoldereed. Fitted a new switch. But i discovered a wire coming from the connector to the box which runs up to the speedo are in th edash. (Its either white with a red strie or red with a white stripe, cant quite remeber now.)

I cant see anywhere at all for this wire to go, could anyone enlighten me? There dont seem to be any empty ports on the back of the speedo or anything. And the wire is missing its connector anyway.
I've looked at wiring diagrams and not found a wire like this mentioned, but the diagrams i looked at are on the US site so i wonder if the wiring is different anyway.
 

television

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There is a red and white going to the brake pedal,,look on the diagram above in the link
 
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Onne

Onne

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All good, thanks for the hijack, I'll look at that while I'm there.

Cheers
Onne
 

ianrandom

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Ah, cool thanks Malcolm and Onne. And what are those modules then? Left control unit and base control module. Are those in the fusebox, like relays or something?

Interesting that wire i mentioned could be for the brake pedal. When i first got the car it just did full throttle when you engaged cruise, nothing else, just flat out. Pretty scary on the motorway! Since then, nothing at all and I've tried 3 different boxes.
 
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Onne

Onne

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Hi Ian, the box is directly above the brake pedal, mounted on the bulkhead on a bracket with two 13mm bolts. You'll need to remove the cover under the steering column.
Mind the vent tube that's attached to it, it's a bit (but not a lot) of a pain.

I hope to get mine sorted later this week, I should have all the bits.

Has anyone got an idea where the actuator sits under the bonnet? I assume on the inlet side?
 

ianrandom

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Onne, thanks. I know where the control box itself is, i've changed it in and out quite a few times. But that wiring diagram suggests some other little boxes, and my cruise is completely dead so i wondered if they could be at fault. I've changed the actual switch a couple of times too but no joy. It went from crazy to nothing. (Nothing being a lot safer than what it was doing before!)

The actuator, you cant miss it. On mine it sits kind of behind the offside headlamp. Box with an arm and a cable attached, the cable is attached also to the throttle linkage.
 
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Onne

Onne

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I'll have a look tomorrow, you must have a bigger engined model :), mine is the humble 300SE.. Checked my spare amp box btw, nothing looks melted or burnt, so that must be a good thing.

Anyone got a photo of the whole affair underbonnet?

Thanks!
 
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Onne

Onne

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Well, it's faster than the Morris Minor I had...

I run mine on LPG, so relatively affordable to run. Have to say that I am loving the build quality, bolts that undo after 21 years... Amazing!

Thanks for that, hopefully that'll clear it up :)
 

ianrandom

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Here you go. I assume it'd be the same for the 6 cylinder..
 

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Alex Crow

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picture in post #18 above are of retrofitted cruise control, not sure if the wiring will differ for this.

fctory fitted actuator on M103 engine is under intake manifold IIRC.
 

ianrandom

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Pretty sure mine is original. But its a 420 so maybe location is indeed different on the 300.
 


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