Cruise control codes

[multigrade]

I've had the W124 Coupe for six months now and the cruise control has never worked and still doesn't work. So now I'm trying to understand what the problem is.

I understand that there's no separate CC module for this model year as it's all under the management of the ECU. So I built a reader and received the following fault codes on pin 14 (which is the CC pin): 2, 5, 7, 11

The description of these faults is:
code 2: Cruise control/idle speed control module
code 5: Stop lamp switch
code 7: Data bus (CAN)
code 11: Fuel safety shut-off to LH-SFI control module

Supplementary information: I've just rewired the throttle body harness (eco insulation!) in the hopes that this might fix the problem. It didn't.

So I think I can figure out what two of the individual codes mean but the stop lamp switch fault is odd because the brake lights work, and I don't know what the CAN bus fault means. So can anyone say what this combination of faults is telling me, please?

Thanks.

 

[Wearsafoxhat]

I think the brake light switch operates two circuits, one for the lights and one as an input to one of the CPU’s. They are know to cause odd faults if defective. Touching the brake pedal will cancel the cc..so hopefully just this.

 

[multigrade]

Thank you. I'll be found with head upside down in the footwell...

 

[multigrade]

Quick update: I cancelled all the fault codes. Only one persisted and couldn't be cancelled: code 5. It's the brake light switch, so a new one is in the post.

 

[mioba]

Good luck. Let us know how you get on

 

[multigrade]

Here we go: I replaced the brake light switch today.

Before fitting the replacement switch, I checked its operation: it has two sets of terminals, one set NO, the other, obviously designed for higher current with fatter terminals, NC. Once I'd replaced the original, the fault code had disappeared. However, a quick check with the multimeter showed the original switch I took out worked identically to its replacement. And a test drive showed the C/C still doesn't work. On my return, the fault code was back.

So where do I go from here? Having rewired the ETA, I don't think that is the problem. But the rest of the wiring harness is still using the old degraded wiring. Of course, there could be other causes for non-functioning: the switch or the ECU, I would guess.

Thoughts?

 

[Doug1234]

Have a look on this forum below for
Information

Mercedes-Benz Discussion Forum, DIY Technical Advice, Parts and Cars for Sale - Welcome to the PeachParts Forum!

Mercedes-Benz Forum and Technical DIY Discussions for Enthusiasts. PeachParts is NOT affiliated, sponsored nor authorized by Daimler AG.

www.peachparts.com

 

[multigrade]

Thanks Doug. There's a lot of stuff there - a lot of it sadly now linking to dead sites - but having crawled down a number of those rabbit holes no-one seem to have encountered the specific problem that my car is experiencing. Think I might have to check the operation of the CC stalk switch, even though my suspicions are aimed more in the direction of the engine harness.

 

[mioba]

The W124 harness was known to degrade after time. There was a company that was making them up. If it needs doing it will need taking out and certainly you could get one made up.

PP used to have a great forum for the 124 . i used it alot years ago.

 

[multigrade]

The W124 harness was known to degrade after time. There was a company that was making them up. If it needs doing it will need taking out and certainly you could get one made up.

PP used to have a great forum for the 124 . i used it alot years ago.

Yes, mine is one of the biodegradable harnesses. I plan to replace it when it starts to show signs of misbehaving...

 

[multigrade]

Another wrap-up from me. After replacing the brake light switch, which made no difference, I dropped the car into Merc specialist John Haynes in Worthing and asked them to sort the c/c. They found it was a faulty over-voltage protection relay. Bit annoying as I could have replaced that myself had I known! Still, it now works...