New towbar relay causes electrical woes

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Welshmerc

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Mar 7, 2011
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10
Car
mercedes e270 cdi
Hi, just had a new towbar relay fitted on my W211 E270 cdi today. It appeared that following the fitting the new relay did not work and the electrician is returning tomorrow to fit a replacement.
In the mean time, the car has an SRS warning on the dashboard, a general dashboard warning, intermittent climate control cutting in and out, no main beam, no indicators, no speedometer etc etc.

My question is, can these be reset easily by an auto electrician and why should the fitting of a relatively simple towbar relay cause these faults in the first place?

Thanks for any help
 
I’m afraid you’ve answered your own question in the phrase “…a relatively simple towbar relay…”.

The modern Mercs are not intended to simply have additional electrical loads added around the vehicle without the primary vehicle systems being aware.

Also, in some cases, loads are both high side and low side switched which means that without adding diodes around your relay coil circuits you could end up drawing power in unintended directions. It also means that direct connections to chassis are unwise.

I'd suggest that you’d be far better to get a proper CAN Bus electrics pack which advises the rest of the vehicle what is going on. An example is here:
Erich Jaeger wiring set MERCEDES-BENZ E-CLASS (W211) Saloon | In the online shop of rameder.eu | Rameder

I’ve no connection to Rameder other than being a satisfied customer: I’ve fitted one of these packs to my CLS which has the same wiring as your car. It is probably quicker to install one of these than it is to add a generic kit.

I'm sure that someone will be along to tell me that I'm talking rubbish, and that they successfully used a relay from Maplin, but I would suggest that they succeeded more by luck than judgement...
 
I’m afraid you’ve answered your own question in the phrase “…a relatively simple towbar relay…”.

The modern Mercs are not intended to simply have additional electrical loads added around the vehicle without the primary vehicle systems being aware.

Also, in some cases, loads are both high side and low side switched which means that without adding diodes around your relay coil circuits you could end up drawing power in unintended directions. It also means that direct connections to chassis are unwise.

I'd suggest that you’d be far better to get a proper CAN Bus electrics pack which advises the rest of the vehicle what is going on. An example is here:
Erich Jaeger wiring set MERCEDES-BENZ E-CLASS (W211) Saloon | In the online shop of rameder.eu | Rameder

I’ve no connection to Rameder other than being a satisfied customer: I’ve fitted one of these packs to my CLS which has the same wiring as your car. It is probably quicker to install one of these than it is to add a generic kit.

I'm sure that someone will be along to tell me that I'm talking rubbish, and that they successfully used a relay from Maplin, but I would suggest that they succeeded more by luck than judgement...

I note from that particular kit, that the cable and fuse for pin 9 is only rated at 15 amps which means it is unsuitable for towing a modern caravan with the ALKO ATS system as that requires 20 Amps.

Has anyone found a kit with a 20 amp supply on pin 9?

Steve
 
What kind of relay did you (or your fitter) use? I've used a smart bypass relay on a number of Mercedes towbar electrics installs, with no problems at all.

It sounds like the fitter has damaged or interfered with wiring beyond just the wires he needed to connect into for the lighting - is this for a 13 pin socket, or 12S (grey) 7 pin socket?

A starting point would be for him to remove all the wiring/relay he has done so far, and see if things start working or not.
 
PS the dedicated electrics kit seems to be priced at 115 euros, a smart bypass relay is typically £15 and 12N electrics £10.
 
It sounds like a CANbus issue, i,e as paul_c says he has done something bad - either cut into the CANbus wires instead of the correct thing or broken an ECU.
- The CANbus is the car's network and the different bits of the car talk to each other via it.

Unplugging whatever he plugged in should leave the car working - i.e if the relay being added fails you can't end up with an un-usable car (imagine what happens on a motorway at 70mph if your car ends up in that state)

Do not let him leave you with the car in that state - he certainly won't be able to reset the SRS light without a diagnostic tool, and if he is a mobile general towbar man he is unlikely to have one.


Richard
 
I note from that particular kit, that the cable and fuse for pin 9 is only rated at 15 amps which means it is unsuitable for towing a modern caravan with the ALKO ATS system as that requires 20 Amps.

Has anyone found a kit with a 20 amp supply on pin 9?

Steve

To resurrect this, today I recd a new S212 Westfalia towbar loom. OEM allegedly and £117 when ordered with a towbar from PJ Jones.
Pin 9 has a larger than most red but seems to be still governed by a 15A fuse despite the allocation drawing giving it as 30 (?)
However, why does the Alko ATS supply have to come from the car directly?
Surely the correct way would be to wire it from the caravan battery using suitable cable and a 20A fuse if reqd and allow the car to charge said battery in the normal fashion. This would be a more reliable supply anyways given the notorious voltage drop over the length of car/caravan electric connections.




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