Fuel pump wiring route ?

[Greybeard]

My 190e auto has suddenly developed a fuel pump problem, non-starting, with no sound of the pump whining when the ignition key is turned on.
Reading previous posts on the subject, I removed the pump relay and substituted a shorting link across the 87/30 pins. Still nothing.
A check on the voltage at the pump terminals, with one wire removed and the link still in place, shows a voltage of about 1.5 volts across the supply.
The resistance measured across the pump is about 2 ohms, which sounds ok.
I presume from this that there is a partial short circuit somewhere in the wire between the relay socket and the pump terminals.
(We're on a farm and predation by rats is not unknown !)
Can anyone give me any guidance on the route the loom takes, and where there might be any access to it. I can see that it disappears into the bodywork close to the pump, so I'm hoping that I can get to it from inside the car.
Many thanks
John

 

[television]

Hello again, this is the best that I can do, it would be open circuit rather than short circuit

http://www.detali.ru/cat/oem_mb2.as...&L=765.902&CT=F&cat=500&SID=54&SGR=555&SGN=10

 

[Greybeard]

Hello again, this is the best that I can do, it would be open circuit rather than short circuit

Thanks Malcolm. It at least shows me a connector inboard of the grommet in the bodywork, so something to look for. If that had come adrift, that would give me a high resistance point with the consequent voltage drop across it.
You're right about the possibility of an open cct. Partial open/partial closed will give me similar symptoms, I think, but either way, that connector looks like a good starting point.

John

 

[television]

In order to find breaks in wires, I use a pin and push it into the center of the wire and take a reading from the pin, this way no damage is done, most cable go open circuit near the connectors rather than in the middle if the cable cannot move

 

[Greybeard]

Nice idea, Malcolm, I'll give that a try tomorrow, and post the results.
John

 

[Greybeard]

Well, I thought I'd best start by checking the resistance measurements I'd made yesterday, and the result is that the measurement across the fuel relay socket pin 87 to earth is ~40 ohms, and that at the fuel pump to earth is about 20 k ohms. These are the opposite ends of the same wire, so I started to trace the loom looking for any signs of problems.

I've got the back seat out, then all the floor covering, then the panels under the passenger front area, and nothing to be found.
That is I found the wires, but no connector to come loose, so onward and upward.

Standing on my head under the glove box, I traced the cables out through the bulkhead, with nothing untoward.
The only place left seemed to be the relay socket, and having had the problem of removing the relay, getting at the wiring between the socket and the bulkhead seemed to be utterly impossible.
However, wiggling some wires in its vicinity, I noticed that the socket base moved a little, so I didn't give up. I then decided to try and identify the actual wires to the pump out of the nine or so that were there, and the good news was that there they were. The bad news was that the 12v wire from relay to fuel pump had been cut, and another pair looped into it.
I followed this forward into the engine compartment, and it led to another relay ! AAAAAAAAGH.

This turned out to be a "Gas Timer relay", part of the LPG control system for switch over from gas to petrol.
Normally closed, the contacts open after a few seconds of switching to lpg, thus disabling the fuel pump when running on gas.
Currently I have a tuning problem with the gas system, so I haven't been running on lpg anyway.
Tomorrow, when the sun has cleared the frog and fost, I shall short out the contacts and hope to report that I am up and running on petrol again.
Then off comes the relay for a closer examination.
It has shown up an unexpected weakness in the design of this dual fuel system, and has certainly taught me a lot about the layout of the car.

But then as a passing neighbour said, "John, you always like a problem."

Merry Christmas all.
John

 

[Bolide]

Sounds a bit like:

MB original wiring loom: 1
Aftermarket LPG wiring: 0

Nick Froome
www.w124.co.uk

 

[Greybeard]

Thanks Nick, I could do with the
One bright note was to discover that the loom ties can be released by bending back the little tag and pushing. Not a design that I've seen before, and it did help, so perhaps another goal in extra time for MB.
John

 

[Greybeard]

Well the fog cleared but the problem didn't.
With some difficulty I managed to get at the wiring loom that goes into the relay socket, and the two joints are well soldered, so that doesn't look like the answer.
Everything is now back in place, including this evening's fog, so more thinking time is in order.
I did try starting the engine again, just in case sod's law was in effect, but no such luck.

I'm sure I'm missing some piece of the jigsaw that relates to the resistance measurements I made. I checked the measurement between the socket pin 87 and earth and it's still ~40 ohms. While looking for the fault, I tried wiggling the loop of wire where it was connected into the original, and briefly it changed, but that might have been the meter leads moving (prods only,no clips).

I'll have another go tomorrow, when I plan to connect a separate 12v supply across the pump as a first step in starting the fault finding process again, but I'd welcome any other ideas.

:idea: I've just realised that one thing I haven't tried is a shorting loop across the gas timer relay contacts, as well as a loop across the fuel relay socket, so that will now be the first step tomorrow.


John

 

[Greybeard]

Sorted. A shorting link across the gas relay wires and away she went.
So a quick Christmas wash and we've got respectable mobility for the dash across to the rest of the family.

The relay is now on the bench for a post-festive post-mortem.
John

 

[television]

Pleased that you got there, and so it was someone else's work.

 

[Greybeard]

Thanks, Malcolm, and all the best for Christmas.
John