Mercedes Benz E-Class W124 Estates - Electrical problem with fuel pump | Technical matters | Back Room Forum

Thu 15 Apr 2010 15:16

Mercedes Benz E-Class W124 Estates - Electrical problem with fuel pump - snbatra

Hello,

Can you help?

My 1993 280E Estate has an intermittent problem.

Every now and then there is no current to the fuel pump. The engine cranks over ok but no start. At other times there is current and the car starts first time and runs perfectly.

I've done the following checks and actions:

1 When it fails there is no current to the pump live terminal - the earth is ok.

2 Changed the green fuel pump relay - problem persists

3 Took out and cleaned the following relay terminals - OVP, fuel pump and the black relay which is marked KD.

4 Cleaned all the fuse box terminals and fuses.

The problem persists so I'm not using the car at the moment to avoid getting stranded.

I've not checked the ignition switch as the relays and other elements always click and work and I don' know how to access the ignition switch.

Help!

Regards

Suresh Nakra

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Thu 15 Apr 2010 18:06

Mercedes Benz E-Class W124 Estates - Electrical problem with fuel pump - WellKnownSid

Sounds like you've tried all the right things. Looking at the Haynes manual, I can see the fuel pump is powered from pin 87 of the fuel pump relay, whose supply appears to be fed from the OVP and controlled from several other points.

I think checking pin 87 on the fuel pump relay has +12V is a start. If it doesn't, then checking whether the input feed from the fused output of the OVP (pin 30) has 12V may clarify things. It looks like this is the fuse that's mounted on the OVP itself.

The OVP itself seems to be a common source of head scratching on these cars.

Hope that helps!

Edited by WellKnownSid on 15/04/2010 at 18:12

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Wed 21 Apr 2010 09:13

Mercedes Benz E-Class W124 Estates - Electrical problem with fuel pump - snbatra

Dear WellKnownSid,

Thank you for your reply of th4e 15 April.

Checked pins as you suggested - all ok but I presume they would be when everything is working ok.

After I had cleaned all the contacts and terminals all was well for a few days and then yesterday it was back to no current to the fuel pump. However, a new symptom appeared, there was a pulse to the speedometer and the needle jerked upward and then back when the igniton switch was turned on. the fault cleared itself after turning the ignition on and off a few times.

Any ideas as to the cause.

Thanking you in anticipation.

Regards

S Nakra

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Wed 21 Apr 2010 18:21

Mercedes Benz E-Class W124 Estates - Electrical problem with fuel pump - elekie&a/c doctor

The Ovp relay (the one with the fuse in the top) is also the fuel injection relay.It is silver in colour and usually has a plastic protection sleeve over it.These suffer with dry solder joints on the printed circuit causing a non start/no fuel pump condition.These can be repaired but require the aluminium casing to be de-crimped to allow the circuit board to be slid out.Other wise replace it.From memory I think they cost around £80.hth

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Sun 25 Apr 2010 10:23

Mercedes Benz E-Class W124 Estates - Electrical problem with fuel pump - snbatra

Dear elekie&a/c doctor,

Many thanks for your response.

The OVP relay was replaced with a new one about two years ago when the ABS light came on - this sorted out the ABS problem.

I am not competent to re-solder the ovp relay. If it has dry joints then I shall have to get a new one - is it worth getting a used relay?

I also get a strange pulse on the speedometer with the needle jerking when the ignition is switched on. If it is going to work then this does not happen. Is this also an indication of the OVP relay malfunctioning?

Thanks again

Regards

S Nakra

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Sun 25 Apr 2010 10:57

Mercedes Benz E-Class W124 Estates - Electrical problem with fuel pump - elekie&a/c doctor

It would be unusual for an ovp relay to fail after 2 years.Was this a genuine MB part?Also has the car been fitted with an alarm system,either MB approved or other,as sometimes the immobiliser circuit can interrupt the fuel pump relay.I would only replace the relay with a new genuine MB part.As for the speedo needle,I am not sure why it should jerk as it is a completely mechanical item driven by an old fashioned cable with no electrical connection at all.hth

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Tue 27 Apr 2010 11:02

Mercedes Benz E-Class W124 Estates - Electrical problem with fuel pump - snbatra

Hello and thanks,

Yes, the OVP was a genuine MB part purchased from MB.

Yes, there is an alarm system but it is switched off as the car is unused for long periods and the alarm system drains the battery. Would the alarm system be causing the problem even while switched off? How can one test if this is the problem?

Thanks

S Nakra

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Tue 4 May 2010 08:59

Mercedes Benz E-Class W124 Estates - Electrical problem with fuel pump - snbatra

It would be unusual for an ovp relay to fail after 2 years.Was this a genuine MB part?Also has the car been fitted with an alarm system,either MB approved or other,as sometimes the immobiliser circuit can interrupt the fuel pump relay.I would only replace the relay with a new genuine MB part.As for the speedo needle,I am not sure why it should jerk as it is a completely mechanical item driven by an old fashioned cable with no electrical connection at all.hth

Hello,

Sorry about my error - the needle jump is on the tachometer not the speedometer! Does this help you to identify what may be the problem?

There seems to be some confusion about which relay(s) need repair or renewal - the OVP or the black KDN. I've already replaced the green relay behind the KDN relay. Can you clarify?

Tkanks

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