New Fuel Pump Relay Fault

[Quirrel]

The W126 300se I am picking up has fueling issues.

When bypassed the FP runs constantly as expected, however it does still cut out under load so suggesting an issue with fuel flow/pressure.

The previous owner (an old old friends dad that I have just caught up with after 15 years saving the car from the scrap man next week) has bought a new FP relay, which didn't solve the problem.

The car won't run without the relay being jumped.

So do the FP relays often turn up broken when new? He paid 80 quid for it and is miffed, but didn't have time to do anything else to it.

The car is solid, it started (on jumpers) for the first time in 6 months straight away.

Just the fueling issue to deal with just now and getting an MOT.

 

[grober]

The fuel pump relay is not a simple animal. It performs several functions. It will run under the following conditions. 1. For couple of seconds on switch on= pressurise the system 2. During engine cranking= may also power the cold start valve
3. If it gets a constant engine RPM signal between 400 and 6540 rpm = a safety feature to prevent fuel being pumped to a stalled engine as in an accident.

If for some reason it fails to get an engine running signal then it will shut the pump off.

Since the car has been sitting some time I would be inclined to drain the fuel tank and use new fuel and check the exit strainer is not blocked- renew the fuel filter - make sure its the correct way round. It may also have 2 fuel pumps in series - make sure both are working.


ps are you certain he got the correct FPR there are several altho the later ones tend to be backward compatible

 

[Quirrel]

I'm not convinced.

It's a 1991 but he has the black relay with no knob, I am sure the newer relay is the MAS one with the knob on top.

 

[grober]

The relay with knob on the top is only for the 5 speed automatic cars afaik

 

[Quirrel]

It is an autom no idea if it's a 5 speed or not though.

 

[219]

Your 300SE will be a 4 speed .