johndregg

Active Member

• Nov 29, 2015

• #1

Defender 110 v8 on twin SU carbs.
I'm having an intermittent fuel pump problem. One day it works, the next it doesn't. But on the days when it doesn't work my oil pressure warning light does not come on when I switch the car to start with the key.
I have read that the fuel pump only switches on when the oil pressure rises with the engine cranking.
Could there be a problem with the oil pressure switch? Or is there a relay that controls this?
I'm a bit stumped.

 

Flossie

Well-Known Member

• Nov 30, 2015

• #2

I have no knowledge of this at all buuut... If you have 1 wire to the oil pressure switch? Try pulling it off and earthing it and see if the pump works then.?

 

P

Pete-B

Member

• Nov 30, 2015

• #3

There is no direct connection between the oil pressure circuit and the fuel pump circuit as far as I know. I have a 1988 110 V8 and that has started and run normally with a faulty sender unit which indicated no oil pressure.

The wiring diagram suggests that a fuel cut-off relay was connected to the oil pressure circuit but only for the German market. If you also have a 1988 model then you're likely to have the headlight dim-dip circuit in place and there is an indirect connection between the ignition circuit (and then on to the fuel pump) and the oil pressure switch through that, but sees unlikely.

I can't remember where the earth point is for the fuel pump. If it goes all the way back to the engine area then it's worth checking the engine to chassis earth strap and the battery to chassis earth strap as it could be one of those. Alternatively a poor connection within the ignition switch itself which you should be able to check by testing whether other ignition control circuits work (indicators, brake lights etc).

 

norseman

Well-Known Member

• Nov 30, 2015

• #4

The fuel (pressure) pump on an EFI only activates when the engine is cranking, but not on carbs - to the best of my knowledge.

 

jamesmartin

Well-Known Member

• Nov 30, 2015

• #5

johndregg said:

Defender 110 v8 on twin SU carbs.
I'm having an intermittent fuel pump problem. One day it works, the next it doesn't. But on the days when it doesn't work my oil pressure warning light does not come on when I switch the car to start with the key.
I have read that the fuel pump only switches on when the oil pressure rises with the engine cranking.
Could there be a problem with the oil pressure switch? Or is there a relay that controls this?
I'm a bit stumped.

Click to expand...

oil pressure has nothing to do with the pump in a std set up ,if there is a connection between the 2 its were they get there positive feed from

 

johndregg

Active Member

• Dec 2, 2015

• #6

Is there a relay for the oil pressure warning light.
New oil pressure switch arrived in the post, but it will be Sunday before I can fit it. I don't have a wiring diagram for a military 110, hopefully this weekend I can check the output to the pump while creaking the engine.

 

johndregg

Active Member

• Dec 2, 2015

• #7

CRANKING!!

 

P

Pete-B

Member

• Dec 2, 2015

• #8

Not sure about military ones but there's no relay on non-military ones. The oil pressure switch just provides an earth for the warning light when there's no pressure.

One reason they wouldn't link the oil pressure switch to the fuel pump is to avoid problems where the engine would cut out if the switch developed a fault and incorrectly indicated no oil pressure. A friend of mine had this problem on a Rover 800 many years ago. He was in lane three of the motorway, the oil pressure switch failed so the EMS shut down the engine at 70mph!!! He wasn't very impressed.

 

T

texpis

Active Member

• Dec 23, 2015

• #9

I had a 1989 genuine V8 90 and it did have a relay that tripped the fuel pump if the light was on, I believe it's a safety feature if the oil pressure fails the pump stops and the engine stops eventually. Easy to check remove the wire from the oil light and the pump will run connect it and it will stop obviously with the engine stopped and the ignition on.

Mick

 

P

Pete-B

Member

• Dec 23, 2015

• #10

Mick, was that a factory fit relay? I've had two 110 V8's and neither had a relay. Also the wiring diagrams in the workshop manuals don't show one. But maybe they were fitted briefly as the safety laws changed.

 

T

texpis

Active Member

• Dec 23, 2015

• #11

Pete I don't realy know I never looked for the relay it was behind the dash somwhere, I just know it was there, I had changed the engine to a 4.2 and was wanting to prime the carbs and noticed the pump wouldn't run when the ignition was first switched on, disconnected the oil sensor light went off and pump went on. The rest of the car was. Very origional and the previous owner had owned it since early 2000.

Mick