Hi everyone,

I have a 76 XJ6C. I'm trying to get this old jag running after sitting a while. I replaced both fuel pumps (submersible type) and now get fuel delivery to the carbs. After starting the engine it will run for a moment then die. I noticed the fuel pump hum had disappeared once the engine started. After tracing the electrical I found that the fuel pump relay is wired to an oil pressure switch? When there is no oil pressure, the switch grounds the relay input and voltage is delivered to the pump. When there is oil pressure, the switch opens and the relay cuts off voltage to the pumps. Can any body tell me why the oil pressure is controlling the pumps?

Thanks,

Steve

 

YES.

That is the WRONG switch in that position, common error.

The original switch had NO ground with NO oil pressure, and ground WITH oil pressure. They were notorious for leaking/failing (Lucas switch), and most of us simply earthed that wire under a convenient screw/bolt, and plugged the switch orifice.

This fixed 2 issues.

1) The fuel pump stayed running as did the engine.
2) Another oil leak source fixed.

 

Grant,

Thanks, but that puzzles me too. If the switch is backwards, when starting the car, there is no oil pressure until the car is running, so the pumps would be turned off and no fuel is pumped, so the car can't start. Arghhh... how did this ever work?

Thanks again.

Steve

 

MAAAAAATE.

The carby fuel bowls ALWAYS have fuel in them. That is enough for about 2 minutes of running, even for a Jag.

Bear in mind these cars were DAILY drivers, and this sitting for 6 months or longer was unheard of.

We collectors etc have created this issue in the real world of motoring.

SOOOOOOO, that wire earthed is the simple fix. The pump runs when the ign is ON, so after a winter lay over, the pump will refill the bowls, and away we go.

The original switch was a 4psi unit, so even whilst cranking, enough pressure would be sensed, and the pumps would switch ON, and the rest is pure pleasure.

 

Yeah, a anti idiot protection. Supposedly, so one can not run an engine with no OP and thusly avoid disaster.

I have my doubts. If it will run a bit on the fuel in the bowls, running sans OP will mess things up quick enough.

My lump's GM harness had such a provision. I eliminated it. I thought it was a knock sensor til I studied the schematic.

I wonder if the ground might be repurposed with a different switch as an inertia fuel cut off. Those make sense to me and I included the one on the right A pillar in my project. A sort of antitheft device as well as a safety feature.

Back a few years, my luckless 88 Tbird was rear ended on the freeway by a little Asian truck. The engine shut down!! Fortunately, I was only one lane away from the shoulder. I coasted over and stopped. The fellow puilled in behind me. Stand up guy.
Apologies. We agreed that the CHP was needed. After the paper work was done, he left. But, my bird wasn't ready to fly. Nice trooper. We consulted my driver manual and found the location of the inertia switch behind a panel in the boot. Click and zoom. He was pleased as well. he told me he had a Mustang. Both built on the FOX platform. The manual will be in the car after perusal were his words.

Carl

 

Its a safety provision. The idea was that if the vehicle was in a frontal collision which stopped the engine the electric fuel pump would stop too and not keep pumping fuel on to a potential fire through a severed fuel line. I'd suggest a working replacement. Many have perished in hot rods with minor fires or leaky float valves and hard wired fuel pump when dazed by a minor crash. Given a few more minutes they could have stumbled away from the danger.

 

Grant,

So, if you go out and turn the key to your '83 to the on position, you don't hear a pump humming?

Thanks everyone.

Steve

 

Correct.

The S3 6 cylinder EFI system is totally different to any other. Bosch K Jetronic.

The airflow meter has 2 contacts at the front (located under the plastic cover on the airflow meter) operated by the 2 prongs of the variable airflap. These close as soon as the flap moves (suction from cranking), and that closes the contacts, thus earthing the fuel pump relay and the rest is engine activation.

I dont have an '83 anymore, hated that car.

I have an '85 V12, a '76 V12, an '84 V12, etc, and they ALL have the hum at ign ON, as designed, as that is a feature of the V12 cars.

All my S2 6 cylinder cars (including the one that arrived tonight) have the factory enertia switch on the LH A Pillar. I see the sense of that oil switch and its "safety" feature, but I have yet to have an engine fire, the fuel system under the bonnet is ONE area I NEVER joke with, it is RIGHT, or the car stays put, take one of the others.

Some S2 6 cylinder cars in the US of A have fuel injection, whoopee, something to do with California emission controls ??????, we never got that, we had carbies, up until the S3 intro.

 

Right!

Except the fuel pump circuit was also powered-up from the strater relay.....so the fuel pump runs whenever the starter is engaged. Once the engine begins to fire the suction part takes over








I often wondered why the 2-second-prime at 'key on' wasn't incorporated on the Ser III 6-cylinder cars. It makes perfect sense to have that feature although, in reality, they start perfectly well without it





Right again. The USA market got the F.I. on 1978 model year Ser II XJ6s. And, yes, it may have been an answer to California requirements which, at the time, were much more stringent then most other states.

Cheers
DD

 

Does anyone have a picture of the switch and part that needs to be earthed?

 

Anfield,

here's a link to the part....

C42200,JAGUAR C42200 Oil Pressure Switch for AUSTIN,ROVER