I'm a nubee to this forum with an 84 XJS 3.6 coupe, one of the 24 US spec cars with a Getrag 5 speed brought here for evaluation. Most of the systems are identical to the US spec 5.3 V12s but the federal spec 3.6 engine has only 8.2:1 compression compared to the high compression European spec engines. It has 46k on the clock and I have owned it for 15 years after buying it from a Jag engineer who worked at the US HQ. It has been off the road since 2000 but up until 3 years ago in dry storage and started regularly. When I lost the storage garage it had been in and needed to move it, it wouldn't start and it then sat outdoors for two years. The fuel pump wasn't working and there was about a gallon of old fuel in the tank which had had fuel stabilizer added to it. I replaced the pump with a Bosch 69536 pump, this has threaded posts not spade connectors, one post is approx. 4mm in diameter and the other about 5mm. Unlike the original pump there are no + or - markings so I connected what had been the smaller 1/4" wide spade connector lead to the smaller post and the larger 5/16 spade lead to the larger post. Pump now spins up for 2 sec when the key is turned on as it should. I also replaced the fuel filter, added 6 gallons of high test fuel, also a can of octane boost. I removed the air filter and if I open the butterfly valve on the throttle body and spray ether into the intake, the engine fires and will rev to 2k smoothly but then quickly dies once the ether is consumed. Even though the pump runs there seems to be no fuel in the FI rail. I pulled the return hose from the pressure regulator and attached a new length of hose to a bottle and when I turn on the ignition and crank the engine I don't get a drop of fuel from the relief valve. I also replace the FI relay but no change. My question is, can I have accidently reversed the polarity on the pump? Will that pump spin the opposite way with reversed polarity and deliver no pressure? Before I experiment and swap the leads on my new $160 pump and end up frying it, I thought I would ask here first.

 

Firstly since you are new, WELCOME.

I am surprised a Bosch has no indicators, but, so be it.

I would reverse the wires.

NO harm will come of the pump, been there, done that, a looooong time ago.

Simply got it wrong when fitting the wires, SAME symptoms as you have. Simply swapped the wires, action, V12 sweet as. Same pump as yours, and being a DC motor, as I said, NO harm.

 

I swapped the leads and 'BINGO' problem solved. I also discovered that the new fuel pump is a heck of a lot quieter when spinning the right way! The engine fires immediately and is idling smoothly with good oil pressure. Next project is an oil and filter change then brake overhaul since the left rear caliper is leaking. I also need to flush all of the hydraulics lines including the clutch. Then follow that with a cooling system flush. Thanks for the help.

Bottom line for anyone replacing an original Lucas 4FP (aka Bosch) pump with spade connectors, the smaller 1/4" blade is marked ground and the wider 5/16" blade is marked positive. The replacement Bosch pumps have no markings and the smaller threaded post is positive and the larger post is ground. The pump spins up either way but you don't get a drop of fuel to the FI unless the polarity is right.