Mechanics

INTERMITTENT FUEL PUMP PROBLEM

1994 Jaguar XJ40

Electrical problem
1994 Jaguar XJ40 6 cyl Two Wheel Drive Automatic 86000 miles

I have an electrical problem where, from time to time, the fuel pump does not activate when I first try to start the car. I have 12 volts to the fuel pump relay. I have continuity all the way to the pump from the relay (about 3 feet). I have jumped the relay directly to the fuel pump power in wire and the pump does not run. However, if I jump that wiring directly from the battery, I get the pump to run. It is as though there is not enough voltage to initially get the pump started.
Once I jump the wiring directly to the battery for about 1 second, all the problems seem to go away and the car will start and the electrical system operates as it should. Ie. Jumping the system from the fuel pump relay activates the fuel pump.
Some times I can go for several days before the problem resurfaces. I then jump the pump directly to the battery for a second and I can then be on my way again.

I am enclosing some information from a jag forum.
It may help you with giving me directions to my resolving this problem. I have it in the form of a pdf file here, and on my website imertz. Com/downloads/Fuel system check. Pdf


Thanks,

Wayne
Avatar
Wemertz
October 24, 2010.



Hi Wayne,

Thank you for the donation.

Symptoms description indicates the fuel pump is most likely to be failing.

If it is insufficient voltage at the relay, it would show less than 12 volts.

Did you try connecting power direct from battery to the relay terminal that goes to the fuel pump and did it work?

KHLow2008
Oct 25, 2010.
The fuel pump is fine. When I started having the afore mentioned problems, I took the tank, pump and all out of my other 93 XJ6. I know the pump is fine. And yes, as I indicated in my original question, I did jump it directly to the battery. It did work.

This is an intermittent problem. I've driven the car now for three days without a problem. (Lucky)

I also supplied a link to my website that gives you some insight into what I've tried on my original question.

If you need that information, I can resubmit the link.

Wayne

Tiny
Wemertz
Oct 25, 2010.
What I meant was apply battery voltage to the relay terminal leading to the fuel pump, not directly at the fuel pump. Jumping the relay terminals did not work, did you test the voltage at the fuel pump when this was dione? Did jumping the relay and adding power source from the battery to the terminals works the fuel pump?

You mentioned using the tank and pump from another vehicle and problem persisted?

Intermittent conditions can be due to loose connections and when this occurs, the voltage to the relay could drop and not activte the relay or the power supply from source to the relay and fuel pump drops resulting in insufficient power to work the fuel pump.

The link that you mentioned listed how to go about testing the relay and wire connections, etc. It is not going to be of help as your problem is intermittent and the only lead you have is applying battery voltage to the fuel pump and it works.

We would have to work from here. I need to confirm that I understand the actual situation and answers to the above questions are important.

KHLow2008
Oct 26, 2010.
The fuel pump and tank from the other Jag worked fine in the 93", but after install into the 94' the problem still persisted.
I replaced it as a unit as not to mess with the wiring, etc. When I went to install it in the new car.
Yes, I tested for voltage at both sides of the relay and was getting 12 volts. I was also getting 12 volts at the lead into the pump. However, the pump did not start activation until I jumped the wire directly to the battery.
Then, the car works as normal, until the next time the it decides not to start.

I am thinking that although the unit worked fine in the 93' Jag, there must be a loose connection at the terminal to the fuel pump on the top of the tank?
I'm not looking forward to, once again, pulling down the tank to verify it one way or the other.

The whole thing just doesn't make sense.

Wayne

Tiny
Wemertz
Oct 26, 2010.
Sometimes wiring problems do not make sense.

How about the grounding circuit for the fuel pump, did you try cleaning and retightening it?

KHLow2008
Oct 26, 2010.
Checked everything except confirmation of the connector on top of the tank. As the problem was happening with the other tank and pump, connector didn't seem like a viable source of the problem.

Tiny
Wemertz
Oct 26, 2010.
The tank connector could be good but there is the possibility if the vehicle harness being the cause.

KHLow2008
Oct 26, 2010.