1987 Cherokee 4.0

I've run into a problem with the fuel pump in my XJ. When I turn the ignition on the pump doesn't prime. I removed the relay and jumped it and it pumps so it's an electrical problem.

I swapped the relay out twice with relays that are known to be good and the problem persists.

I put a multimeter on the socket pictured below I got the following readings.

Clockwise from the top with the ignition off.
O.00
0.00
0.12
12.62

Clockwise with the ignition on.
11.88
.005
.165
12.41

When I jumped it I used the top and the next one going clockwise. As I said that was successful.

Can anyone point me where to look?

 

Hmmmm. Having it removed like that makes it pretty hard to know which is what, but maybe we can figure this out just from the info you have provided and some basic relay knowledge.

We should have 4 contacts:

12v from battery
Out to the fuel pump.
Ground for the coil.
12v from ignition switch (in Start & Run positions) to the coil.

So, with the key OFF, we would expect to see 12v on ONE contact. Check.
With the key ON, we would expect to see 12v on TWO contacts. Check.

That sounds right, so where's the problem?

It could be the ground isn't grounding properly. I don't have info on the ground points for the earlier XJs available, but Cruiser54 probably does. He is Mr. Renix. You definitely want to check that out. One of those contacts should have very good continuity to ground. You can check that with your meter. That would be the first step.

If that's all okay, or fixing a bad ground there does NOT solve the problem, then we can assume that the 12v we are seeing isn't good enough.

"Huh? What in the world do you mean, "not good enough"??? Are you crazy???? 12v is 12v!"

Well, no. If there is corrosion somewhere upstream in the circuit, you may measure a nice 12v with your meter, but once a load is supplied, that 12v may drop to nothing. Try the same tests with a test light and see what results you get.

 

Thanks, Mark. If the problem is upstream would I still be able to jump it to get current to the pump? Wouldn't that problem not let me jump it?

 

If the problem is in the main 12v supply, no. But it might be in the coil supply.

 

Try backprobing the connector with it plugged in. The +12 to the coil of the relay comes from the ignition switch and the computer controls the ground leg. From your description and the picture, I think you actually jumped power from the ignition switch instead of the normal power feed from the fusible link at the starter relay. Try jumping the left and right pins together in your picture, which is what the relay would normally do. I would check the fusible links at the starter relay on the passenger fender. I would also check that the fuel pump ballast resistor on the drivers fender is intact (assuming your 87 has one as I think it was a retrofit in later years)

You do need to take care jumping power at connector, btw. If you jumped power directly to the pin going to the computer (the bottom pin I think), you will blow that circuit in the computer.

 

+1 on jumping the left and right pins to see if it primes

Top ignition on to tell relay to work (Pin 85)
Right - to fuel pump (Pin 30)
Bottom - Ground (Pin 86)
Left - Constant 12v from battery (Pin 87)