What sends power to the...

What sends power to the Fuel relay? I'm trying to figure out why my jeep fuel pump sometimes does not get power. About half the time, there is no power to the relay. There is power from the battery, but not the side that triggers the relay.
88 4.0

Right now, the jeep runs perfect when it's cold. Once it gets warm, it just shuts down. No sputtering... just dead. any ideas?

jrowell said:

What sends power to the Fuel relay? I'm trying to figure out why my jeep fuel pump sometimes does not get power. About half the time, there is no power to the relay. There is power from the battery, but not the side that triggers the relay.
88 4.0

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I believe there are wiring diagrams up on my RENIXPower group (groups.yahoo.com/group/RENIXPower - you'll have to sign up, if you haven't already,) but I don't recall directly. If you can't find it there, remind me in a little bit and I'll dig out an FSM for reference (probably be for a 1990, but the only real difference would be the colour of the wire.)

i would think the relay would get power by the signal from the cps. if the cps is weak(magnet), the heat could have an affect on it.

nope, i take that back, as soon as the key is turned the relay should have power. could still be a bad cps.

Do I have any more votes for the CPS? Has anyone had problems with it only working when it was cold?

The CPS would generate a signal that tells the ECU to turn on the relay, but it won't turn it on directly.

The FPRelay will also be a different signal than for the fuel injectors - the fuel injectors aren't fired until (at least) the CPS gets a 300rpm signal to the ECU. The ECU also looks for a valid SYNC signal - but it can (and do) start without one by "guessing".

jrowell said:

Right now, the jeep runs perfect when it's cold. Once it gets warm, it just shuts down. No sputtering... just dead. any ideas?

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Hot short/open from a sensor, CPS, TPS, MAP, distributor, coil, or ?? The fun part is catching it the act, or testing the component at underhood temps.

Run the RENIX sensor diagnostics at the operating temps - carefully - you should find a part that just doesn't respond warmed up.

It is a hassle finding this one.

That's why I've long kept a hair dryer in my toolbox - it's great for simulating engine heat on a suspect component...

jrowell said:

What sends power to the Fuel relay? I'm trying to figure out why my jeep fuel pump sometimes does not get power. About half the time, there is no power to the relay. There is power from the battery, but not the side that triggers the relay.
88 4.0

Click to expand...

You have four or five pins on the relay (this position only uses four).




The thirty pin is constant power, red wire, most all the red wires are through a fusible link and always hot.
The 87 pin passes on power when the relay is activated. Orange wire, many orange wires in the Renix are relay (switched current) outs.

The 85 and 86 pins are the activator coil, an electro magnet that switches the contacts.

The coil *in* wire (86 I think) is yellow, most all the yellow wires in the Renix are hot when the ignition switch is in the run position.

The coil out wire goes to the ECU. the relay coil is activated through the ground side at the ECU. Orange wire with a stripe (white I think). many of the colored wires in a Renix with a strip are function wires.

The fuel relay is bypassed when the ignition switch is in start. The fuel relay closes for a second or two when the key is first turned to run, to prime the fuel system and then shut's down (opens). During start the fuel pump relay is bypassed and the fuel pump gets it's power the same place the starter solenoid does, the starter relay. When the motor starts the ECU closes the fuel relay.

If the CPS doesn't send a signal (the motor isn't running) the ignition coil and the fuel pump relay will be shut down by the ECU, along with the injector drivers and some other stuff.

I've had my 88 refuse to start hot and start fine cold on a few occasions. I had a brain storm and actually poured a bottle of water on my CPS one day to get my XJ started. I had a suspicion the CPS and heat were the problem, this confirmed my suspicions.

The CPS may not be the whole problem, the signal gets weak from corroded or dirty connectors, the CPS wires cooking on the manifold and also sometimes when the CPS itself gets hot.

That is a huge help 8Mud! Thanks for taking the time to respond. That give me a direction to start looking.


Thank you!

So, we changed the CPS, TPS, and the Map Sensor... No luck. Does the same D#$% thing!

Going to throw a different ECU in it.

Any other suggestions!

Do you have spark? If yes, then the CPS sensor is good.

Stop throwing parts at it would help your wallet. You might as well throw on a new rear bumper and see if that fixes it.

Narrow it down and isolate the problem before buying parts. A faulty TPS wouldn't case a no start condition. Fautly MAP would most likely just cause it to run rough and sputter, but it can stop it from starting at times.

Sounds to me like when it won't fire up, you should take a spark plug out and ground it against something, crank it up and have someone look and make sure it's giving off a blue spark. If it is, then it's definitely a fuel issue.

On that note, you can smell the spark plug when you take it out and see if it's wet with fuel. Also you can check your fuel pressure on the schrader valve on the fuel rail. 30 PSI at idle, 40 PSI with fuel regulator vacuum hose disconnected. Should also keep fuel pressure for awhile when it's shut off.

Also, you can splash a tiny bit of fuel into your throttle body - it'll act as a TB Injected system and it should fire right up if you have spark.

Thanks for your help. Not throwing parts at it, just narrowing it down sensor by sensor 8>) I'm not sure a bumper will help... But I guess I'll try that out too. Thanks for the suggestion!

It has spark and It has and keeps fuel pressure. Once it gets warm and shuts down, if we keep trying to get it running, eventually it will... but it will run VERY badly- and anything over half throttle will kill it. Let it cool off and it will start right up and run perfectly!

So you're saying it has spark after it heats up and shuts down? is it a bright blue spark like it gives off when it's cold?

Splash some fuel down into the throttle body when it won't start, and see if it runs normally. Do you have a heat shield in between your fuel rail and intake manifold? It might be heating up and doing bad things to your gasoline.

As another thought, disconnect your Manifold Air Temperature sensor, AND your Coolant Temperature sensor. The RENIX system will use data from charts, so it should run semi normally. Sounds like one of those might very well be your problem.

Howsabout we try this.


The fuel pump relay only is needed during startup. No relay on start, no start, just crank. try getting the XJ going then yank tat relay. Wont shut down.

Now, I had the same issue. My issue ended up being the fuel pump ballast on the fenderwell.

(this was on an 89 XJ with a 4.0)

Now we are checking out the Fuel pump. Will swap it tomorrow (already have a spare, so no waste of money-only time)

I hate these frustrating electrical issues!

How about the fuel pump resistor on the drivers side fender( white ceramic) that could be shorting out at a heated state. What you think Jon?

90Blue_XJ said:

How about the fuel pump resistor on the drivers side fender( white ceramic) that could be shorting out at a heated state. What you think Jon?

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Possible. I got rid of mine years ago - connect the two wire ends directly together, or make a 14AWG (or larger) jumper to take the place of the resistor.

It was never on my 87, I've removed it on my 88 and on one of my 89's, and haven't had any trouble. It was added as a voluntary recall for NVH to cut down on fuel pump noise (which I don't mind anyhow - for the same reason I like a little rumble in my exhaust. I like to know my engine is running. If I wanted quiet, I'd get a Cadillac...)