Starting

GENE HALE

MEMBER
1989 JEEP CHEROKEE
4.0L
6 CYL
4WD
AUTOMATIC
170,000 MILES

The two second timer to the fuel pump relay does not active when key is on to run position. It will crank over in start position. It will start with a twelve volt test light connected to ground and terminal number 35 ( relay plunged in ) test light draws 0.10 amps, light is dim, turning key off light goes out. I have driven it around town that way. Fuel pressure comes up to around 39 number and cuts back to 31 number running. It all started when I jumped number 35 and number 36 at the female area of the relay, I got mixed up and should have jumped 30 and 87 to get pump to turn on.

2 Replies

STEVE W.

MECHANIC
13,576 POSTS

Check the fuel pump relay and its socket. The contacts can get weak and the relay does not operate.

Was this

answer

helpful?

Yes

No

Sunday, April 23rd, 2017 AT 9:36 AM

STEVE W.

MECHANIC
13,576 POSTS

Had any luck with your Jeep?

I was looking at your description closer. Do you mean you jumpered terminals 86 and 85? If that's the case it may have damaged the ECM.

To see if that's the case, connect the test light to battery positive, then touch terminal 85 (this is switched ground from the ECM) It should light when the key is turned on (prime) and when the engine is turned over (that is triggered from the tach signal)

If it doesn't send the prime signal or activate the ground when cranked the fault is inside the ECM. Basically the ground transistor was burnt out due to excessive current flow from the jumper.

To have it operate as factory you would need to replace the ECM. OR if that is the ONLY issue, you could add a second relay, connect the relay to switched power that is hot in run/start only. Then use the contacts to switch the ground signal at terminal 85 on/off. You would lose the automatic shut off if the engine stalls but it would run just fine.
Another option would be to pull what GM did on some engines. They used an oil pressure switch to provide a ground to the pump. You could just T in a switch for an oil idiot light and use it as the ground for pin 85. It would ground the pump as soon as you had 5-10 pounds of oil pressure and shut off the pump once the pressure dropped.