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Odd starting running issue, need help.

G

GarageAlchemist

NAXJA Forum User
Location
Fargo, ND
1989 Cherokee 4.0

About a week ago i was rollin down the road, at like 25, and the jeep shut off. No sputtering, just shut off. I figured it was the fuel pump, as i had spark, but no fuel pressure. I put in a fuel pump, and still nothing. The jeep would crank and crank, but never fire. Then i noticed the wire loom that runs over the top of the IM, and includes the 02 sensor, oil pressure, and something else, had melted to the exhaust mani!

I took and separated the wires, and soldered in new ones, and now the jeep wire fire and run fine, AS LONG as i am holding the key on engaging the starter, if i let the key go back, it shuts off....

Pulled the fuel pump and the wires are in the right spots, but it doesn't prime, only works when im holding the key on... HELP!
 
theres a ballast resistor on the drivers side firewall its ceramic and has two leads hooked to it take those leads and jump them your pump will run direct
the resistor is just there to reduce fuel pump noise any way or so i've read
by jump them i mean take a cotter pin and stick the wires on it if its the resistor them the jeep will stay on
i just twisted my lines together with a button clamp and thats i have been running without the resistor for like a yr and change try it and get back
 
I jumped the ballist resistor and it did the same thing. Ran with the key on, died when i stopped cranking. With both wires un-hooked it wouldn't fire at all. So that is not the problem. I pulled the fuel pump again to triple check i had the wires in the right spots. The next thing i pull is going to be my hair! Any more suggestions?
 
Go to the passenger side fenderwell.

Identify the three or four relays there.

From front to rear, a/c clutch relay (if a/c equipped), power latch relay (aka ASD), fuel pump relay, oxygen heater relay.

Try switching them around to see if that helps.
 
I know you said the problem started from melted shorted wires, but years ago, I had an old Pontiac -same switch design as yours- fail the same way. Starter cranked fine, but you had to find the run position 'sweet spot' before the engine stopped spinning. If all the other stuff fails, I just gave you one more lead to chase down!
 
Check the wiring at the ignition switch. Ignition switch may be going bad, and or the wiring connectors to the switch may be going bad. Check the fuse box for loose connections, in addition to the other suggestions below.
 
The key switch is where you put in the ignition key. It has a coat hanger sized wire that runs way down the column to a slider style multi throw, multi pole switch that switches power to everything in the jeep. The switch is way down the column, on the top side of the column, and not easy to reach. There are some good OEM forum threads here on how to service the ignition switch, some links in those threads and some images. Try a search in the OEM forum for "switch" or "ignition switch"

The switches do go bad, they are also know to melt and corrode the bulkhead wire end connectors that attach to the switch, and there is also a rod connector to the switch, from the key lock mechanism that can get out of adjustment and cause problems.
 
Unless you've got X-ray vision, take and check with a meter for continuity across the approriate poles, checking for smooth action and for dropouts, or inconsistant contact.
 
i have them, but my knowledge of electrical systems is pretty nil. They might as well be written in Russian. But i can follow instructions. And, try as i might, x-ray vision doesn't seem to be one of my gifts.
 
I'll try to explain it to so it's understandable :). When cranking the engine the *starter relay* supplies power to the fuel pump circuit after (bypassing) the ballast resistor. When the motor starts the ECU closes the fuel pump relay, which then provides power to the fuel pump, through the ballast resistor.

As best I can figure, the three second prime comes from the O2 heater relay (Renix). But I've never actually tested it.


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Last edited:
me too!! would like to know if bypassing the ballast worked. i'm a little hesitant, this is my first go-around with the renix engine, and they seem kind of particular with their electronics...
 
FWIW 87 did not use a ballast resistor. By passing the ballast resistor allows you test for a bad resistor. All the resistor is drop the running voltage back to static battery voltage, to extend the life of the fuel pump.
 
8Mud, what makes you think the O2 heater relay has anything to do with timed fuel pump prime? I would thing the Renix computer is powering the fuel pump relay on an internal Renix computer virtual timer and still using the fuel pump relay. There must be an ECU interface between the fuel pump relay and the starter run switch IMHO.
 
Ecomike said:
8Mud, what makes you think the O2 heater relay has anything to do with timed fuel pump prime? I would thing the Renix computer is powering the fuel pump relay on an internal Renix computer virtual timer and still using the fuel pump relay. There must be an ECU interface between the fuel pump relay and the starter run switch IMHO.
Click to expand...

B latch relay is the source for the 12v to the fuel pump when the key is ON.

Starter relay is the source for the 12v to the fuel pump when cranking.

Fuel pump relay, through ballast resistor, normal engine operation.

oxygen heater relay is the source for the 12v to the fuel pump at WOT.

This is from the 90 FSM.
 
WOW!


That is simply bizzare! I wonder if 5-90 knows this. The one that is so strange is the O2 heater at WOT powering the fuel pump! What were they thinking of? Is it a typo? Is it to boost fuel pressure and bypass the ballast resistor at WOT? Maybe so.

Very interesting.


joe_peters said:
B latch relay is the source for the 12v to the fuel pump when the key is ON.

Starter relay is the source for the 12v to the fuel pump when cranking.

Fuel pump relay, through ballast resistor, normal engine operation.

oxygen heater relay is the source for the 12v to the fuel pump at WOT.

This is from the 90 FSM.
Click to expand...
 
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