my 1990 4.0 just will not run. it'll crank and it will attempt to start, but once the key is in the run position, immediately the engine just dies. everytime, i try to start it, the same issue crank tries to catch, let go of key, nothing. i've replaced the ignition switch due to a burnt connection and the lock cylinder followed suit, checked power to the ballast, tps, cps, did relay swap with another jeep, checked the power to the ignition switch, and still having the same issue, could this be related to a ecm or the neutral safety switch? if not any suggestions, also does anyone have a good wiring schematic for the ignition switch and engine control system for the 1990 year? haynes is brainless!!!!!!

 

Check the fuel pump ballast resistor, right fender forward.

The ballast was added mid-1988 to address NVH issues (Noise, Vibration, Harshness) by ChryCo, it was a voluntary recall for prior releases.

Generally, when there's a ballast in place, there end up being two circuits - the ballast is bypassed for starting, then cut in for running. So, if the ballast blows, you'll usually not be able to run once you release the key to RUN from START.

The nominal resistance for the unit should be stamped into one of the tabs near the ceramic, but it sounds like it's "open" entirely. Disconnect at least one wire before checking resistance, so you don't get a distorted or false reading.

The part may be replaced with one of like resistance, or bypass it entirely (use a short section of 14AWG wiring with 1/4" spade-style quick-connects - insulated - on each end. This is my usual "repair" for the thing, but I think trucks should make some noise anyhow.

 

You should have battery voltage to the front terminal of the ballast resistor and about 10 volts at the rear of it. Have you tried jumpering the ballast, effectively eliminating it?

 

Update: I have replaced and bypassed ballast, replaced cps, replaced ECM and ICM, and I have cleaned and relocated dipstick grounds to engine battery ground location. The jeep is acting better with wanting to start, but it still will not. It's making a solid effort at running, but still will not run. Oh, I did check spark plugs and distributor cap. The distributor looked slightly but still good, and the plugs had slight carbon foul, but stilll looked fine. I cleaned the plugs and put them back and still nothing. Any new sugestions or am I in serious trouble? Please help.

 

Time to check your fuel pressure on cranking.

 

I have checked the pressure: With the key on, the pressure is around 40, and when I crank the engine, the pressure drops to between 32 and 34 psi.

 

If the map sensor is unplugged it will die the second it starts. Just a thought

 

Burn connection, what about plug and did you adjust it ?

 

Good thought. The wire in the plug that burns needs to be cleaned up and tightened up so it grabs the connector on the switch or this condition will repeat itself.