ELC_AMX

Title cuts to the chase a bit so people know what I'm REALLY looking for, but I'll give some description here:

First off, I believe my problem has to do with the ignition switch not supplying power through the SINGLE WIRE that is supposed to send signal to the ICM, the o2 sensor relay, and the fuel pump relay. My problem is that I have a crappy Haynes manual (I know that I should throw it in the trash but it's all I have for wiring diagrams) and it shows me practically nothing, for what I need.

So here's what's happening.

I have an '89 4.0L 5spd manual Cherokee that crapped out all at once one day. It will crank when I turn the key, but it gets no fuel or spark. The fuel pump never primes at any position on the key, and while cranking there's no spark, even off the coil wire. HOWEVER, I can put a jumper wire into the 87 pin of either the fuel pump relay or o2 sensor relay sockets (both of which control the fuel pump, just at different points in time) and the fuel pump cranks on AND builds fuel pressure in the rail (I've tested). This also works both with and without the ballast resistor in the circuit, so the ballast resistor is good. All of this means I know the wiring from the relays back is good. I haven't fiddled much with the ignition control module since I know it's tied into the fuel pump wiring, so they're probably the same issue here.

I have ALSO taken a long test lead and jumped power alllll the way from battery positive to the wire at the ignition switch that is supposed to feed into all the aforementioned relays, and got... 3 volts back at the relays. So there seems to be a short/partial open somewhere, even IF that wire would ever send any power without me jumping it... Which it doesn't.

Crank Position sensor is brand spankin' new, I replaced it last week even though the one in the jeep was only two months old and worked fine. Throttle position sensor is new though that shouldn't matter. I've swapped all sorts of relays out with a new test relay I bought. I've been all through the wiring harness under the hood and cleaned all the connections, I even popped the fuse panel out and checked it out, it's clean, but I went ahead and refurbished all the connections anyway. I pulled the ECU (engine control unit/(module?)), checked the connections, then opened it up and visually inspected the board for any visible damage, it looks perfect. I haven't plugged the ECU back up to try everything out with it in again, but before I did that I figured it wouldn't hurt to ask here:

Does anyone have any wiring diagrams that show exactly where the power is routed through to get from battery to ignition and back to the fuel pump relay/o2 sensor/ICM? Any knowledge on where the wires come and go under the hood? My multimeter's continuity function is broken (it WILL NOT STOP BEEPING on continuity setting even though everything else is fine), so it's hard for me to properly trace without yanking wires out physically to check, and that gets messy fast.

I have one final thing. This is the only wiring diagram I have related to the relationship between ignition switch and fuel pump relay/o2 relay/ICM. This is it. The yellow wire coming from the ignition switch is what I've been staring at.



There's no way it's as simple as this diagram tries to tell me, I've tried tracing from the ignition switch to the aforementioned relays/modules and got sent to... the back end of the jeep? Not only is that wire NOT yellow, there isn't a single yellow wire originating from the first connector the ignition switch feeds into - the one clipped onto the side of the steering column. Big wide one. The best I can tell, power for those relays actually comes from the ECU based upon where the physical wire routing goes leading back from the fuel pump/o2 sensor relays and ICM, but... Who knows. I just hope I can get some help here.

TL;DR for those who want a short explanation: There's a single wire that switches on the fuel pump and ignition when I go to crank, but that wire never sends any power to any of those components, so no fuel or spark in key-on position. I am relatively certain that that wire is my problem, but I am not sure where it comes from exactly, so I need a better wiring diagram than I have right now to make sure since my multimeter is broken. Or, even better, someone with experience in troubleshooting this to tell me what they've found in the past.

cruiser54

Have you ruled out the ignition switch?
The c100 connector which is on the firewall side of the fusebox can cause low voltage issues.

As for a diagram, go to my website and see if there is a downloadable manual there that has it.

ELC_AMX

The ignition switch itself is new too... I just finished removing, inspecting, cleaning, and re-installing the fuse box today, and it looked okay too...

ELC_AMX

Okay, so after re-installing all the stuff I removed and cleaned, I am now getting fuel pump activation while cranking. Still no priming, but it's working while cranking - 40 PSI of pressure, according to my pressure gauge. I pulled the ignition coil and control module today and ta-da, they are probably original. I'll go ahead and replace those too, just because.

Oh, I'm also considering installing a fuel pump switch as an anti-theft device - then I'll have a starter killswitch AND a fuel pump switch... I should probably run the relay's coil power off of a circuit that kills itself upon rollover so the fuel pump doesn't keep running if I roll... Does a renix jeep have a rollover switch for the fuel pump, or is that a later innovation?

cruiser54

fuel pump shuts off when engine dies.

ELC_AMX

Alright fellas, I found the problem. Turns out that the switch I used for my kill switch was too low of an amperage, so it burnt out inside while feeling fine to turn on and off.

Jeep's fixed now, I've been doing some rewiring for the past few days, making up relayed circuits for some heavy load items. My fog lamps are now relayed, my cooling fan is now relayed, and all the other accessories I've added are properly fused (they weren't before, which was stupid of me, and just asking for a fire!)