I am working on a 91 1/2 ton with the 5.2l. The truck will run for awhile but it seems after it warms up a little it will die. When this happens I have no power at the coil or the asd relay. I have taken the wire harness loose at the firewall looked and cleaned terminals which looked fine and put back together. I did this to the asd relay and the computer as well. All seemed to look very good. No pushed out pins or anything else. The truck then started and died a short time after. I pulled the harness from the firewall again and reinstalled. Truck then started. I am thinking this was a coincidence that it started after I did that. It stayed running for a good long time after I did that and it did not die.

I am going to test drive it tomorrow to see if it dies again. What I am wondering is what might of caused this. The owner already put a coil and asd relay on it before I looked at it. The wire that goes from the coil and the asd is green with black stripe. It leads into the wire harness at the firewall. My schematic says it is a battery supply wire. I am not sure what to check next. Does this truck have a crank sensor on it. Should I start there or could it be in the ignition. Like I said it runs and then all of a sudden it will die. It does not start until you let it set for a good while. I highly doubt I fixed it buy just cleaning the terminal but wont know for sure until I drive it tomorrow. Can you guys lead me to what might be causing this irratic behavior. Thanks

 

Crankshaft position sensors were introduced in 92. In 91 the Hall Effect Switch inside the distributor accomplished this function.

Power to the ASD should be the 14 gauge red wire. Power out should be a 14 gauge dark blue/black that changes at a junction to a 16 gauge dark green/black that feeds the coil. When it quits you need to check power on all 3 of these wires to isolate the problem area.

If you are loosing power to the ASD red wire I would check the fusible link that supplies it power. If you loose power on the blue/black the ASD or its actuating circuit is faulty. If you loose power only on the green/black it is probably corrosion at the junction point or a broken wire.

 

At the asd relay there is a red wire, green/black. blue/yellow, and two drk blue wires that run into the same side of the relay. When I lose power there is still power on the red wire but not at the green/black wire. My schematic shows that there is a junction somewhere. I have not tracked it down yet, I am assuming it is somewhere the relay and the firewall all wrapped up. I also think this green/black wire feeds the fuel pump and the injectors.

 

Seal, since I dont have the blue/black wire at the asd and it the green/black wire and I think it is opposite off the red wire on the relay, when the truck dies and I still have power at the red wire shouldn't I still have power coming out of the relay on the green/black wire. If I am understanding this then red brings power in and the gr/bl wire takes power out. If so then there should be power on the gr/bl wire where it comes out of the relay. But there is not power there on the gr/bl wire. If it was corrosion then I would assume it would be right there at the relay since the coil has no power to it. Is this right? I cleaned the terminals on the asd relay connection and seen no corrosion. Am I missing something. Where does the green/black wire go to when it goes to th emain harness at firewall. Does it go inside and then to the fuel pump? I am going to drive it today and if it dies again then I will track down the juntion for this wire and go from there. Thanks

 

The green/black wire feeds the injectors, the coil, the O2 sensor, the 3 vacuum solenoids, the fuel pump, the ignition switch, and the overdrive transmission if equipped.

The blue yellow and the 2 dark blue ones activate the coil inside the relay. One dark blue wire goes to the ignition switch and the other goes to the computer. The blue/yellow also goes to the computer. There should be power at the dark blue when the switch is on and this should activate the coil and close the contacts. Closing the contacts should let power cross from the red to the green/black thru the contacts.

If there is power at the red and the dark blue wires when the key is on and no power on the green/black the relay is faulty.

 

I just had an 89 that was doing the same thing. Because the distributer had been replaced about a year before I by passed looking at the Hall Effect switch. After diagnosing everything I could think of including repacement of the SMEC, I replaced the Hall switch and it that took care of the problem. The SMEC needs to see the Hall switch, if it doesn't, no power on the green wire.

Buster