'93 with 4.9 electrical problems
#1
10-23-2011, 03:25 PM
Join Date: Jan 2007
Location: Kings Bay, GA
Posts: 24
Likes: 0
Received 0 Likes
on
0 Posts
'93 with 4.9 electrical problems
After 2 days of troubleshooting I'm still not sure, but I think the PCM is bad on this truck. My friend was backing out of the driveway and the truck died. It will not start now.
I initially checked every fuse and relay. I thought it was the fuel pump because when we turn the key forward, the pumps don't run, but it is not. I can jump 12 volts from the fuse block to the tank selector switch, and the pumps run.
We are getting no spark and no fuel. Even with the pump running via a jumper it will not start. I pulled a plug to confirm that there is no spark.
The fuel pump relay does not click, nor does it supply proper voltage to the inertia switch or tank selector switch. The green/yellow wire coming to the inertia switch only has 5.5 volts. The hot wire on the tank selector switch is only reading 5.5 volts also.
I have tested as many things as I can In accordance with the EEC and fuel pump circuit in the Haynes manual, and it seems that all things point to the PCM. I replaced the fuel pump relay as well.
On one wire that goes to the fuel pump relay, there is supposed to be >11.5 volts applied for 1-2 seconds as soon as the ignition is switched to run. That wire reads 1 volt instead of >11.5
Any help would be appreciated. Like I said, I think it's the PCM, but since it's not my truck, I don't want the guy to buy one and that not be it. I have checked fusible links, looked for loose connections and broken wires, and haven't found anything out of the ordinary. I just didn't think pcm's went out often. The truck does have about 180,000 miles.
I initially checked every fuse and relay. I thought it was the fuel pump because when we turn the key forward, the pumps don't run, but it is not. I can jump 12 volts from the fuse block to the tank selector switch, and the pumps run.
We are getting no spark and no fuel. Even with the pump running via a jumper it will not start. I pulled a plug to confirm that there is no spark.
The fuel pump relay does not click, nor does it supply proper voltage to the inertia switch or tank selector switch. The green/yellow wire coming to the inertia switch only has 5.5 volts. The hot wire on the tank selector switch is only reading 5.5 volts also.
I have tested as many things as I can In accordance with the EEC and fuel pump circuit in the Haynes manual, and it seems that all things point to the PCM. I replaced the fuel pump relay as well.
On one wire that goes to the fuel pump relay, there is supposed to be >11.5 volts applied for 1-2 seconds as soon as the ignition is switched to run. That wire reads 1 volt instead of >11.5
Any help would be appreciated. Like I said, I think it's the PCM, but since it's not my truck, I don't want the guy to buy one and that not be it. I have checked fusible links, looked for loose connections and broken wires, and haven't found anything out of the ordinary. I just didn't think pcm's went out often. The truck does have about 180,000 miles.
Last edited by Trip99; 10-23-2011 at 03:28 PM.
#4
10-24-2011, 03:58 PM
Join Date: Jan 2007
Location: Kings Bay, GA
Posts: 24
Likes: 0
Received 0 Likes
on
0 Posts
I'm going to remove it tonight and see if it smells or shows any other sign of damage. I'll repost when I finish.
Yes, the relay is getting 12V. I just think it is strange to lose fuel and spark all at once. What else could control both??? It doesn't look like anything else would control fuel and spark just going off the schematic??? I don't know, driving me nuts!
Yes, the relay is getting 12V. I just think it is strange to lose fuel and spark all at once. What else could control both??? It doesn't look like anything else would control fuel and spark just going off the schematic??? I don't know, driving me nuts!
