Sudden grounding or ECT issue 1994 4.0 4x4

[Fiveowen]

I Daily drive my '94 4.0 4x4 with 165k, started up and drove perfectly all day Thursday, and Friday, parked it at work, outside temperature dropped from 45-30 Fahrenheit before I went to leave Friday afternoon and it wouldn't start without holding at 1000-2000rpm until warm, coolant temp gauge was no longer reading so I suspected it was the coolant temp sensor causing the hard start, but later learned the "sender" and ECT are different sensors and on different circuits, started today with replacing both of them, still no gauge (did verify the wiring from the sender to dash was good) and hard starting still but will fire up and idle poorly at 600rpm voltage gauge is reading 10 volts at 600rpm but jumps to 14 when revved past 900 or so. could this be a bad pcm? alternator? engine to chassis ground? or specific pcm ground? any info is appreciated Thank you!

 

[franklin2]

My 86 was doing this. The colder it got, the harder it was to start till one morning, it would not fire off at all. It tried sometimes just as I would turn the key back.

I never have found it yet. I figured out the voltage to the ignition coil was very low during cranking. I ended up running a wire directly from the battery, around in front of the radiator, to a relay I added. I then took the ignition trigger wire that feeds the coil and the ignition module normally, and use this wire to trigger the new relay, and then the direct power from the battery goes through the relay and feeds the coil and the ignition. Fires right up.

So that told me I have a problem somewhere from where the electrical feeds the truck from the starter solenoid (battery) all the way to the ignition switch and then out to the ignition circuit. But I haven't figured it out yet, it runs good so I left the relay in place.

But I do have the curious alternator problem you have also. I have a very accurate volt monitor on the truck, and when I first start the truck it is not charging. But if I rev the engine up to about 2000 rpm bam, it starts charging and will keep charging after that. It is very consistent about this, though it charges more readily when the engine is warmed up.

I think this is all related. The alternator needs a voltage signal from the keyswitch to start charging. I think the problem I have somewhere is reducing this voltage like it did with the ignition, and the alternator is having a hard time starting to charge.

 

[Fiveowen]

My 86 was doing this. The colder it got, the harder it was to start till one morning, it would not fire off at all. It tried sometimes just as I would turn the key back.

I never have found it yet. I figured out the voltage to the ignition coil was very low during cranking. I ended up running a wire directly from the battery, around in front of the radiator, to a relay I added. I then took the ignition trigger wire that feeds the coil and the ignition module normally, and use this wire to trigger the new relay, and then the direct power from the battery goes through the relay and feeds the coil and the ignition. Fires right up.

So that told me I have a problem somewhere from where the electrical feeds the truck from the starter solenoid (battery) all the way to the ignition switch and then out to the ignition circuit. But I haven't figured it out yet, it runs good so I left the relay in place.

But I do have the curious alternator problem you have also. I have a very accurate volt monitor on the truck, and when I first start the truck it is not charging. But if I rev the engine up to about 2000 rpm bam, it starts charging and will keep charging after that. It is very consistent about this, though it charges more readily when the engine is warmed up.

I think this is all related. The alternator needs a voltage signal from the keyswitch to start charging. I think the problem I have somewhere is reducing this voltage like it did with the ignition, and the alternator is having a hard time starting to charge.

Hey man thanks for the info, I sat in the truck and manually kept it at 1000 rpm until I felt the heat get hot, coolant temp sensor started to read so that rules out a bad engine to chassis ground in my mind for now, my thought process from here is to test ECT sensor wiring to the computer, smoke test for vacuum leaks, and check my alternator. thoughts? anything I'm missing or should do first?

 

[franklin2]

When it's cold and won't start, take a scrap piece of wire, tie it to the red/lightgreen wire feeding the coils. Put the other end on the battery + and see if it will start. If it still won't start,, then look elsewhere. If it does start you have a voltage problem to the ignition.