Warm no-start

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Your rig being a 78 , it "should" have the ballast wire. The ? is . Is the wire to your resistor connected to the ballast wire, or some other direct 12v wire.

I had a 60 that used to run like a dog when hot. After some digging , the PO had wired the ignition module from the coil+, witch was only 10v.
Full 12v fixed it. This must happen a lot IMO.
Only the coil + should be connected to the ballast wire. Any other electrics need 12v. Relays might be ok. They seem to activate as low as 5v sometimes.
 
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all the focus seems to be on ignition here, but op states he always has good spark, so I'll throw this out there...is there any possibility the temp sensor feeding info to the ecm is acting up? you do have one (a gm sensor) running to ecm in addition to the factory one running the gauge correct? I've had very similar symptoms on a couple different gm vehicles which ended up being stuck in permanent "cold start enrichment mode" due to a faulty coolant temp sensor.
 
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all the focus seems to be on ignition here, but op states he always has good spark, so I'll throw this out there...is there any possibility the temp sensor feeding info to the ecm is acting up? you do have one (a gm sensor) running to ecm in addition to the factory one running the gauge correct? I've had very similar symptoms on a couple different gm vehicles which ended up being stuck in permanent "cold start enrichment mode" due to a faulty coolant temp sensor.
You're right. I do feel like this is an over-delivery of fuel (though I'm pretty sure spark isn't healthy either after some digging...more to come).

I put a new ECT sensor in as a part of my troubleshooting. It did not do the trick.
 
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Your rig being a 78 , it "should" have the ballast wire. The ? is . Is the wire to your resistor connected to the ballast wire, or some other direct 12v wire.

I had a 60 that used to run like a dog when hot. After some digging , the PO had wired the ignition module from the coil+, witch was only 10v.
Full 12v fixed it. This must happen a lot IMO.
Only the coil + should be connected to the ballast wire. Any other electrics need 12v. Relays might be ok. They seem to activate as low as 5v sometimes.
So I did a little more investigation with my ohm meter and found some strange things. The igniter is a mystery to me as far as the specs are concerned. Here's a photo with voltages I measured across different points with the key on and with the engine running. The most interesting find was the +1.5V on the wire that attaches to the coil- (at least the way I have it wired). I didn't label it in the pic, but the voltage of the wire going to the ballast resistor measures at 12V to ground when it's disconnected from the resistor.

Coil Voltage.jpg


Also, because I'm guessing someone will ask:
- 3 wires hang off coil-: tachometer, tach feed to ECM, igniter
- the pink wire pigtailed into the wire feeding the ballast resistor is the ING1 source for the ECM
 
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You're right. I do feel like this is an over-delivery of fuel (though I'm pretty sure spark isn't healthy either after some digging...more to come).

I put a new ECT sensor in as a part of my troubleshooting. It did not do the trick.
figures, that would have been entirely too easy of a fix. I was just hoping we were at a "can't see the forest for the trees" situation and this may have been overlooked but sounds like you've covered this possibility already. I don't have much more useful input other than I believe map sensor signal to be crucial on tbi systems so it may be worth a test if you haven't been there already as well.
 
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It could still be fuel related for sure. Intermittent start issues are a real pain.
The 12v reading when disconnected from resistor is normal. A ballast with no load will show no voltage drop. I still get the feeling you have the original ballast harness wire in play. Ideally, if you can get it warmed up and behaving hard to start again, then jump bat+ to coil+ and see if that improves things, at least you can hopefully rule out a low volts to coil issue. 6.8v is a bit low. It's usually around 9v with a coil load.
 
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Looking at your pic, there is only 1 wire going to coil+ With a ballast system, there should be another wire that supplies full 12v when cranking.
If it is double ballasted;) with no ballast bypass cranking wire it would be really hard to start.
Sorry for confusing the situation even more

The ONLY reason for a ballast resistor in your rig is to aid starting . The resistor supplies about 9v to the coil when running. When cranking, the resistor is supposed to be bypassed and the full 12v goes to the coil+. This can be done by using a relay that's powered from the starter solenoid wire, or a direct full 12v wire from the ignition switch that's only live when cranking. [that's how the 40's work I think ??]

Here is a bit about the resistor wire in the 78-80 trucks.

I'm guessing most people would have no idea about the ballast wire in the harness. I certainly didn't before reading about it here. It wouldn't be the first time someone added a ballast resistor and wired it in series with the ballast wire, if that's what's happened here.
 
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Update:

I figured out an issue that would seem like it’s related to fuel delivery. The wire that tells the ECM when the motor is cranking was wired to a constant +12V, which I believe would cause a constant rich condition. Got that sorted and put it through its paces. No change. After a highway spin to get it warm, it would not start after a 15 minute break. When I put my timing light on it during crank, it flashed maybe 1/sec while the tach showed 1200 rpm. Definitely missing a lot of spark.

Given that the wiring is klugey-at-best with a little bit of mystery sprinkled in from 40 years who-knows-what being done, I just decided to simplify the whole situation and drop in a DUI dizzy. Am I proud? No. Will it save me time and [maybe] heartburn? I sure hope so.
 
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Take out the ballast resistor and hook the wire that feeds the resistor to the coil+. It might be all it needs.
Remember, the way you have it wired, it was trying to start with lower voltage through the ballast, instead of getting the full 12v when cranking,
Good luck.:)
 

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