91 HO Shut Off While Driving

[Minuit]

What do you have, as far as electrical test equipment goes?

[acfortier]

1 minute ago, Minuit said:

What do you have, as far as electrical test equipment goes?

Just a multimeter.

[Minuit]

9 minutes ago, acfortier said:

Just a multimeter.

What kind?

[acfortier]

30 minutes ago, Minuit said:

What kind?

I think it’s part of this set: https://www.lowes.com/pd/Southwire-Electrical-Multimeter-Digital-90-to-1000-V-Test-Meter-Battery-Included/1000860316

[acfortier]

Haven't tried starting it again, but may stop at the parts store to pick up a Napa ignition coil to keep with me as a spare... I'm still pretty baffled by this issue.

 

Does it need to crank awhile for a new CPS to read properly? I can't think of any other reason why I wouldn't have spark for a few minutes then go back and it turns on like nothing happened.

[acfortier]

Happened again today, sigh. Not sure what to replace next. Maybe I'll pick up a spare coil?

 

@Minuit did you have a suggestion/recommendation to check?

[AdroitKirby]

I was having intermittent shut off while driving issues with mine, but since I have a rod bearing issue, I ended up just having the engine rebuilt. Only electrical component that was changed was the TPS (replaced with MOPAR part) and the distributor. I had previously already replaced the distributor but they told me something was wrong with the one that I had installed. 

 

Hasn't shut off since

[acfortier]

4 hours ago, AdroitKirby said:

I was having intermittent shut off while driving issues with mine, but since I have a rod bearing issue, I ended up just having the engine rebuilt. Only electrical component that was changed was the TPS (replaced with MOPAR part) and the distributor. I had previously already replaced the distributor but they told me something was wrong with the one that I had installed. 

 

Hasn't shut off since

 

I've isolated my issue to spark--pre-distributor. Actually, forgot to check the input voltage on the ignition coil when it wasn't working yesterday. Can't tell if it's a faulty coil, or if there is something else that controls spark is going bad.

[Minuit]

On 10/2/2022 at 3:58 PM, acfortier said:

Happened again today, sigh. Not sure what to replace next. Maybe I'll pick up a spare coil?

 

@Minuit did you have a suggestion/recommendation to check?

 

Here's the problem. The ignition coil (as well as the fuel pump and injectors) are fed by the ASD relay circuit. That circuit is energized when the ignition is on and the ECU is receiving a valid signal from both the distributor sync sensor and the CPS. So just checking the coil for voltage at any old time without the engine running (which is kinda the point of this whole troubleshooting exercise) wouldn't tell you anything, because the coil is only energized when the ASD relay is, which is either cranking or running with signals from the CPS and sync sensor.

 

Normally for me, this is about when the expensive lab gear comes out. What I would do with your equipment is to buy these:

https://www.amazon.com/AST-Labs-Banana-Stackable-Yellow/dp/B07FKPJ7RR/ref=sr_1_15?crid=210QNYODBKVMT&keywords=banana+extension+cable&qid=1664925651&qu=eyJxc2MiOiIyLjM2IiwicXNhIjoiMi4yMSIsInFzcCI6IjEuNjkifQ%3D%3D&sprefix=banana+extension+cable%2Caps%2C94&sr=8-15

 

https://www.amazon.com/Tool-Aid-23500-Back-Probe/dp/B00G9C3RSU/ref=sr_1_11?crid=YG9B1ZPI9F2S&keywords=backprobe&qid=1664925577&qu=eyJxc2MiOiI0LjEzIiwicXNhIjoiMy44NCIsInFzcCI6IjMuNjQifQ%3D%3D&sprefix=backprobe%2Caps%2C108&sr=8-11

 

The first is a set of extension leads so you can connect your test leads in the engine bay, close the hood, and run them inside the truck so you can leave your test leads setup any time you want to drive the truck, and so you don't have to set anything up in a hurry when the problem rears its head. The second is a set of backprobes. You stick these up the butt of a connector so you can take measurements from it while it's plugged in. I own both of these items, and they're good enough quality to work for occasional use.

 

Connect a backprobe to the coil's positive wire, then connect enough of the extender leads until you can have the meter in the truck. Then connect an alligator clip to something with a good ground and run it to your meter. Verify that you have a reading of 12V with the engine running normally. If not, check connections. Once you're set up, just wait for the problem to happen again. Once it happens, you've gotta be ready, because this seems like the kind of thing that disappears in a hurry. Try to crank the engine and see if the 12V has disappeared. If not, suspect coil or connection issue between coil connector and coil. If yes, suspect CPS, sync sensor, or circuit fault in ASD relay circuit. That doesn't narrow it down much, but it's what you have to work with now.

 

Have you recently checked the OBD1 flash codes? Cycle the key on, off, on, off then on and stop. Count the flashes of the check engine light. They are read as follows:

 

[acfortier]

1 hour ago, Minuit said:

 

Here's the problem. The ignition coil (as well as the fuel pump and injectors) are fed by the ASD relay circuit. That circuit is energized when the ignition is on and the ECU is receiving a valid signal from both the distributor sync sensor and the CPS. So just checking the coil for voltage at any old time without the engine running (which is kinda the point of this whole troubleshooting exercise) wouldn't tell you anything, because the coil is only energized when the ASD relay is, which is either cranking or running with signals from the CPS and sync sensor.

 

Normally for me, this is about when the expensive lab gear comes out. What I would do with your equipment is to buy these:

https://www.amazon.com/AST-Labs-Banana-Stackable-Yellow/dp/B07FKPJ7RR/ref=sr_1_15?crid=210QNYODBKVMT&keywords=banana+extension+cable&qid=1664925651&qu=eyJxc2MiOiIyLjM2IiwicXNhIjoiMi4yMSIsInFzcCI6IjEuNjkifQ%3D%3D&sprefix=banana+extension+cable%2Caps%2C94&sr=8-15

 

https://www.amazon.com/Tool-Aid-23500-Back-Probe/dp/B00G9C3RSU/ref=sr_1_11?crid=YG9B1ZPI9F2S&keywords=backprobe&qid=1664925577&qu=eyJxc2MiOiI0LjEzIiwicXNhIjoiMy44NCIsInFzcCI6IjMuNjQifQ%3D%3D&sprefix=backprobe%2Caps%2C108&sr=8-11

 

The first is a set of extension leads so you can connect your test leads in the engine bay, close the hood, and run them inside the truck so you can leave your test leads setup any time you want to drive the truck, and so you don't have to set anything up in a hurry when the problem rears its head. The second is a set of backprobes. You stick these up the butt of a connector so you can take measurements from it while it's plugged in. I own both of these items, and they're good enough quality to work for occasional use.

 

Connect a backprobe to the coil's positive wire, then connect enough of the extender leads until you can have the meter in the truck. Then connect an alligator clip to something with a good ground and run it to your meter. Verify that you have a reading of 12V with the engine running normally. If not, check connections. Once you're set up, just wait for the problem to happen again. Once it happens, you've gotta be ready, because this seems like the kind of thing that disappears in a hurry. Try to crank the engine and see if the 12V has disappeared. If not, suspect coil or connection issue between coil connector and coil. If yes, suspect CPS, sync sensor, or circuit fault in ASD relay circuit. That doesn't narrow it down much, but it's what you have to work with now.

 

Have you recently checked the OBD1 flash codes? Cycle the key on, off, on, off then on and stop. Count the flashes of the check engine light. They are read as follows:

 

Thanks for this! I did that similar testing with another person, but using those leads will definitely make it a bit easier. I'm not sure if the issue went away too quickly when I tested with another person, because after I verified voltage at the coil plug, it fired up again.

 

I had in the past, and nothing came up. I'll check again.

[Dragos]

Good day ,

i had this stupid problem at my 88 Laredo MJ. I have a 4 L Renix.

When the engine is hot or driving  continuously for30-40 minutes , engine stop. After a 20-40 min break , engine starts.

I read all that you describe before and i check all. What i find is that when the engine is not starting I don't have spark.

Any way , after 1 moth of looking and changing including ECU , I discover that the problem came from Ignition Control Coil module.

I hope that this information will help you .

 

[acfortier]

So, after changing the o-rings on my oil filter adapter, it won't start. I did remove the coil to dist. wire, since it was in my way (I have relocated my coil to the firewall, so the wire runs over the top of the oil filter/in front of the dipstick).

 

And now it won't start. Took out my spark tester, and I'm getting spark from the coil. So, that should rule out the coil?

 

But it does seem odd that the only thing I touched electrically on the truck was the wire going from the coil to the distributor. It's fairly new--had to get it when I relocated the coil. Could it really be intermittently bad?

[Minuit]

An intermittent circuit of some kind would be at the absolute top of my list of suspects, and the wire from the coil to the distributor having an intermittently bad connection would cause every single problem you've seen. Unfortunately, we are now in a climate where "I replaced it" does not mean you can automatically assume "it is good", and just as often means "it is bad right out of the box". I often find myself preferring 30 year old NOS or good used parts to the brand new stuff you can buy these days, and bad aftermarket parts are at least 10% of why I'm bald at age 26.

[acfortier]

9 hours ago, Minuit said:

An intermittent circuit of some kind would be at the absolute top of my list of suspects, and the wire from the coil to the distributor having an intermittently bad connection would cause every single problem you've seen. Unfortunately, we are now in a climate where "I replaced it" does not mean you can automatically assume "it is good", and just as often means "it is bad right out of the box". I often find myself preferring 30 year old NOS or good used parts to the brand new stuff you can buy these days, and bad aftermarket parts are at least 10% of why I'm bald at age 26.

Yeah, I was just hoping I'd be able to see the wire from coil to dist. not working, which I haven't. I tested it at two separate times yesterday during my no-start issue, and both times, spark was coming through consistently.

 

I went out this morning to see if it'd start, and go figure, started. It sputtered a bit, like you would expect a carb'd engine to start after sitting for some time, but caught and idled fine after a little while. This seems slightly different than my other no-start conditions, so IDK if it's related in anyway to spark... still scratching my head!

[acfortier]

OK, I'm a bit confused now.

 

Backprobing the coil connector, and according to this, I want the DG/RD wire (which goes to the alternator) and the GY goes to the PCM.

 

 

Does it make any sense to get voltage on both? I get 14.5V (same as the battery) when I backprobe the DG/RD wire, and like 13.8V when I backprobe the GY wire. That doesn't seem to make sense to me... Shouldn't that be a switched ground to the PCM?

[Gjeep]

Try replacing the fuel pump relay in PDC. When they get old/ overheat/ die they can cause issue.

[acfortier]

3 hours ago, Gjeep said:

Try replacing the fuel pump relay in PDC. When they get old/ overheat/ die they can cause issue.

I guess I could--not sure if my symptoms are an indicator of a fuel pump relay going bad. The engine doesn't sputter or anything, just straight up shuts off.

[acfortier]

OK, found this resource and it makes a lot more sense to me now. Going to try to get something set up to test next time this occurs.

 

I did measure resistance, primary was 1.5 - 1.9 ohms, secondary was around 13.5k. The primary seems a little high, but I'm not sure if that could contribute to the issue?

[acfortier]

Still trying to track this down, happened to me today, and it didn't come back.

 

If I'm understanding this correctly, the DG/RD wire that comes from the alternator to the ignition coil should be battery voltage when the key is turned on? This supplies power to the coil, while the other GY wire is to tell the coil when to fire?

 

It seems if I'm not getting any voltage on that DG/RD wire at the coil, so it still could be a suspect CPS, sync sensor, or circuit fault in ASD relay circuit? What would be the next steps to narrowing it down further? @Minuit tagging you because you gave some great information previously--hoping you have something else to help me