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Discussion Starter #1
Hey Guys - been struggling with this issue for a few weeks now. Truck starts on the first crank and runs great for about 15-20 minutes then just dies. I pulled the coil wire and there is no spark. Let it cool down for a bit and she fires right back up. So it seems like an ignition issue vs fuel. Thinking it was the coil, I put in a new bosche unit and get the same result. Put in a new ignition amplifier module and same result. My question is... what would kill the spark when the engine warms up?

I ran through the ignition preliminary checks in the manual and found this anomaly. Test 3 - Check Amplifier Switching. Manual says "connect the voltmeter between battery positive terminal and coil negative terminal: the voltmeter should register zero volts. Switch the ignition 'on' and crank the engine. The voltmeter reading should increase just above zero, in which case proceed with Test 5." With the key in the off position I am reading battery voltage. I do have an ignition amplifier module relocation kit in my rig if that matters.

Thanks in advance.
 

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Discussion Starter #3
Haven’t thought to check that. I’ll be down at my shop this weekend and will get back to you. What will that tell me?
 

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Discussion Starter #5
Yep, I was following that thread closely but not sure it’s the same issue. For sure my truck loses the spark from the coil wire when it warms up and takes about 30 minutes of cooling down before it starts up again. I don’t think the other post ever discerned if the problem was fuel or spark. I’m thinking that the engine and fuel temp sensors aren’t in play here as this is spark and not fuel. I believe the efi system doesn’t manage spark. Makes total sense that the ignition amplifier module is failing but I swapped it out and got the same result. What else tells the coil to fire??
 

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I had similar issue issue when I had my 94 D90. It would start fine cold and would run fine for a while. When engine was warm, the engine would just cut off. After cooling down it ran normal. Lucky I was near Trevor Griffith’s shop. It turned out to be a bad O2 sensor.


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If it's good when cold probably no codes. As I mentioned in that other thread, replace the coil. BTDT. At worse you'll gain a spare.
 

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Discussion Starter #13 (Edited)
What brand do you recommend? I replaced a new(ish) Lucas coil with the Bosch. Also, the problem manifested when I had the Lucas coil installed. The first thing I did was replace it with the Bosch and continue to have the same issue.

------ Follow up post added April 25th, 2019 08:54 AM ------

Which brings up a good point— did you check for codes?
I did... no codes.

------ Follow up post added April 25th, 2019 08:55 AM ------

Doesn't the coil get it's instructions from a pulsated signal created by the distributor pickup? 25 yr old dizzy pickups: GUILTY until proven innocent!
I replaced the distributor about a year ago with one from Power Spark in the UK. I got the one with the amplifier relocation kit on it.
 

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Something else to try. I had a similar fault on a 3.9 RRC, it would cut out after 20-30mins of driving and refuse to start. I found that the coil was generating a spark just fine but when it got to the distributor it would just disappear. Everything was set-up correctly, it was very odd. Eventually I realized that there was a bad ground and the spark was travelling down the rotor arm and grounding through the distributor to the block rather than the the HT leads to the plugs. I put it down to a poor earth between the block and the heads. I added an earth strap from each head (on an exhaust manifold stud) to each suspension turret mounting bolt which fixed the issue. When I inspected the rotor arm body and the distributor shaft closely there was definitely signs of arcing between the 2. Not sure how the plastic rotor arm body did this?? A quick test is to carry a spare rotor arm with you. When you get the vehicle to a failed state (I was normally on the side of the Southern State Parkway!) pop the distributor cap and replace the rotor arm with the spare and it should fire right up, it did in my case. This will keep you going for another 30 mins until the problem occurs again. I think the heat build up in the distributor and build up of carbon from the arcing created a better conductor than the proper route to the plugs/heads, in addition as the engine got hot the resistance between the block and the heads increased. Fitting earth straps bypassed this and fixed my RRC..
 

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George- Have you checked your voltage coming into the coil when it dies to see if you have 12v coming from the ignition switch to try to isolate if the issue is at the coil/distributor or power supply issue?
 

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Discussion Starter #16
OK Guys... thought I would close the loop on this one. Turns out that after the engine got to operating temperature, the coil pick-up in the distributor would stop sending its signal to the ignition amplifier resulting in no spark from the coil. The way I found it was by measuring the resistance on the pick-up leads when the engine was cool and again after it would warm up and die. When cool, the resistance was 3,300 ohms... within spec. Right after the engine would die, I checked again and the resistance was measuring open circuit. To prove my hypothesis, I swapped in a spare distributor that I had and the truck ran fine for 35 minutes before I shut it off. With the old distributor, it would run for 15 minutes or so and then crap out.

Thanks to everyone who provided input!
 

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OK Guys... thought I would close the loop on this one. Turns out that after the engine got to operating temperature, the coil pick-up in the distributor would stop sending its signal to the ignition amplifier resulting in no spark from the coil. The way I found it was by measuring the resistance on the pick-up leads when the engine was cool and again after it would warm up and die. When cool, the resistance was 3,300 ohms... within spec. Right after the engine would die, I checked again and the resistance was measuring open circuit. To prove my hypothesis, I swapped in a spare distributor that I had and the truck ran fine for 35 minutes before I shut it off. With the old distributor, it would run for 15 minutes or so and then crap out.

Thanks to everyone who provided input!
Congrats on sorting out the issue. Good that you had a spare. There are a few modern and reliable dizzy options out there. Give Mallory a look.
 

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Discussion Starter #19
I've got a little more troubleshooting to do in order to determine where the failure is in that loop. My gut tells me it's in the relocation bit that I got from Powerspark in the UK. Here is a picture of the relocating wire from Powerspark. The frayed end that attaches at the distributor is what concerns me. I had a shop do some work that required pulling the distributor (i know... I know...) and I think they may have stressed the wire getting it out. Figure I'll use a heat gun to warm up the wire and then the pick-up and see which point fails.

I've seen a few threads here that call the Mallory setup into question so I haven't thought about using it. Glad to hear it's been working for you, Russell.
 

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Don’t go Mallory, the ignition module will give you problems. Good distributor, incompatible module. Just my experience and I’ve tried every module option they had.

That relocation kit maybe adding to your problem, those cloth-wrapped British wires are garbage. They look pretty and cost a lot but, they’re not as good at handling the harsh engine bay environment (especially, heat) as modern plastic sheathing.

Just try attaching the module at the distributor, where it belongs, run two new jumper cables to the ignition coil, etc


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