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No spark , no fuel. 1988 tbi

Snoop dogg

Saturday Night Special
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It's a 1987 2wd 4.3 truck, auto.

But everything I've seen under the hood looks like the 88-93 ignition system.

Icm inside distributor cap. 4 prong plug and 2 prong plug on icm, with 2 wire pickup coil going to it.

No spark, no fuel, but cranks. I believe that eliminates it being a neutral safety switch.

The no fuel and no spark leads me to believe ICM OR/AND ECM.


I have a couple icms, and apparently no parts house tests them anymore...at least not the places I frequent. (So they could all be bad for all I know) I bought a new coil to have as a control. (Ac Delco but I know it could be bad out the box)

I've swapped in a new coil and old coil, older icm, and older icm relay . All one by one till it was all swapped over. Still no spark and no fuel.

It's my understanding that the icm can control spark AND fuel?
Am I incorrect in that thought?

And then ECM takes over and controls timing and fuel?

What I have left is to jump 12v to pick up coil on distributor and test coil for spark. If there's spark than the pickup coil has gone bad, correct?
How I was thinking about doing this was jumper wire from pos. Battery post to one of the coil pick up wires....crank over motor and see if the ignition coil sparks to my screwdriver. Should that work?

Trying to understand how the distributor and icm interact to signal fuel and spark.

I *think (lol). I'm narrowing it down to bad pickup coil or bad ECM. Not toooo much grey matter in my dome, and the Chilton's book is ok for help but doesn't really elaborate on troubleshooting. I have not spent the money on a new icm and would like to figure out the root of the issue before I by another part bc it seems like it's either gonna be a new distributor , new ECM, or new icm.

Ive only found the relay for the icm that is on the intake manifold, is there any other relay on that circuit? I Have voltage for the ign and ECM fuses at the fuse block , drops a little when cranking. Maybe ign. Switch?

Thanks!
 
Check the oil psi for the comp(not the gauge). It will kill both fuel and ign.

Should be able to jump/bypass the fuel relay to get fuel psi for pump test.

If it was just ign you should at the very least get a intial fuel psi.
 
The ecm has to see oil psi also or it shuts the fuel down.

On odb1 veh they typicaly have a sendor for the dash and a sensor for the ecm
 
This same thing happened on my 1989 4.3l this swapped toyota. The coil has 2 plugs. One came undone and then no spark AND no fuel from injectors at tbi. I've also heard a bad coil causing that as well but no experience with that
 
I had this problem with a '89 7.4l the magnets inside the distributor weren't strong enough after 30 years.

I got help troubleshooting it from the old board (after doing everything the factory service manual said to do) I ended up plugging in a new distributor and spinning it by hand with the key on the the air cleaner off to confirm spark and fuel.
 
M92PV4U said:
I had this problem with a '89 7.4l the magnets inside the distributor weren't strong enough after 30 years.

I got help troubleshooting it from the old board (after doing everything the factory service manual said to do) I ended up plugging in a new distributor and spinning it by hand with the key on the the air cleaner off to confirm spark and fuel.
Click to expand...
Well that's kinda where I'm thinking too, lol.
if you seen the way the magnets look, it's probably the cause. But I hadn't even tried to check it's magnetism :homer:

Just tryin to get better at troubleshooting these trucks, typically it's the common stuff...but I think* it's the pickup coil magnet things. I've got a little more fussin to do with it tomorrow , I got a broken hand right now so its all slow going anyway.

I think I might have a spare nos dizzy laying around too to give that a spin.
 
Snoop dogg said:
Well that's kinda where I'm thinking too, lol.
if you seen the way the magnets look, it's probably the cause. But I hadn't even tried to check it's magnetism :homer:

Just tryin to get better at troubleshooting these trucks, typically it's the common stuff...but I think* it's the pickup coil magnet things. I've got a little more fussin to do with it tomorrow , I got a broken hand right now so its all slow going anyway.

I think I might have a spare nos dizzy laying around too to give that a spin.
Click to expand...
Mine were still magnetic to a screwdriver... but I guess that the force wasn't strong enough.
 
I tested the pickup coil .

Voltage did not get above 0.14 volts...I've found one place on the internet that says it should be between .3-1.8 v.


Ohms was 136...not sure if that's within spec but I believe the pickup coil is bad.
 
The voltage should fluctuate between .3 and 1.8 as the engine turns over... Not sure about ohms but 136 seems like a common value...
 
I threw the icm, coil , and icm relay onto another truck, from the non runner and it fired up. And also confirmed with the icm unplugged injectors won't spray.

Gonna put a new dizzy in the truck , the magnets on this dizzy look gnarly corroded so it's no surprise that pickup coil not getting enough voltage to signal icm.

Thanks for all the other insight on these trucks. I'll follow up when I throw a new distributor in it
 
I had an issue similar with my 90. Lost fuel and spark. Ecm b fuse was popped. It would keep popping new fuses. Then it would take one and run a few days. Then do it again. Ended up being the oil pressure switch ultimately.
 
M92PV4U said:
I had this problem with a '89 7.4l the magnets inside the distributor weren't strong enough after 30 years.

I got help troubleshooting it from the old board (after doing everything the factory service manual said to do) I ended up plugging in a new distributor and spinning it by hand with the key on the the air cleaner off to confirm spark and fuel.
Click to expand...
Yo yo! So after pretty much pinning it down to the pickup coil , I pulled a good dizzy from another good running 4.3. to confirm. You must have some damn fast hands, maybe fastest in the west to spin the dizzy quick enough to spray fuel and generate spark lol.


I used a drill with a flat head screw driver in it and put it in bottoms of dizzy to spin it. It's aliiiive! Fuel and spark. Ordered new distributor today.

Thanks for all the fun!
 
I think that ign system uses "hall effect" I don't know what that means but you may have to spin it faster.
Here is a totally tore down 7.4l dist.
20200409_092924.jpg


Here is my testing set up, I spun the gear with my middle finger and thumb in a snaping motion.
20200413_154242.jpg
 
M92PV4U said:
I think that ign system uses "hall effect" I don't know what that means but you may have to spin it faster.
Here is a totally tore down 7.4l dist.

Here is my testing set up, I spun the gear with my middle finger and thumb in a snaping motion.
Click to expand...
Hall effect sensor doesn't give a shit how fast you spin. You're thinking of a VR sensor.
 
M92PV4U said:
I think that ign system uses "hall effect" I don't know what that means but you may have to spin it faster.
Here is a totally tore down 7.4l dist.
20200409_092924.jpg


Here is my testing set up, I spun the gear with my middle finger and thumb in a snaping motion.
20200413_154242.jpg
Click to expand...
Well I've got the same ignition system you got there. 1987 models have the same, but the book will reference 1988-1995. I'm working on a 1987 model and it's the same as the 1994.
I think there were the HEI with the coil in cap(carb motors?), and then the style like we have with coil mounted on intake. Vertical towers on cap (opposed to the 96-98 caps that have the horizontal looking type) And all the dizzys are adjustable from 4.3 to 7.4

The book says 1995 went from the HEI, to the DI system (hall effect/camshaft sensor /knock sensor/crank position sensor) and new looking dist. Cap?? But it notes that it is an adjustable distributor as well for the 4.3

1996-1998 notes the 4.3 distributor is NOT adjustable. There is a specific process to setting the dizzy. And the dist. Mounting clamp will not allow adjustment.
 
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