How to troubleshoot erratic tachometer? (TBI)

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DerekTheGreat

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Hello everyone,

Truck is a 1989 K1500, factory five speed 350 TBI but with the moonies to needles swap. I ran an auxiliary wire from the coil to drive the tach within the cluster I pulled from a treasure yard some four years ago.

Story time/background:

For about the past year and a half the truck has been running great, with no tach flutter or stumble issues. Before that, the tach would usually flutter around on start-up for about thirty seconds or less. What seemed to make the problem go away were new ignition components once the ICM supplied with the distributor I stabbed in it died. At that time the components were a different coil from my parts stash (Tested good), a Standard Motor Products ICM, distributor cap, rotor, wires and NGK spark plugs, ones that were equivalent to A/C Delco CR043's. I had zero complaints with that set-up until a little over a month ago when I noticed the tach fluttering around. Occasionally the engine would stumble or miss when it was really erratic. These episodes would come and go, weren't always constant. Seemed to act up the hotter it was outside and with the engine fully warmed up. Well, the truck finally quit this past Friday. I had already fired the parts canon preemptively & ordered a new coil (Delphi) and an ICM (A/C Delco). So I tear it apart on Sunday and find the button inside the dizzy cap had gotten so hot it disintegrated. I didn't find the button piece, all that was left was the spring, so not sure how it was running before that or if it came apart once I took the cap off. Dizzy was new about 2.5 years ago- no slop in the shaft and the pointed magnet deal wasn't broken.. Had to go to several part stores and ended up with an el cheapo Master Pro cap and another Standard Motor Products rotor as the Accel cap I bought at Advance Auto was trash- the rotor was chewing up the inside of the cap right out of the gate. But before I swapped caps & rotor I fired the truck up and the tach fluttered right away. So then I swapped the Delphi coil out for the old coil- no more flutter. Swap coils again to the Delphi unit- tach flutter. OK, so I put the old coil back in place, return the junk Accel stuff and then get it all put back together with the Master Pro Cap, SMP rotor and old coil. No flutter. Checked and set the timing, let it run for a while and then buttoned everything back up. This morning though, I start it up to head into work and the tach began to flutter a few seconds after it fired up and then fluttered around for thirty seconds or so like it did before all the SMP products went on it. No tach flutter for about fifteen minutes until it twitched a bit, and I felt it when it did it. So now the truck seems like it's back to where it was just before it failed. Oh, I got about 1.5 years and 12k trouble-free miles out of the SMP stuff. Kind of a record for me and this truck.

Questions:

1) What else could cause tach flutter/erratic ignition?
2) Is it just more bad aftermarket krap?
3) Who makes a reliable distributor cap that won't buckle under pressure? (Not my first go-around with this issue, only my Firebird with it's A/C Delco cap seems to be OK as I haven't had to touch that for over six years. That car has also been converted to an MSD ignition, with their distributor, 6AL box and one of their coils. Their distributor caps are also junk, by the way.)

I'd love to hear from folks who have actually had and solved my type of issue. Not really looking for the armchair quarterback approach, but I suppose all suggestions are welcome. I need to go back and look at the electrical troubleshooting manual I have to better understand the circuit path, but I'm still pretty convinced there is something going on between the ESC module, ICM, dizzy magnet thing and coil. I don't understand why or how the coil would impact the tach signal though- seems the thing is driven by the ICM, so it either works or it doesn't. I also don't remember what role the ESC plays into things, but remember reading it does have something to do with the ignition. My truck is old enough such that it has the outboard ESC module right next the throttle body on the IAC & TPS side. I'm ready to be schooled by those in the know, so please help.

Derek
 
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DerekTheGreat

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Sometimes you feel it, sometimes you don't. With these parts, it seems I feel it any time the tach flutters around. So that has me leaning toward an ignition issue. I felt it this morning cruising steady at 35 when the tach fluttered momentarily. And this morning, the truck was a bit stumbly as it idled while the tach fluttered.

..Would using dielectric grease hurt things? I applied a generous amount to all the connectors and plug wires I touched, as I always do. I'm paranoid about corrosion, especially in wiring harnesses. Most of my tools are Mickey Mouse approved, so I think I'd really be out of luck trying to troubleshoot a weird wiring issue.
 

Schurkey

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First Guess: Pickup coil including the magnet riveted to the distributor shaft.

Second Guess: Failing ignition module.

Third Guess: Wire harness problem--wires broken inside the insulation, or corroded connections that occasionally don't make contact. Could be anywhere in the ignition wiring--including the ignition switch, or firewall bulkhead connector.

Beyond that, I suppose it could be an ECM issue or EST, but that seems very unlikely.
 

DerekTheGreat

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First Guess: Pickup coil including the magnet riveted to the distributor shaft.

Second Guess: Failing ignition module.

Third Guess: Wire harness problem--wires broken inside the insulation, or corroded connections that occasionally don't make contact. Could be anywhere in the ignition wiring--including the ignition switch, or firewall bulkhead connector.

Beyond that, I suppose it could be an ECM issue or EST, but that seems very unlikely.

Hmm. I wish I had a known good ICM to experiment with, along with a distributor with a good magnet and pick-up coil.

Just brainstorming here, but I wonder if a failing pick-up coil can cause an ICM to nuke itself. I was cruising fine on Friday some fifty miles before the truck just shut right down on me. Tach fluttered only once at idle when I made my drop off. Could a bad pick-up coil cause that erratic tach thing? Seems like it's possible, as isn't that what drives the ICM? That and given my problem is exactly the same as just before it quit, I'm finding it hard to think that both the coil I ordered and the ICM are also bad right out of the box. Is it possible to test the ICM somehow?

I've had the bulkhead connector apart a few years ago. How the shell do you clean out all of that goop that's in there? I did the best I could at the time and regooped it with dielectric grease. What's a good way to test for broken wires inside the insulation?
 

PlayingWithTBI

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That and given my problem is exactly the same as just before it quit, I'm finding it hard to think that both the coil I ordered and the ICM are also bad right out of the box.
Sorry to hear you're still having this issue since the last time we talked. IMHO just one of them being bad can/will take out the other, especially a bad coil. Here's an example of a 7 pin ICM and wiring to the ECM, coil, and pickup (the ground wire is in your 4 pin connector to the ECM on an 8 pin ICM, everything else is the same). Maybe check them for corrosion and connectivity? Dielectric silicone grease is great on the connectors but, you don't wanna get too much on the electrodes.

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Other than that, I don't have anything else :33: HTH
 

Schurkey

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Hmm. I wish I had a known good ICM to experiment with, along with a distributor with a good magnet and pick-up coil.
I grab spare distributors from the Treasure Yard. They go for five dollars and up. In that a new module is ~$45; and you get the housing, driven gear, potentially a usable pickup coil and mainshaft, etc. it's a bargain.

Grab the ignition coil, cap 'n' rotor if you can. Sometimes they're usable, sometimes not...but you can always pitch what isn't good later.

Just brainstorming here, but I wonder if a failing pick-up coil can cause an ICM to nuke itself.
Possible. Not common. The ignition coil would be more-likely to murder an ignition module than the pickup coil would be.

Could a bad pick-up coil cause that erratic tach thing? Seems like it's possible, as isn't that what drives the ICM?
Absolutely. Failure to produce an acceptable signal would cause the module to ignore it, leading to engine stalling from no sparking. Producing "false" signals would have the module working overtime; sending sparks that the engine doesn't need. Again--potential misfire, and hard on the distributor cap 'n' rotor.

Is it possible to test the ICM somehow?
GM has "special tools" for testing ignition modules. I've never used one.

If you connect an ignition oscilloscope, you could see the primary and secondary waveforms and look for anomolies--failure to vary the dwell being the easiest to see.

For the D-I-Y, there's really no good way to test 'em other than to see if the engine runs right.

What's a good way to test for broken wires inside the insulation?
Ford calls it the "wiggle test". In this case, you'd run the engine and wiggle the wire harness. If you find a place where you move the harness and the engine runs worse--or better--you've found the location of a potential wiring problem. (An open, a short, a grounded wire that shouldn't be, faulty connections, etc.)
 

DerekTheGreat

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I grab spare distributors from the Treasure Yard. They go for five dollars and up. In that a new module is ~$45; and you get the housing, driven gear, potentially a usable pickup coil and mainshaft, etc. it's a bargain.

Grab the ignition coil, cap 'n' rotor if you can. Sometimes they're usable, sometimes not...but you can always pitch what isn't good later.


Possible. Not common. The ignition coil would be more-likely to murder an ignition module than the pickup coil would be.


Absolutely. Failure to produce an acceptable signal would cause the module to ignore it, leading to engine stalling from no sparking. Producing "false" signals would have the module working overtime; sending sparks that the engine doesn't need. Again--potential misfire, and hard on the distributor cap 'n' rotor.


GM has "special tools" for testing ignition modules. I've never used one.

If you connect an ignition oscilloscope, you could see the primary and secondary waveforms and look for anomolies--failure to vary the dwell being the easiest to see.

For the D-I-Y, there's really no good way to test 'em other than to see if the engine runs right.


Ford calls it the "wiggle test". In this case, you'd run the engine and wiggle the wire harness. If you find a place where you move the harness and the engine runs worse--or better--you've found the location of a potential wiring problem. (An open, a short, a grounded wire that shouldn't be, faulty connections, etc.)

I do have a spare distributor, but I didn't try it as it was labeled "Bad ICM." So it either has a bad ICM or pick-up coil, in my frustration I was not about to go stabbing that thing in there only to add more monkeys to my barrel... But yes, definitely a bargain. I think it came from the ex's '92 when I put a new distributor in it.. Hmm, I think I have the receipt still, that thing had a lifetime warranty on it. It's still in the box, how would they know it's not the one they sold me?? Derek thinking he might go get himself a free distributor to test.

Your logic makes a lot of sense here about that pick up- perhaps that's what it/was doing, sending erroneous signals.. Perhaps that's why the old cap was burned up inside.

"..the ignition coil would be more likely to burder an ignition module.."
I didn't bench test that new Delphi coil I tried, but when I initially did all the work, the Delphi one kept making the tach jump the majority of the time I had the engine running, occasionally it would run smooth. When I threw my old coil back on, no tach flutter. I did that twice with the same results so I thought that's what it was and I was good to go. I even let the bass-**** run for about ten or fifteen minutes and held it at ~2k RPM for 10 seconds or so at times with no issues. Then of course it sits overnight, I go to hop in it to go to work and the tach starts fluttering around.. So perhaps both are bad, I could bench test mine again.

Next test is the jiggle wiggle thing while someone watches the tach. Perhaps I've got too much schmoo in the harnesses, I do tend to use a lot of that stuff.

I wish I had tools to back probe wires like I've seen done on South Main Auto and Pine Hollow Auto Diagnostics, could see what the quality of my wave forms were as the thing acts up. Not to mention see exactly which component is causing the mess.

Sorry to hear you're still having this issue since the last time we talked. IMHO just one of them being bad can/will take out the other, especially a bad coil. Here's an example of a 7 pin ICM and wiring to the ECM, coil, and pickup (the ground wire is in your 4 pin connector to the ECM on an 8 pin ICM, everything else is the same). Maybe check them for corrosion and connectivity? Dielectric silicone grease is great on the connectors but, you don't wanna get too much on the electrodes.

You must be registered for see images attach


Other than that, I don't have anything else :33: HTH

Yeah, me too. This truck was supposed to help me road trip and pick up some iconic speakers!

Hmm, so it seems the ESC module does have a critical role to play with the ignition system. Seems it's function is just to check for knock? What's the fuel injection switch on pin B? Is that the signal which causes the injectors to fire?

Thank you for your help, fellas. It's appreciated. I'm sure I'll get it squared away eventually.
 
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