GMC No Start, Weak Spark

Vehicle-1996 GMC Savana Van 1500 5.7L Vortec Engine Code R

Probelm- Died while driving, and cannot restart.

Troubleshooting- Voltage on battery is 12v, voltage on ICM connector 12v, voltage from ICM to Coil while cranking is 10V. NO spark at the plugs, and WEAK orange colored spark at the coil (tested from the high tension coil wire, and the coil itself). The rythm of the spark at the coil seems good, just weak. I purchased a new Duralast brand coil from Autozone for testing, and had the same result. I had O'Reillys test the ICM, and they said it was good. The cap, rotor, plugs, and wires are all 8 months old.
60 PSI fuel pressure at the rail.

A troublshooting guide instructed me to "repair the short to ground in the tachometer circuit between the coil and the instrument cluster". This vehicle does not have a tachometer on the intstrument cluster.

Any advice? Should I try another coil in case the one I changed out with is bad?

Do you have access to a scope, or low amp probe? I ask because I'd be very interested to know how much current the coil is drawing while it's making weak spark.

ScannerDanner talks about this in a couple different videos, but the idea is that the ignition module fails in such a way that it starts current limiting the coil WAY before it should be. So, instead of drawing six, eight, or ten amps like it should be, it ends up drawing one or two amps. Then you've got weak spark.

There's a great example of this in Section 22, page 20 of Engine Performance Diagnostics:



If you don't have a scope or low amp probe, then it becomes tougher to prove a failed module. Given that you've already replaced the coil, I think my next move would be to recheck power and control at the coil itself before replacing the module. I see that O'Reillys tested the module, but I wouldn't place a lot of faith in their bench tester. We know that the module is 'working', or switching ground to the coil, but may not be carrying enough load to produce strong spark.

It may be worth watching this video, too, if you haven't already. It's an older Chevy, but the testing procedures are the same.

I actually saw this video, and the case you are referencing with the ICM limting current. So, part of me wants to just replace the ICM and assume the transistor isn't grounding properly during the switching signal (hence low voltage and weaker spark). Before I do that, I want to confirm if the ground wire for the ICM is weak, or
if the (-) control wire from the ICM to the Coil is bad. Do you know how to properly diagnose this using a test light (I also have a logic probe) or multi-meter? I am pretty weak with electrical testing. I would not mind if you explained like you were talking to a toddler.

Thanks!

Sorry for the delay in getting back to you!

Do you know how to properly diagnose this using a test light (I also have a logic probe) or multi-meter? I am pretty weak with electrical testing. I would not mind if you explained like you were talking to a toddler.

Thanks!

:lol: No problem. I grabbed a diagram of this system, mostly for my reference. :blush: But also for anyone else following along.



Kinda blurry, I know, but it'll work. The ground for the ICM, pin C black wire, can be checked with a test light connected to B+. Crank the engine and look for the light to go dim. Do this with the connector plugged in and backprobed. The brightness of the light will change with starter current, but should NOT go out or get dim. I believe you already checked ICM power, correct? You can recheck it if you like, same as above, with the test light connected to B-.

The coil control wire, pin D white/black, can also get checked with the test light, connected to B+. Connected and backprobed, the light should pulse rapidly during cranking. The 'Tachometer Signal' label on that pin is kinda confusing... It's definitely the coil control, but it's also the source of the tach signal on some models. Here's an example of how the circuit looks on a pickup with a tachometer:



Probably worth checking the coil power feed as well, pin A pink wire. Test light connected to B-, should remain lit solid during cranking. Any significant dimming or going out is a red flag.

Let me know how it goes! Please feel free to ask more questions as needed.

I was able to test the ICM with the instruction you gave in the previous reply. Here are my test light results:
Battery(+) to Black (C) wire on ICM while cranking = Bright Light, no dimming
Battery(+) to White/Black (D) wire on ICM while cranking = Bright blinking light
Mult-Meter red lead connected to Pink (+) power wire (Back-probed Coil Wire), and black lead connected to engine ground while cranking= 10v DC supply.

So after these tests, I decided to pull the cap and clean it out. It wasn't bad, but when I installed this cap 10 months ago, I used a little dielectric grease on the cap terminals. I was thinking that it would decrease corrosion, and increase conductance. Well, as I looked at the cap terminals, I noticed that the grease had turned into a hardened paste like substance. So I decided to clean them off a little and reinstall. I hooked up my spark tester directly to the cap at number 7, and got spark while cranking. Then I tested all of the other points on the cap and had spark. Next, I hooked up my tester to the plug wires, and had spark. So cleaning the cap, really helped get the spark to the plugs. I still couldn't get it to start, but it was acting like it really wanted to. BTW, I had 60 PSI fuel pressure at the rail. I decided to take the battery out and put it in my other vehicle to give it a charge. I will be trying some more this afternoon.

csipes1983.. wrote: I was able to test the ICM with the instruction you gave in the previous reply. Here are my test light results:
Battery(+) to Black (C) wire on ICM while cranking = Bright Light, no dimming
Battery(+) to White/Black (D) wire on ICM while cranking = Bright blinking light
Mult-Meter red lead connected to Pink (+) power wire (Back-probed Coil Wire), and black lead connected to engine ground while cranking= 10v DC supply.

Looking good to me! The last thing you could add in would be to repeat the ICM control wire test at the coil itself. Backprobe pin C at the coil and use the test light connected to B+. Again, there should be pulsing, just like at the ICM.

So after these tests, I decided to pull the cap and clean it out. It wasn't bad, but when I installed this cap 10 months ago, I used a little dielectric grease on the cap terminals. I was thinking that it would decrease corrosion, and increase conductance. Well, as I looked at the cap terminals, I noticed that the grease had turned into a hardened paste like substance. So I decided to clean them off a little and reinstall. I hooked up my spark tester directly to the cap at number 7, and got spark while cranking. Then I tested all of the other points on the cap and had spark. Next, I hooked up my tester to the plug wires, and had spark. So cleaning the cap, really helped get the spark to the plugs. I still couldn't get it to start, but it was acting like it really wanted to. BTW, I had 60 PSI fuel pressure at the rail. I decided to take the battery out and put it in my other vehicle to give it a charge. I will be trying some more this afternoon.

Interesting... Is the spark out the coil itself still orange/weak? How long of a gap will it jump?

It could be that you've got multiple issues at work here - weak spark, secondary ignition resistance (which you fixed), and washed out piston rings from all the cranking done during testing. If you have time, it may help to remove an easily accessible spark plug and see if it's wet with fuel. If so, you may need to remove them all and dry them off.

Let us know how things go with a freshly charged battery!

Still no go.
Yesterday, I installed my newly charged battery, pulled the cap and rotor off and gave it another cleaning with wire brush and a small amount of battery terminal cleaner. I made sure it was clean, dry, and free of debris before reinstalling. I checked for spark at the coil wire, and had spark. It was an orange colored spark, but it jumped a gap of like an inch or more. So then I hooked my spark tester to the cap and checked for spark at several points, and got nothing! So the day before, I was getting some spark, and yesterday I was getting none. I would also, like to add that when I first acquired this vehicle, I found that the cap had a zip tie around it and the distributor, and as I was messing with it yesterday, I finally realized why. The screw hole on the distributor (the hole where the cap is screwed down on the dist.) has a hair line crack, so it doesn't allow the cap to be tightened down well. So I tried the earlier mentioned cranking test with a zip tie around it for better connection.

As to the coil... I replaced the coil a couple of weeks ago with an off brand cheapy, and when that didn't fix my issue I returned it to the store. My plan was to buy an AC Delco coil, if my cheap coil replacement worked.

This has been a really disappointing experience. So I need to refresh, and make a new plan. I hate to be a parts changer, but I think I am going to purchase AC Delco replacements for the coil, the cap, and the rotor. I already have a new AC DELCO ICM. By the way, should the ICM get really hot while cranking? I noticed that the ICM was pretty hot to the touch after only a few cranks. I was really afraid that I was about to blow my new one, so I pulled it off. I'm really concerned that the wiring is crossed or shorted to ground where it shouldn't be.

Thanks for your help, I appreciate it.

Just wanted to follow up on this repair! As I was troubleshooting the ignition coil and ICM with my test probe while cranking engine, it fired up and scared the hell out of me. Turns out the wire connector to the ICM was worn out and making intermittent contact with the spark signal on the ICM. Replaced it for $25 and it hasn’t had trouble starting since. I will always remember to check the wire connectors on all my troubleshooting from now on.