mattquerio

So as the title states I have no spark after head swap. I know logic says ok a wire has been damaged, or not plugged in. Let me give you the situation, all 4 wires going in to coil have power. The coil bracket is grounded to firewall using original ground strap. The coil does not output spark to the distributor, as tested with a digital timing light, it however does show some constant voltage with key on using a simple led test light.

Any thoughts at all??

Vehicle is a 1988 Iroc Z L98

mattquerio

I was told there should be a blinking signal at one of the wires when the key is in the start position, however i checked all 4 wires, none blink, they all show constant signal.

bphage

Can you give more info? Year TBI TPI etc.

AlkyIROC

Changing the heads means you had the intake and distributor out.
Did you time the distributor when you put it back in?

Pull the #1 spark plug and bump the engine over until you hear the rush of air indicating you just went past TDC on the compression stroke. If you look at the timing mark on the balancer, it should be at or just past the timing pointer. Do whatever you can to make sure the timing mark on the balancer is right at the pointer for #1 TDC, not #6.

Now pull the distributor cap and look to see where the rotor is pointing. It should be aimed at the #1 plug wire. If it isn't, you need to pull the distributor and realign the rotor.

The easiest way to tell if everything under the cap is working even if it isn't timed properly is to put a timing light onto any plug wire and crank the engine over. If the light flashes then the module, pickup etc are working. If there is no ignition spark, continue to ignition diagnostics.

There's no way to properly test the ignition module. Assume the coil is still good. Unplug the magnetic pickup from the module under the cap and connect a multimeter to the plug. Crank the engine over and you should have AC voltage as the field fluctuates. If that's good, replace the module.

mattquerio

Sorry totally slipped my brain!! Added to post!

mattquerio

Yes I put it on #1 tdc and aligned the distributor as I re assembled. I'm going to check the module under the cap, good call. Thank you! I'll keep you posted!

mattquerio

All I see going into the ignition control module is 2 plugs, one with 2 wires, one with 4. I checked both for voltage, and the 2 wire plug shows 10.3 ac volts on the meter but it does not show fluctuation, although its ac volts so.... it is always alternating right?

so if it shows 10.3 ac does that means its alternating properly and that may control module may be failed?

AlkyIROC

The AC voltage check is on the pickup wires on the plug under the cap.

On the outside plugs, the 2 wire plug goes to the coil. The 4 wire plug is used by the computer. As long as there is power and ground, the ignition will work. The computer is only used to adjust the timing and to detect the collapsing field to modulate the ground circuits for the injectors.

Follow these instructions

http://easyautodiagnostics.com/gm/4....ed-icm-tests-1

mattquerio

That's a great website thank you, very thorough. I tested it all the way back to the pickup coil on the distributor, it tested a constant .1 ac volts, instead of the .3-1.8 ac volts is supposed to fluctuate back on forth from, so i just said **** it and bought a new distributor at oriellys, obviously with a new ignition control module installed.

Still no spark.

New distributor, ignition control module, coil, plugs and wires already just from the swap...

There must be a ground, or something bad somewhere... but why am I not generating anything from the pickup coil on the distributor....what the hell would make that fail.... even with bad wires wouldnt that still generate the alternating signal for spark??

mattquerio

Ok just went out and checked one more thing... The 2 plug connector going into the ignition control module... one wire is 12v key on power, and the other is the pulse signal for the coil... the 12v key on power is only registering 5volts... i checked battery.. its 12 volts, and to be safe even attached a charger, still 5 volts at the 12 volt power on wire to the ignition control module.... I would assume this means there is a problem or potentially a short in this circuit.....???

Any thoughts?

Could I cut the wire and run 12 volts myself on a jumper wire to the ignition control module to see if that gets me spark??

AlkyIROC

The module can get fried from bad grounds. It's not uncommon to have them fail in a very short period of time. We had a truck in the shop not too long ago for a no start. After just about everything in the ignition system, including the module was replaced and it still didn't start and they wanted to put in a new ECM so I got put on the job. I diagnosed it to be a bad magnetic pick so the distributor was replaced (includes new module). Engine fired right up. Took it for a test drive and about 10 miles later, the module failed again. Replaced the module and it fired right up again. I went through every circuit and can not find any reason why it failed. The truck is still sitting in the shop (low priority repair) waiting for another road test.

Check for continuity from the alternator body to Distributor body, Block, Frame/body. A missing ground wire can cause the module to fail. If you do get it started, make sure the alternator isn't overcharging. That can also fail a module.

mattquerio

Problem solved. Here's what happened, in case anyone ever searches and finds this thread. There is a sensor on the underside of the upper intake plenum that readily interchanges with the two prong plug on the distributor. I switched the two and boom. Thank you all for the advice.

AlkyIROC

Although disassembly is easy, reinstalling everything can cause problems if you don't do it on a regular basis. Harness plugs, vacuum lines, hoses etc can easily be forgotten where they go when you put things back together.

One trick I learned decades ago was to mark everything. Masking or duct tape works but dirt and oil can easily cover up words or the tape tags get torn off.

Track down a local telephone repair truck and ask them for a 1 foot chunk of multi strand phone cable. I think the chunk I have has at least 100 wires in it.

Pull out 1 wire. It will have a unique color code to it. Cut it in half and put one half on the harness plug and the other half on the sensor. Now when you go to reassemble things after a day, week, month, year, you'll know exactly what connects to where.

mattquerio

Yeah those 2 plugs are the only that are interchangeable. I did label everything, but mis labeled those 2.