: HEI ignition module problem
Nov 26th, 06, 11:25 PM
What can cause module failure?'76 Chevy truck.Long story short-had to use an old HEI today to break in a cam,fired up and ran about 10 seconds then poof.Picked another up at Auto Zone and it ran fine for the break in.Fired it up later for a test drive and about 10 seconds later-poof.
The old cam chewed up the gear on the Mallory(Complete Mallory ignition system) so I had to fall back on the HEI (factory ignition system).
edit:after using the search feature on this subject I'll add a little info,the coil is from Hypertech,I got that for nitrous use,and the tach is a very old Sun tach-made in USA and don't know when,it was on a buddies dad drag car in the '60s.Sounds like it may be just a cheap module (the 2nd one),but the first was an original GM (delco) module.Not much grease on it though.Ground strap is connected and there is 12 volts at the bat wire.
Nov 27th, 06, 11:01 AM
Coil could be shorted. Is it an in-cap coil? Get some grease, it's cheap.
Nov 27th, 06, 11:40 AM
In cap coil.I'll check the coil.I'm about to go get a Napa ignition module,TP45.Think I'll get the pick up coil as well.If this fails I might try to wire it up to my Hyfire IV box,but I don't have an HEI harness for it,like the wiring diagram shows.
Nov 27th, 06, 11:56 AM
Had this same problem once with my 402. Kept blowing modules. A buddy and I switched out HEI's, still runnin' on that one 3 years later.
Nov 28th, 06, 6:12 PM
It's fixed now.I think the culprit was a loose screw for the ground wire.It looked good visually,but when I removed the coil,the screw where the ground is fell out,threads in hole were stripped.A little evidence of some arcing looks like.Anyway,a new Napa TP45 module and new pick up coil with a good cap and rotor and all seems well.Put 80 miles on it today with no problems.
Nov 28th, 06, 6:37 PM
I've always gotten long life service out of the genuine GM modules. Those cheapie parts store brands ain't worth a flip. :sad:
And be sure to use the heat sink grease...
Nov 30th, 06, 1:46 PM
Spoke too soon.This thing is driving me crazy.It doesn't burn up the ignition module anymore.The coil tests good.I put a new pick up coil in it.Now I have put in a new RFI suppressor and harness that goes from the module to cap.Have 12 volts at the bat connector that goes to the cap,voltage never drops below 13.4 +/- when running.
It will fire up (when it wants) and run for any undetermined time,sometimes short,others long,and then it will just shut down.I would mess with the harness' going to the cap after it shuts off and it usually fires right up,going into the same scenario.That's why I replaced the RFI and harness thinking the connector or wires may be bad.No change.Last time I got it running (where it quit on me this morning-and after replacing RFI/harness) I unhooked the tach lead.I let it idle for awhile,test drove it some in the parking lot,then drove it home.Everything is new on the dizzy now,and still have the problem.
The only thing I can think of is the bat connector,it looks good visually,as does the wire.
Noticed the water temp gauge(elec.) in dash freaking out also,don't know if it's related or just because it got wet at the connection this morning.
I've got an extra MSD6a,anyone know how to test these?don't know the condition,worked when I loaned it out,got it back and they said it was bad,been on the shelf ever since.
Nov 30th, 06, 2:15 PM
Do you have a good ground from engine to chassis? And, dash (gauges to chassis)?
Nov 30th, 06, 4:35 PM
Everything is grounded good.This just started.It's sleet/snow outside right now,if it lets up,or maybe tomorrow,I'm going to get between the engine and firewall to see if I can find a short/arc or something.Going to switch coils also,even though this one tests out o.k.
Dec 2nd, 06, 8:42 PM
Checked all wires,from starter,alternator,distributor,tach,all good insulation and connectors and/connections.Have grounds from battery to block,block to frame,cab to block and frame,from instrument cluster to chassis,loosened and retorqued all of them.Rechecked internal distributor wiring/connections-pick up,capacitor,module,grounds.Swapped on a different coil.Fired it up and it ran maybe 10 seconds before it blew the TP45 module.Swapped to yet another coil,found a GM 990 module in the garage on another dist. core.Fired it up for another 10 seconds or so until that one blew.Got another module from Auto Zone,DR100,changed cap and rotor,pulled the wiring harness up and away from the firewall and block and inspected wires yet again,and zip tied them up away from anything.Checked wires on starter solenoid,again.Ran almost 20 seconds this time.
I've wasted 2 GM 990,2 AZ DR100,and 1 Napa TP45 ignition modules,I've got over $100 and 100 hours now tied up in this.I really need to know how to test the MSD 6a box,while I hunt down the bushing I need for my Unilite.
I can't get mt Mallory stuff back in soon enough.
What causes modules to blow?If I know I can remedy this.
I need any help with this ASAP,need truck for work tomorrow.
Dec 16th, 06, 1:34 AM
Have you verified that the power to the B terminal is not interrupted when the engine dies? I'm wondering if you have a loose connection at an ignition fuse--or somewhere else in the harness or ignition switch--and the power is intermittent. "Maybe" you aren't blowing the module, you've just got a lack of power to it??? In screwing around with the cap to replace the module, you jiggle the wires and the connection is made again--so it appears the new module is OK and the old one was bad???
If not that, there are some unlikely possibilities:
First Guess: Tach (including the wire connected to the tach terminal in the distributor cap) is defective or inappropriate for use with an HEI. You do NOT want the Tach terminal in the distributor cap to ground out--it will prevent spark but might overheat the ignition coil.
Second Guess: (long shot) Shorted ignition coil. 'Course, this has happened with three different coils. Seems unlikely that all three would be bad.
Third Guess: (long shot) Missing ground strap from metal frame of the ignition coil to the center terminal of the three-wire harness which routes from the distributor body up to the cap. [often the engine won't run without this strap]
Fourth Guess: (long shot) Wrong kind of grease used under the module. Needs to be heat sink compound, not dielectric grease or wheel bearing grease. Ten seconds shouldn't build up enough heat to fry the module, though.