"Myth 10: Not necessarily a myth but a fact. A defective tachometer, can cause an HEI to malfunction."

Over the 5 years I've had this car I have gone thru at least 3 modules every year. Wondering if that Myth is true or not.

My '80 left me stranded again yesterday and today I went thru a cap with a new coil, rotor, module and still no spark. Only piece not replaced is the pick up coil(think that's what it's called).
There is power going to all the right places.

Now, my tach has not worked from the day I got the car. It has a burned out spot on the board itself. Found that when I went thru the interior a couple years ago.

IF I keep this car, I plan on getting a new tach board in Carlisle.

 

I have found many module "failures" weren't failures at all but rather a real bad connection on the ends. Over time the 1/4" female blade terminals in the connectors open up and fail to make a good contact with the module's 1/4" male blade terminals. Use an ordinary needle nose pliers to squeeze those female connectors so they plug onto the male terminals firmly.

 

Thought about those, checked them, still tight. Distributor was bought new back in the spring.

 

Where are you getting your modules? I have been buying Standard Blue Streak for several years now and they hold up a lot better than the no-name brands (probably made in China).

 

Got them from Advance and Auto Zone.

Tomorrow I'm going to go and see Dub and see if he has any suggestions. Talked with him before and his idea was correct. Bad connection at the plug from the starter, but this is completely different.

Thanks for your thoughts BTW.

 

I'm not sure how a bad tach could have anything to do with the stock GM HEI module going bad.

but not sure, so let's talk through how it works.

The HEI module is a NPN transistor, and a signal converter from the pickup (reluctor). (it is basically the Motorola MC3334 chip)
The Reluctor sends the signal, the chip in turn tells the transistor to power on/off the coil.
The tach reads off the coil.

pretty simple setup, and not a lot to go wrong.
Common issues for the HEI unit are thermal breakdown, or over voltage.

The max temp is 257 F. anything over that will kill it.
Overvoltage protection is 30v.

Are you using the heatsink/thermal paste when you replace your HEI module?

What condition is the pickup? I have posted this before, the pickup moves during advance, the wires get old and brittle and can cause issues.

Finally, what is your voltage feeding the module? Spikes from bad diode in alternator?

is the grounding screw and attachment proper and not corroded?

Again, not saying the tach is not responsible, but I don't see how it could be but maybe I am missing something. The tach could ground out, but then it would seem to affect the coil...15 modules in 5 years is more than a lot...1 module in 5 years is a lot, in fact.

do you know how to test the module out of the distributor, and if so, what are you seeing? any hot spots?

I only post this because the HEI module is so robust, and has such a low failure rate, people adapt and use it in all kinds of applications where stock replacement CDI modules are hard to find or very expensive...including Ford's, Chryslers, Honda's, etc. I have even adapted them to motorcycles...

 

last thing to note about the ground, This is from the spec sheet for the module:

"General Layout Notes
The major concern in the substrate design should be to
reduce ground resistance problems. The first area of concern
is the metallization resistance in the power ground to module
ground and the output to the Rdrive resistor. This resistance
directly adds to the VCE(sat) of the IC power device and if not
minimized could cause failure in load dump. The second
concern is to reference the sense ground as close to the
ground end of the sense resistor as possible in order to
further remove the sensitivity of ignition coil current to ground
I.R. drops."

 

Can't you just disconnect the tach wire to the condenser/capacitor since the tach isn't working.

 

I would have assumed he did that after the 3rd or 4th failure...maybe not. I am just really curious on that high of a failure rate...I can count on one hand the # of HEI modules I have replaced in the last 25 years...when I worked at a parts store, we sold them almost daily. an hour later, most would bring them back for a return and buy the part they really needed...rare was the defective module...but it takes a lot of blame for other issues.

 

Had a similar issue in a Trans Am years ago. Turned out to be bad grounding. Take a digital volt meter set it to voltage DC-. put the positive lead to the red wire lug on the altenator and the negative to a good know ground with out the engine running note the reading. Next keep the positive lead on the red alt wire put the negative on the distributer base note the reading it should be the same as first test. If not look for corrosion or paint stopping the ground path. I had excessive paint syndrome rather than scrape all my paint I carefuly drilled and tapped the distributer body and added a seperate ground lead never had a issue again, as I recall a bunch of my so called bad modules worked in other cars after I deemed them junk I only kept them cause I keep every thing its a horrible problem. try it hope it helps. if you dont have a volt meter a test light could work but it wont show voltage drop because of resistance. good luck

 

Thanks for the reply.

 

Will try that, Thanks.

 

It will be now until board gets replaced.