HEI module dying

[Breitenbucher]

I had followed Deve’s article for the HEI upgrade for my 1957 GM 235 engine and purchased his kit plus he overhauled my distributor and installed the reluctor and pickup sensor that was going into my forklift. After a short amount of run time on the engine the forklift was put away in the barn for several months and then wouldn’t start. I chased the issue to the HEI module that wasn’t firing and suspected the pickup sensor since I was only seeing, at best adjustment, about 4 volts peak out of it. Deve’s article states the HEI module needs a minimum of 9 volts peak so I purchased a new pick up sensor. The new sensor was going to be very difficult to swap out so I did more testing. I chucked a 1.25” butterfly wood drill bit into my drill press and clamped the new pickup sensor such that the edges of the drill bit would pass very close to center of the coil mimicking the reluctor. Again I’m only seeing at most 4 volts peak on my oscilloscope. My next test was to coble up a small circuit with a couple of resistors and a capacitor to generate a higher voltage spike to drive the HEI module on the work bench. Raising it up to 14 volts peak, I’m still not seeing the HEI module output switching while driving a 10 ohm resistor load. So off to auto parts store to purchase a new HEI module and using my aforementioned test circuit and load I see the new module switching all the way down to a 1 volt peak drive signal. I install the new HEI module in the forklift and drive it for a short time and it fails again. After removing the HEI module from the heat sink I see the heat sink grease I applied had not been squished, it looked the same as when I smeared the blob on to it. Looking close at the back side of the module there are two small plastic alignment knobs sticking out about 3/32” which prevents the module from seating tight against Deve’s modified heat sink. Looking at Deve’s supplied original HEI module he had filed those knobs off. It would be good to mention this in the article. Still not sure why I get such a low voltage out of the pickup sensor, Deve’s oscilloscope trace looks like he is getting 40 volts peak, 10 times what I get but the module seems not to care so hopefully this fixes the issue. Still not sure why the original HEI module failed, perhaps just infant mortality, keeping my fingers crossed.

[Deve]

The quality of these modules is going way downhill. Put in a different brand module and see if that helps. Since I wrote that article, I found that the HEI module reliably triggers at .3 volts. Almost nothing and it still triggers nicely. Adjusting the pickup as close to the reluctor as possible without rubbing increases that voltage. If you turn the distributor by hand, you should get about .25 millivolts AC each time a reluctor star passes the pickup. No need for fancy equipment to check that. Very impressive research on your part though! This issue is universal for all HEI systems of this vintage so its not just my modification, its all of them. It has to do with using cheap module internals. Once you get a decent module, it will run for years without issues.

[Breitenbucher]

The HEI module failed again after very little use. The MSD 8202 coil measures 0.6 ohms which is a bit low. Thinking of ways to reduce the heat dissipation of the HEI module and what I had in my junk box I inserted a 1 ohm 20 watt wire wound resistor between the plus side of the coil and the incoming 12 volts and B terminal of the HEI module. Effectively making the coil primary 1.6 ohms. With the HEI providing constant current of 5.5 amps this will now drop 8.8 volts across my 1.6 ohm resistor/coil load leaving a couple of volts for headroom even when cranking to start the engine. The original 0.6 ohm coil would only drop about 3.3 volts leaving the HEI to dissipate the remaining 10.5 volts times the 5.5 amps times 50% duty cycle equals about 25 watts. The 1.6 ohm resistor/coil load drops 8.8 volts requiring the HEI to dissipate 5 volts times the 5.5 amps times 50% duty cycle equals about 13.75 watts. Running the engine with this modification showed no difference but of course it is in a forklift not a hot rod, hopefully will prevent more failures.

[Breitenbucher]

This forklift isn’t used very often but after loosing another HEI module it finally dawned on me that the forklift was always put away in the barn and wouldn't start so it either failed while trying to start or failed during shutoff. So I put an oscilloscope probe on the incoming 12 volts to the B terminal of the HEI module. With the engine running I set the scope trigger level to 25 volts and shut the engine off. Bam, the scope triggered showing an over 60 volt spike which maybe the culprit taking out the HEI. I happened to have a TVS diode SA18A-E3/54 that I installed from the HEI B terminal to ground which clipped off this high voltage spike. This maybe just associated with my, very unique, homemade forklift ignition wiring. The TVS diode can be purchased from Digikey, part number SA18A-E3/54GICT-ND.