cfair

I've burnt through 3 HEI modules and a coil over the past 3 days. I converted my 66 98 from points to HEI years ago - maybe 20 or more. And it's been great except for the occasional (and expected) module failure.

Suddenly it's unreliable. The distributor is older - maybe 8-10 years? I redid the wiring to it about 3 years ago with an 8 gauge to the BAT cable, just to minimize resistance.

There are 12.31 volts to the BAT terminal when the key is in run position. The wiring system shows a stout 14.3 volts at the battery when it's running. 14.3 at the junction block too.

One weird reading - when the car was running I wanted to check voltage inside the BAT connection in the cap. At one point I got -1.

I'm wondering if that means the distributor is aging and shorting out electrical parts by grounding in some way I've can't see.

Could my key switch be aging and dropping voltage to the distributor? I would have expected more like 13 or more there, although I believe 12.31 is perfectly within spec.

Any help is appreciated, I'd like to get it back to rock solid reliable again.



cheers
cf

oldcutlass

12.31 is acceptable with the key on. Where do you have the HEI power wire connected? Try these troubleshooting steps if it fails again. Check your feed wire for chafing and shorting. If you have a tach disconnect it, sometimes they can cause problems.

1. Connect an ohmmeter between the TACH and BAT terminals in the distributor cap (at the coil on the L4). The primary coil resistance should be less than one ohm.


2. To check the coil secondary resistance, connect an ohmmeter between the rotor button and the BAT terminal. Note the reading. Connect the ohmmeter between the rotor button and the TACH terminal. Note the reading. The resistance in both cases should be between 6,000 and 30,000 ohms. Be sure to test between the rotor button and both the BAT and TACH terminals.


3. Replace the coil only if the readings in Step 1 and Step 2 are infinite.




These resistance checks will not disclose shorted coil windings. This condition can only be detected with scope analysis or a suitably designed coil tester. If these instruments are unavailable, replace the coil with a known good coil as a final coil test.

4. To test the pick-up coil, first disconnect the white and green module leads. Set the ohmmeter on the high scale and connect it between a ground and either the white or green lead. Any resistance measurement less than infinity requires replacement of the pick-up coil.


5. Pick-up coil continuity is tested by connecting the ohmmeter (on low range) between the white and green leads. Normal resistance is between 650 and 850 ohms. Move the vacuum advance arm while performing this test. This will detect any break in coil continuity. Such a condition can cause intermittent misfiring. Replace the pick-up coil if the reading is outside the specified limits.


6. If no defects have been found at this time, and you still have a problem, then the module will have to be checked. If you do not have access to a module tester, the only possible alternative is a substitution test. If the module fails the substitution test, replace it.

A link with more info:
http://www.angelfire.com/realm2/84ca...coil/coil2.htm

Ozzie

If you literally mean "burnt through" then look for some stray high voltage going where it shouldn't. One of the shortfalls of the GM HEI is that everything is compact in close quarters. If something gets out of specs the high voltage can go somewhere where it can do a lot of damage.

cfair

I tried stabbing in a new distributor today, but it was badly machined (perhaps offshore) and just wouldn't fit. So I cannibalized the non-fitting one for electrical components and retrofitted my old aluminum housing with new bits.

It worked. Nicely. Then I connected the tach lead.

That stopped the motor dead in it's tracks. Twice.

Would you agree I likely have a short in the wire from the HEI to the tach that's grounding the distributor?

I've never rebuilt an HEI before. Interesting design. Fun morning.

thanks in advance for your guidance

cf

oldcutlass

You have an issue with your tach or tach wiring. Thats why I mentioned it above, seems there have been a rash of these type problems lately. All tachs are hooked to the negative side and monitor firing pulse. If there is a short there, the engine shuts off.

cfair

If there's a rash, Are you seeing grounded wires or Olds tachometers going bad?

When I pulled my tach wire today, i saw no obviously broken insulation, but I'd be astounded if the olds tach suddenly gave up after 50 years. I'm hoping it's a wiring problem I can't see...

I'm installing a new HEI tomorrow and hoping the new tach wiring doesn't kill it since that leaves me with the need to check out the tach itself which is considerably harder to replace than wires and connectors...

oldcutlass

Both, most often its a chaffed wire that feeds through the bulkhead with non stock tachs.

droldsmorland

Not reading deeply through the entire thread I can offer one important piece of advice...grounds, grounds, grounds. If a negative ground automotive electrical system does not have a clean current path back to the battery it will cause all sorts of transient currents trying to find its way back to the battery. The first thing to go is electronics. Clean all grounds. The main neg wire should be grounded to the block the block should be grounded to the body and frame etc...The VR and the alternator need good grounds as well. A 40-50-60+....year old metal vehicle surely has many bad grounds that need your attention to work as intended.

cfair

Just to finish this out, I replaced the brown tach wire and the problem is fixed.

oldcutlass

Good to hear

Fun71

Any idea why the wire was the problem? Compromised insulation? Shorting to something?

cfair

The wire had 2 or 3 solder points and seems to have been rubbing on the firewall or somewhere else. Anyway, a new wire solved it. Interesting that a short in the tach wire killed the engine dead

oldcutlass

Same principle as a lawn mower.

MDchanic

Will do it every time.

Glad you got it fixed!

- Eric

Fun71

As oldcutlass said, it's the same principle as a lawn mower - ground the coil and no spark. The difference between a lawn mower magneto and an HEI distributor must be that the HEI electronics don't like driving current into a low resistance short. That is a certain way to kill an output transistor -think of an audio amplifier.

oldcutlass

Quite possible that the other modules are still good.

cfair

Is there a way to bench test HEI modules using a multimeter?

I tossed 2 old modules (possibly good), but have a couple more in unknown condition which I'd love to confirm as good or bad, but ideally without the hassle of popping them in and out of a 455...

MDchanic

Just take 'em to a local chain auto parts store and have them test them.

- Eric