Trouble with underhood harness with HEI conversion

[TBONE1964]

This has got me so curious that I went out and checked my own truck (1972 Chevy 350 w/HEI and external regulated alternator (factory config)

Using a DVOM, I have 12.4 volts with key on engine off.
During the crank which is very short, I see a voltage drop to 10.8.
While running, I see between 13.6 and 14.1 volts at idle with no acc on or lights.

The battery voltage on the hot wire to the distributor for an HEI should be whatever the alternator is putting out at any given time.

I believe it is possible for a weak ignition module to require more than 10 volts to light the engine up. Is it 10 or 10.8 and is that enough to make a difference??

Normally when an HEI ignition module goes bad it is due to heat and not voltage.

I am curious to see what the fix is for you.

[Bigdav160]

The specification for starter motor operation is a maintained 9.6 volts at the battery.

It would not be unusual to only see 9 volts at the HEI under those circumstances. (cranking the engine)

[VetteVet]

I'm late to the party here but if you just want to get 12 volts to the Hei to start it you can just run a 16 gauge wire from the "R" terminal on the starter to the HEI BAT and you'll get battery voltage during the start. That's what the points ignition does. That will solve your problem until the module goes tango uniform.

[franken]

Duh, what does back probing mean? Measuring Voltage while cranking? That being the meaningful measurement.

The OP mat want to measure the resistance from the junction block by the battery to the connection at the HEI, and from the same spot to the firewall connector. Of course, measuring resistance doesn't give an indication of current capacity.

Tango uniform. Funny...

Interesting. I guess the R terminal gets 12V when the solenoid engages.
The S terminal I guess means start. What does R mean?

[VetteVet]

Originally Posted by franken

Duh, what does back probing mean? Measuring Voltage while cranking? That being the meaningful measurement.

The OP mat want to measure the resistance from the junction block by the battery to the connection at the HEI, and from the same spot to the firewall connector. Of course, measuring resistance doesn't give an indication of current capacity.

Tango uniform. Funny...

Yes the solenoid has a copper washer inside that connects the R terminal with the large terminal that the battery cable is connected to thus giving it battery voltage. The two most popular names for the R terminal are the resistance terminal because it bypasses the coil resistance wire or Relay because it acts as a relay.

here is a picture for anyone who cares.

[TBONE1964]

Originally Posted by VetteVet

Yes the solenoid has a copper washer inside that connects the R terminal with the large terminal that the battery cable is connected to thus giving it battery voltage. The two most popular names for the R terminal are the resistance terminal because it bypasses the coil resistance wire or Relay because it acts as a relay.

here is a picture for anyone who cares.

Attachment 1141970

Correct me if I am wrong but if the "R" terminal has battery voltage supplied through the copper washer by the battery cable lug, it would have constant battery voltage(12volts +/-), true? If that is true and you wire the distributor that way, the ign module is going to have constant voltage regardless of ign switch position.

This is very simple. You need battery voltage on the "BAT" terminal of the distributor when the key is in the on/start position. If the connection is good, wiring is not the problem. I would try a different distributor or ign module. Is the dist you have 100% new including the pick up coil? Even new parts are prone to premature failure.

[Bigdav160]

No, the copper washer is only hot while cranking. It will see battery voltage. Your's was 10.8v which indicated a healthy battery. The minimum specification is 9.6v. However, I've seen, many-many times, a vehicle start even when the battery voltage drops lower than that.

Since the OP said he only had 8-9 volts at the HEI connector (I assume unhooked) andd he can run the HEI connected directly to the battery indicates excess resistance in the ignition circuit. It could be on the power side or the ground side. It could be junction block, ignition switch, or any number of connectors or connections.

[VetteVet]

Originally Posted by TBONE1964

Correct me if I am wrong but if the "R" terminal has battery voltage supplied through the copper washer by the battery cable lug, it would have constant battery voltage(12volts +/-), true? If that is true and you wire the distributor that way, the ign module is going to have constant voltage regardless of ign switch position. The R terminal is wired in the end of the solenoid and is not connected to the large battery cable terminal until the solenoid is energized by the start switch. Then the copper washer is moved back to contact the two, sending 12 volts to the distributor. When the key is released the copper washer moves away from the two contacts and disconnects the R terminal from the battery cable.






This is very simple. You need battery voltage on the "BAT" terminal of the distributor when the key is in the on/start position. If the connection is good, wiring is not the problem. I would try a different distributor or ign module. Is the dist you have 100% new including the pick up coil? Even new parts are prone to premature failure.

I agree with you that if he has battery voltage at the BAT terminal the distributor should fire the coil. I know of a few guys on here that have less than that and the truck still starts. A lot of guys have made the conversion from points and used the same resistance wire that the points use and their trucks still start. They usually don't run very good but they still start.
One guy has been running his that way for years and he claims his truck runs perfect.

Digital electronics are funny critters, they are more sensitive to voltage than analog stuff. The old stuff would run on less than 12 volts but slower, such as a fan or a motor. His pickup coil in the distributor probably needs more voltage because of greater resistance or the module won't switch just like the fellows have said.

My post is only a band aid fix and was not meant to be the correct way to do it.

[WIDESIDE72]

I started the morning off by swapping in a known good ac delco coil. Still no fire at the plugs. I then swapped in a module out of the same dusty crusty dist thT has been on a shelf in my barn for at least 15 yrs. It has "GM" stamped on it. Put it all back together, hit the key and she fired right up! The generic black module with no i.d. Went sailing across the garage in the general direction of the trash can.
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[WIDESIDE72]

I have had trouble with junk chinese modules in the past, but this is the first time i have had one not make it to the end of the driveway!
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[Longhorn 70]

good troubleshooting..... I had an Accel super duper module do that to me. I was able to drive around the block and it stalled right before my drive way.....I don't know if you've ever tried to push a Longhorn UP a drive with a good slope to it by yourself in AZ in the summer...

Consider yourself lucky it didn't make it out of the drive.

[WIDESIDE72]

I am now getting 10.5 volts when back probing the pink HEI power wire at the dist while cranking. I'm not sure if the low voltage ruined the original module or what.

Thanks everyone for the input. It was a big help!

Now to get the brake bled and start on the exhaust!
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