I have one of these to check spark but I also have used my timing light to check for spark. Good thing using the timing light is you can check all plug wires quickly.

 

Yup! Easy enough to check, and the likely cause of a sudden failure.

That loud pop could have been the distributor gear roll pin shearing, or the gear skipping teeth. Can a timing chain break in these cars?

 

I have determined that I have no spark and the alternator isn't charging correctly. I'm replacing the alternator later this week in hopes that will solve the problem. I know DUI distributors require high voltage and I am thinking that the voltage dropped to low, spark stopped, fuel "popped" with a backfire through the exhaust. Hopefully a painless repair, aside from the time spent on the side of the road.

Thanks everyone. If I determine something else is the cause, I'll report back.

Keith

 

While you are waiting on that alternator, you might want to consider one more free test.

Pull the distributor cap, and with a helper (or a camera), confirm that the rotor is turning when you crank the engine.

If your battery has the energy to crank the engine, it can supply 12V when running, enough to keep that DUI HEI module happy.

What did the "pop" sound like? One big bang, or multiple?

BTW, "backfire" in the exhaust is known as "afterfire", and is unburned fuel igniting in the exhaust, often due to a lean miss. I can make my 4-speed do it when I downshift and coast down a hill in gear. It sounds like I'm making popcorn in the sidepipes.

 

Thanks. I’ll check that as well. The “pop” was a backfire, hopefully from lack of spark and unburned fuel in the cylinders.

Keith

 

Just curious as to how did you determine the alternator wasn't "charging correctly?"

 

And, was the vehicle flat-bedded home or is it barely running now?
Some parts of this story are missing.

To test the output of the ALT, it needs to be running, using a voltage meter at the rear lug.
About 12.6V

Hate to see anyone just throw parts to repair nothing needing repair.

 

Obtaining an Answer to the question if rotor is turning, is quick n easy and free.
Your answer to same may be very helpful in accurately answering Your core question.
But, if, IF Your rotor isn't turning properly, All the sparks and ether in China can't help You. I do hope Your issues are universally resolved by a new alternator.

DUI's HEI requires virtually same input voltage and current as does OE HEI (MY '75-up). But, It Cannot run well IF it's supplied via same old resistor wire as did supply old OE points dist. However, it seems you've run a dedicated supply from fuse panel to dist cap BAT+ terminal; that's a good thing.

 

I'm currently out of town and unable to work on the car, but I can tell you my thoughts on why I think the alternator is the issue.

Prior to reversing the polarity on my battery, the car was running great. When I reversed the polarity, I blew the fusible link at the starter and saw smoke near the alternator. I replaced the fusible link and battery and cranked the car. I never tested the alternator, but the dash gauge made it appear to be working. There are occasions my gauge jumps around, so I can't trust it completely. I test drove the car and it was fine for the 5 miles I drove.

After my test drive and determining that I had it running properly (minus checking the alternator), I swapped out the existing HEI to a new DUI HEI. Everything went together perfectly. Not my first time installing a distributor. I set the timing and took it for a longer test drive. During that drive, the engine backfired loudly (pop), big puff of black smoke and the engine died. Flatbed ride home.

Once home, I checked for spark using a timing light and have nothing. I checked the voltage of my new battery and it is at 11.5. This, and the smoke I saw at the alternator, is what I'm basing my diagnosis on. Considering I saw smoke near the alternator, I know something is not right. I'm okay with replacing the alternator, even if I determine that is not the cause. It obviously is not charging like it should. Once I replace the alternator, I'll check for spark again. If no spark, I'll look into the distributor.

I don't believe in coincidence. The car ran fine before I reversed polarity on the battery. I'm sure I caused this issue and damaged the alternator or something else.

Thanks
Keith

 

IIWM I would be checking out the wiring harness very carefully before I would attempt to fire it up again.
Continuity, Volts, resistance, etc.
You may have damaged other wiring.

FWIW the car will start and run without a working alternator at all, for quite a while, till the battery poops out.
I got 30 minutes out of one car, once. Then it started sputtering and slowly getting worse. It never "popped".
5-10 minutes is not long at all.

The alternator nor battery would not be my first "go-to" move, as to "cause".
You likely have other electrical damage.

Your backfire sounds like sudden & total spark failure, while running.
My symptom was entirely different, with slowly increasing misfires over 10-15 minutes.
Mine was a dead alternator.

I have a 72 wiring diagram if needed.

 

To me it sounds like the Ignition control Module took a crap. Same thing happened to me. They do not like high or low voltage,

 

Yes, sbc (including C3 era sbc) Can break a timing chain. However, that's not routine and is rather rare. Much more frequent is when Nylon-Clad OE (& some replacement) Cam Sprockets can & do shed/shuck (aka Strip) their Nylon Teeth; and enough of that action can and does render timing set in virtually same dysfunction as if chain itself had broken. Also, the wasted Nylon bits can and have jammed their respective oil pump. However, it IS worth noting here that OP reports his motor was rebuilt about 5K miles earlier; such a rebuild would typically indicate a New timing chain set was installed. Thus, it would seem OP's motor probably has a near-new timing chain. HOWEVER, some cheap timing chains (most especially the cheapest "double-row" chains) are of Very poor quality and have one or more inherent weaknesses (e.g. split in lieu of solid rollers). Did the rebuilder install a cheap timing set, or clean out the oil pump or replace oil pump screen ? Was the camshaft's rear gear damaged by the gear on the cheap HEI dist that was first installed by OP? Would it have been prudent to freshen the OE points distributor and run it ? Dunno, I wasn't there or making any of those calls. Again, further analytics can only be advanced by OP verifying prior questions about rotor.

 

The owner verified if he had spark or not, I would assume he cranked it over with a Sparkplug tester involved.

If a timing chain did break, the lovely pistons would be kissing the lovely valves making an awful lovely racket.
Lets not go there.

 

In post #29 OP states he checked for spark using a timing light and he had none.
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The OP updated that the "pop" was backfire (afterfire), so everything is pointing to an electrical issue.

OP, when you get a chance, really examine your wiring harness. Make sure the DUI HEI is getting a solid 12 V, and that there are no burnt wires or fusible links. Hopefully something simple. As @leigh1322 pointed out, your car can run on just the battery. You might want to consider testing your alternator before replacing it. If it is a wiring issue, replacing the alternator won't fix it.

A wiring diagram might help (see the link below).

Good luck, Electrical issues are the hardest to diagnose online, but likely the easiest to fix. Please post some photos if you get stuck.

wordpress.keystonestatecorvetteclub.com/?page_id=118

 

Two 72 wiring versions.

 

Thank you, everyone. I have done a slight inspection of the wires and plan to take a much closer look. The harness to the starter was replaced when the engine was rebuilt, so I'm familiar with it. I have a laminated wire diagram as well, although I appreciate the ones provided.

I know I have power to the alternator and power to the distributor. I just have to determine if I'm getting enough power. Hoping I can get back behind this on Friday when I'm back home from a business trip.

Keith

 

*****Solved********

The issue was the control module in the distributor. I figured this out by testing the coil and find that it was good and then I replace the control module with the one from the distributor I replaced and the engine fired. I have DUI sending a new control module to replace it. The only thing I don't currently know is why it went bad. Was it just bad equipment or did something in the car cause it. I don't see how the car could cause it, but I want to be sure so I can comfortably drive without always fearing it will happen again.

**Update** DUI said they had a batch of control modules that were overheating around the time my distributor was built and they have shipped a replacement. Good to know, but would have been nice before I ended up on the side of the road and on the back of a rollback.

Thanks everyone for your help. Happy it was minor, even when it felt major.
Keith

 

Glad you got it sorted!

I'm sorry to hear that even D.U.I. can't source reliable HEI modules.

One trick that those in the Forum have been using to get a bit more life out of the modules is to add some heat sink compound under them when installed (the same stuff you'd use on a CPU in a computer). Did D.U.I. happen to mention that, or send you a small tube of it? It sounds more like a manufacturing defect than anything else, but every bit helps.

 

They didn't mention it and the module is shipping today. I just ordered some heat sink compound from Amazon, just in case they don't include it.

Thank you!