Big bang, then dead engine
 

Hi everybody. Here's my 2nd post about this since it has now happened twice, and I'm obviously not getting something right.

This is a 31 Model A Tudor, stock configuration except for the addition of an alternator, a master fuse mounted atop the starter, and turn signals.

Initial event: driving along, everything normal. Cruising along in town, car begins to lose power. Starts to cough, backfires extremely loudly (or at least that's what it sounds like) and then quits. Main fuse was blown. Replaced fuse on side of the road, no joy. Got her home, found the condenser was blown apart. Literally 60% of it was missing.

Also, the horn and turn signals were fried. So something horrible happened, electrically.

Wound up replacing coil, distributor cap, rotor, points, plate condenser up top and new short-proof condenser. Set point gap, re-gapped the plugs, set the timing and the car fired right up. Took the car out for a test drive and it made horrible power. Would not accelerate at all. Brought it back into the driveway and advanced the timing slightly. Car runs better, makes a bit more power, so I take it around the block.

Car dies 100 yards from the driveway with the same loud bang and blown fuse as before. My neighbor helped me push the car into the driveway and commented the engine seemed quite hot for just around the block. I agreed.

I'm close to throwing in the towel and having it seen by a professional. What could I possibly be missing? I'm so demoralized.

Re: Big bang, then dead engine
 

Charging backwards?

Re: Big bang, then dead engine
 

Change the coil,it gets hot and shorts out.

Re: Big bang, then dead engine
 

gtoger,

First, check the basic stuff. As stated above, is the car a positive ground battery with a positive ground alternator ?

What have you changed since you bought the car ?

Marc

Re: Big bang, then dead engine
 

Yes , It could be the coil. 40 years ago I had the misfortune of standing near a car that the guy just put a high performance coil on. 10 seconds after he started it, the coil blew apart. Obviously he wired something wrong but it was the only time I have seen that happen!! Thankfully no one was injured !! Be careful!

Re: Big bang, then dead engine
 

Hi Marc,

Yes, the car is positive ground, positive ground alternator. 6V.

Since buying the car (before we had the problem) I've added a rock guard for the radiator. That's it.

I put a 6V coil from Macs on it in the course of getting it running between issues.

This wouldn't be somehow battery related, would it?

Re: Big bang, then dead engine
 

You say the turn signals were 'fried' ... perhaps the alternator is outputting too much juice.

Re: Big bang, then dead engine
 

Have you check the voltage the alternator is putting out, Maybe to high.

Bob

typed to slow again.

Re: Big bang, then dead engine
 

Ammeter charging when running?
Wired correctly?
Wiring good? If not, replace it.
Make sure wiring is not shorting anything out.

Re: Big bang, then dead engine
 

Have you checked the output voltage of the alternator. Very suspicious of other things getting fried with the fuse blowing. The amp meter will determine if the alternator is putting out more current than is being used by the vehicle, it does not check the voltage.
The horn will run for years on 12 Volts, as had one installed on several vehicles and still works good. Light bulbs do not like over voltage. Do not think the capacitor would blow if you applied more than 6 Volts as when the points open, there is quite a large voltage spike across the point gap due to the flux field in the coil collapsing. Let us know what the voltage out of the alternator is. Check with an analog voltmeter not a digital one.

Re: Big bang, then dead engine
 

Yes sir, the Ammeter would move up to the charging area when the engine would get some RPMs.

The previous owner (owned the car since the 1960's) restored it in the early 1990's. The running configuration had not been changed since then, and has not been modified under my care. So is the wiring good? Unless something spontaneously went bad. But maybe there is a short somewhere that hasn't been found. A friend and I have gone through it with a multimeter. Maybe we've missed something and it's time to do it again.

I'd like to point out again that this happens when the car is making terrible power. No pep at all. That has to be related somehow.

Re: Big bang, then dead engine
 

First inspect the wiring system. Look for bad wire strands at the connectors and bad insulation. Not likely the problem, but let us start with safety.

Check the alt output voltage.

That fuse you have on the starter does not do much, but when it blows it just disconnects the battery from the rest of the electrical system. So if you have an alternator with a problem (they never fail so people always say) and the fuse blows now you have what ever the alt is putting out keeping things running. So if the alt is throwing out wild voltage that is what is feeding the car.
The condensor will blow up if it sees too high a voltage.
I would get to someone with the tools to diagnose alternator failure problems. Better yet improve the system with a properly rebuilt generator that will likely never fail.
Also get rid of the fuse on the starter. They can fail too, plus they are ususally not correctly sized to actually protect the system. Here it might be what is causing more damage because it keeps the battery out of the circuit and allows for other things to be damaged to be damaged by over voltage (if that is what is happening on your car).

Just an FYI. You should not change points unless you see really bad point blocks or the wear block is gone.

Re: Big bang, then dead engine
 

I would pull the alternator and have it checked out. Most auto parts will do it for free !
Check the battery cable ground at the frame, clean, bright, and tight

Re: Big bang, then dead engine
 

Suggestion:


If taking alternator to big box store for testing ... Be sure that they know that it is SIX volt - positive ground and what that means as far as testing.

Swapping ground to negative, even for a second or two, will usually destroy the internal regulator.

Many young uns never heard of 6volt POSITIVE ground.

If they slap it onto the 12 volt negative ground testing fixture ... the alternator regulator WILL be damaged.

I would find a generator/alt repair shop. They should know how to test it.

Re: Big bang, then dead engine
 

Check the voltage at the battery when running for over voltage..

Re: Big bang, then dead engine
 

Just a thought in relation to the alternator. I have seen (not often) where there is a loose wire on the rotor that under heat and RPM will contact the pole piece and ground out.
If that happens the alternator will full field and put WAY too many volts and cook things.
You might find it on a test bench if things are just right, but you may need to have someone pull the unit apart and visually look for this.

Re: Big bang, then dead engine
 

Their chart most likely doesn't have a listing for a 6 volt alternator, and especially one with + ground, so the store guys will stand there with a dazed look. :eek:

I've see a few alternators that overcharge.
I only like generators and like to install a voltage regulator in them.

The ammeter should stay near the 0 mark except for a minute or two after starting the car, while the battery recharges. If it stays in the 10 zone or there abouts, it's overcharging.

Re: Big bang, then dead engine
 

Thanks everybody. I guess the alternator is something to look into that I had not thought about previously. I do have a couple of questions about that:

#1: How to test if the engine doesn't run? I guess pull it and take it somewhere...

#2: Still doesn't explain why the engine would have such a loss of power right before the audible bang (which sounded like a backfire) and then the electric system would die.

Re: Big bang, then dead engine
 

well to isolate alternator from the system it doesnt need to be hooked up unlike a generator. disconnect all wires going to it so its a glorified idler pully and start the car and run on battery power.

should be able to put a meter between output of alternater stud and frame ground to see what its putting out.


#2 loss of power probably because your running irratically due to weak or shorted connection electrically or dead cylinders. they still suck fuel in and it collects in the muffler till it goes bang from heat or irratic fire from exhaust.

Re: Big bang, then dead engine
 

Today's group 1 batteries don't fit the original type of battery tray as snugly as they should. When the battery slides around, the terminals can short to ground. Huge draw and then slide away -go open with a big arc. The hold downs are a poor fit as well. Metal hold downs also can short to ground. Pure havoc for sure. "V" shaped spacers in the diagonal corners can hold the case securely - good practice.