Technical IGNITION TROUBLE. HELP PLEASE

Hey guys I'm having a hard time diagnosing my 51 merc with 8ba. It's got an older mallory 25 series dual point. I have replaced everything except the distributor itself and the solid core wires.
It starts and runs but will intermittently cut out and then be fine. Just like turning the key on and off. We bypassed the chassis harness and jumped the coil straight off battery. Does same thing. All the grounds are there. Alternator is GM internal regulator and was replaced also. When running the voltage jumps all over the place 9-20+ volts. Also we swapped carbs just to be sure it wasnt fuel. What do you guys think? Anyone ever had this problem? Thanks for the help!

 

Condenser?

 

how is the voltage measured, and where in the circuit? I think this symptom is a clue to the problem, which might be a wiring issue

 

Not really helping but something is loose
Plus or minus positive or negative

 

Rich the condenser was replaced. The car is 12 neg ground and has been running and together for 15 or so years. Squirrel, measured at the battery

 

How does the voltage on a 12 volt battery go to 9 volts when running? A dead short maybe? And then to 20 volts, bad regulator is all that I can think of.

 

asking again...exactly how is the voltage measured? what kind of gauge/meter, what is it connected to (battery posts? battery terminals? battery cables? )

it ain't easy to get the voltage at the battery itself to jump that far. Wonder what's causing it? Is there an ammeter connected between the battery, and the alternator? Intermittent bad ammeters cause mayhem in old Mopar charging systems, for example.

 

Measured with a meter at the battery posts. The car was upgraded to a stewart Warner "wings" voltage gauge when I did the 12 volt conversion years ago.

 

9volts? Try a buddy's battery. Pretty cheap thing to test. Maybe a cell is crumbling and creating a voltage suck. Typically, that kind of thing doesn't just cure itself and a bridged cell puts the car on the shoulder, but stranger things have happened...

Otherwise, I'd trace out that alt wire from end to end.

-rick

 

Yeah, that voltage reading is either a measuring error (digital meter confused by ignition interference for example) or an indication of a very big issue somewhere in the electrical system. Seeing 9V while using a couple of hundred amps to crank the engine may be acceptable, but absolutely not with it running, and 20V is WAY above suitable charging voltage.

 

I'd fully charge the battery, overnight. Then take it to NAPA auto parts. They will load test the battery.
Once you know you have a good battery, then proceed with testing the alternator, etc.
While doing these things keep an eye out for a short, bad or loose or rusty ground connection.
It is probably something simple, as the car had performed fine in this configuration for several years.

 

Something is radically wrong with those readings at the battery. You said you were measuring at the posts, but is it possible that you were actually on the battery cables? A real bad battery terminal connection could yeild reading like you described.

 

a hand held digital meter can give wacky readings if it is trying to 'auto range' I like to test voltage with an old Simpson analog meter...even a Radio Shack one.... what does your dash mounted voltmeter with a fish tail, read?
Do your tail/dash/headlights go out when it 'cuts out' or go bright dim with the engine running?

 

A loose condenser will do that.

I can never understand the bullshit nonsense of "my car wasn't running right so I changed the alternator" though. Unless it wasn't charging correctly why would you change it, if the voltage is all over he place most likely it has a bad internal regulator or you have a bad connection. You can eliminate the alternator from the equation simply by unplugging the plug if it has a plug and isn't a farm tractor single wire unit. No engine needs an alternator or generator operating to be able to run.

Still check connections including each one between the coil and the points. I'd check each and every connection between the battery and distributor but it looks like you did some elimination already.

 

Probably not an issue for old points ignitions, but electronic ignitions (and even more so electronic fuel injections etc.) can get "confused" by outside interference, possibly coming from the pulsating voltage an alternator with a bad diode produces. The alternator still charges, just with reduced capacity which may be fine if you don't consume a lot of current, and the pulsating voltage pulsates way too fast for regular volt meters to pick it up. In other words, unless you hook an oscilloscope to it or test the maximum output current from the alternator you probably won't know it's bad, and the pulsating voltage interference may get into the trigger signal to the ignition electronics (or any other electronics) and cause false triggering. There's a video of this issue on a modern car on youtube, there the speedometer became a tachometer, the alternator interference got into the speedometer sensor signal.

Still, you are quite right, disabling the alternator takes that out of the equation, so that's a better starting point than replacing it and just hoping for the best.

 

Thanks guys. We did test the battery and replaced it and charged it. The Alternator was replaced also and it does this without the Alternator being connected so there's that. Everything so far is pointing to a short inside the distributor. Might try a stock dizzy when I get my hands on one

 

Digital multimeter meter and solid core wires no likey each other.

 

I cannot think how a battery could get to 20 volts WITHOUT the alternator connected, the battery cannot make that much voltage by itself. Low voltage, fine, that high voltage? As winduptoy suggested, I would suspect the meter itself.

 

Okay, when the engine quits, what is the voltage on the coil primary distributor side (key on). If it is close to 0 volts, you have a short in the distributor or the wire going to the distributor. You need to bump the engine over a few times and see if it stays at 0 volts to make sure a short is the problem. If it stays close to 0 volts, you definitely have a short. If it reads 12 volts, you need to bump the starter a few times. If it stays at 12 volts, there is an open circuit either in the distributor or the wire leading to it.

 

?!?

 

Not aware of the internal configuration of your Mallory; but how about the wire between the distributor body and the points plate?

Once had the wire inside another distributor fail where the copper strands fatigued and broke inside the insulation.

 

Get yourself a good analog multi-meter for automotive work. Especially on old cars.

 

Had that happen also; it was a bitch to find.

 

I have solid core plug wires on the avatar car. I can’t use a digital meter anywhere on the car. Not even on the trunk mounted battery. It goes crazy.

 

I found this one in the barn yard. Stole it and cleaned it up. Fresh points condenser and cap. Gonna try it in the morning. Fingers

 

Radio frequency interference?

 

Car has no radio. So no.

 

Ignition problem ehh? Don’t get me started. Hehe

 

Or an "open".

Yeah, this is what came to mind. Not a short in the distributor, but an open.