The Stovebolt Site Forums The Shop Area Engine Shop No fire on '51 GMC after sitting for a year

Feeling pretty dumb on this one, but I'm kind of stumped so I thought I'd throw it out there. Got a '51 GMC that's been sitting for about a year. Ran when parked - sitting in a garage. Battery was dead. Went and put a new 6 volt in it this morning - positive ground, hooked up correctly. Engine spins, getting gas, no spark. I pulled a plug wire and held next to head, no spark. Pulled cap, points gapped correctly - opening and closing. Figured a bad coil. Get new coil - $26. Hooked up new coil pos terminal coming from starter, neg to side of distributor. Used VOM to check voltage coming into coil 6.4 V and out of coil 6.4v. Cranking still no spark. Not so much as a cough. Thought timing might have been bumped somehow. Loosened distributor and swung it cw/ccw while cranking with key on. Not so much as a pop. What could this be, truck was running fine? A bad condenser in distributor can't cause this, can it?

Started getting po'd so I had to walk away for a second.

"Hooked up correctly"? If you say so.
If you have voltage to the coil and all else is correct, Could be: condenser, bad points, bad rotor, bad cap or middle tower wire. Distributor not rotating.
You need to be sure you have voltage at coil during cranking.

It can, however I would clean the points and re-gap them.🛠

Hey bartamos, Yeah what I mean is that it's hooked up positive ground.

More testing or get new points and condenser. This is a bit I found under search: Posted by HRL.
Not sure about how any of this changes due to Positive ground, so use caution.

"A voltmeter will be a better diagnostic tool than an ohmmeter. With the points open and the ignition switch on, you should be able to measure full battery voltage at both coil primary terminals. With the points closed, You'll get battery voltage at the upstream terminal (from the switch) and almost zero voltage on the negative terminal. More than a fraction of a volt at the points when they're closed indicates dirty points- - - - -clean or change them. DO NOT use a file on points- - - -I prefer to clean them with a piece of business card stock, etc.

A bad condenser can cause a no spark condition. Also, you'll need to test the coil output by holding the coil secondary wire close to a ground and have someone crank the engine. You should get a spark at least 1/4" long. If you get spark at the coil secondary and not at the plugs, check to see that the distributor cap and the rotor are the right size- - - -a tall cap and a short rotor will not get a spark to the plugs even if the coil is good.
Jerry"

I will add: Check to be sure rotor is rotating. Be sure the new coil is correct, like internal/external resisted, 6V is internal I believe. No ballast resistor. Something is grounded/shorting out.
Just brainstorming.

6 volt systems don't need a ballast resistor, internal or external. The same procedures apply to positive ground- - - -you need to see battery voltage at the "upstream" coil post anytime the ignition switch is on. The downstream (distributor) terminal of the coil will have either full battery voltage or almost zero, depending on whether the points are open or closed. Dirty points are the most common cause of no spark, by far. Just because they're gapped right doesn't necessarily mean they can flow current. One very good way to clean points is with a pencil eraser, but you'll need to take them out of the distributor to do a good job of it. There's just enough abrasive in an eraser to do a very good job of burnishing points without doing any damage to the contact surface. One of the hard white erasers typists used to use was the best of all, but those things are very scarce these days!
Jerry

99.9% bet on points being the culprit. Pull coil wire out of distributor cap and hold 1/4" from ground, with switch ON, "rattle" the points open and closed or anywhere in between with a screwdriver. If coil wire sparks it's the points.

Thanks for all the feedback. Ended up having to move furniture all day, so I never got back to the GMC. I picked up new points and condenser from NAPA, though and will install in the morning and report.

Back when Sun Electric Co. made the automotive industry's best test equipment, they put their slogan on every machine- - - - -"We test, not guess!" If one of my mechanics changed a part, he had better be able to tell me WHY he changed it, or he would be looking for a job the next day.
Jerry

Sounds like you were hard on him. Lol Mike

I'm not sure this matters, but your original post said you wired the coil negative to the distributor. If positive ground, shouldn't the positive go to the distributor???

Hey, Sweet. I noted the way the old coil was hooked up before I installed new one. Old one wired pos to starter and neg to side of distributor. Truck ran fine this way, but I see your point. Seems like it would be the other way around. Kind or wondering why it was wired this way originally...

Been reading up a bit since I just posted. I think the PO had the coil wired backwards and I assumed it was correct since the truck ran. Don't think that's my problem, but I'm going to switch it to pos to the distributor when I get a chance to work on it today. Thanks for the input. Learn something new every day.

Reversed coil polarity requires about 20% more voltage to fire the spark plugs, since the spark is coming from the "cold" side, the ground electrode, and jumping to the center. Finding that out on a cold, wet morning when the thing won't start is quite a learning experience! I used to use that example for my students when teaching oscilloscope diagnosis- - - - -the difference in the firing line voltage on the scope is very obvious when the polarity is reversed intentionally.
Jerry

I will still run either way. It will not cause a no spark.

Finally got through moving furniture and hanging ceiling fans and got back to the GMC last night.


...and the winner is... condenser/points. Flipped the posts on the coil so positive to distributor, new condenser and points (old ones looked bad when I pulled them. Pulled and checked plugs and tweaked the gaps so everything was right. Popped right off. Got her running by doing shade-tree timing (turn distributor until she starts, set for best idle, then slightly retard). I'll take her out today and see how she goes down the road, Mighty glad to get her going - I've got to move her to another house and it'll be much easier under power.

Thanks for all the suggestions!

Still got timing to mess with, running but wants to conk out at low RPM under a load. At least I can get her moved now. All I've got to do is find a place to put her!

May be selling her - hate to - bought new by my ex's great grandfather in '51. Needs valve job and clutch is pretty weak. If I do decide to let her go I'll post it first here. Basically a rust bucket with good running gear - only had about 30,000 on i t when parked - farm truck.

Thanks again for the tips to get her going.

David, sorry to hear you are thinking of selling. If you do decide to go that route, be sure to post your for sale notice in the Swap Meet Forum along with a price (per Stovebolt rules) and do not post in the regular forums. Just a friendly reminder. Hope you get to keep her!

Setting timing. The true shade tree method is to keep increasing timing until labor knock (pre-ignition knock) sets in under light load in high gear while accelerating. Then back timing off the smallest amount possible to eliminate the ping. The advantage to this system is you'll have optimum timing for the gasoline used. No timing light required.

Thanks, mopar guy. That's what I'll do once I get a little time. My big thing was just to get her running and driving so she could be moved under her own power. Busy moving ex-wife right now.

Got you, Just horsing around - wasn't trying to put up an ad. If I do I'll put it in the correct forum. Hoping we won't have to sell.