Truck Electrical Question

[DRussell]

1971 C40 truck. With the key on how much spark should there be when you crack open the points with a screwdriver? This truck has a big spark when they are opened. Does that indicate a bad condensor? I'm getting no spark at the plugs.

 

[Duner Wi]

You have blue spark? Try a different rotor.

 

[Jim.ME]

1971 C40 truck. With the key on how much spark should there be when you crack open the points with a screwdriver? This truck has a big spark when they are opened. Does that indicate a bad condensor? I'm getting no spark at the plugs.

Hold the distributor end of the coil wire near the block. Do you get a spark from the distributor wire when you open/close the points? If you do the issue would seem to be with the distributor cap and or rotor.

 

[Fritz Maurer]

1971 C40 truck. With the key on how much spark should there be when you crack open the points with a screwdriver? This truck has a big spark when they are opened. Does that indicate a bad condensor? I'm getting no spark at the plugs.

You should have NO SPARK at the points. You are correct. The condenser is open or not grounded.

 

[redtom]

Many times on my old tractors etc. I find after they sit a bit the face of the point must corrode over just a teensy bit. I can open the points with a screwdriver and get a great hot spark because my screwdriver is making the contact. But the points on their own won't connect and make a spark. I sand with flint paper and file, drag paper through, spray with electronic cleaner, etc. Oddly, it usually takes several cleanings to get good contact.

 

[DRussell]

Hold the distributor end of the coil wire near the block. Do you get a spark from the distributor wire when you open/close the points? If you do the issue would seem to be with the distributor cap and or rotor.

I'll try that.

 

[DRussell]

You should have NO SPARK at the points. You are correct. The condenser is open or not grounded.

I was thinking the same, that a big spark at the points indicates a bad condensor. This truck has an internal condensor and an external condensor. On the external condensor the lead is connected to the positive side of the coil.

I tried to check both condensors with my multimeter. On the Ohms setting at 2000K, the strongest setting on the meter, the meter showed gradually climbing readings, starting at about 4 and climbing to 17 or more. I believe this indicates it is good.

The internal condensor tested the opposite, the number would start about 16 and would gradually drop. I believe this indicates a bad condensor, but not positive.

I took the coil off and tested it by running it on a tractor. The tractor ran fine with the truck's coil so I'm thinking the coil is fine.

 

[Fritz Maurer]

Or coil is shorted and can't make a spark

 

[JMOR]

Swap the places of the two condensers and if truck starts/runs, then you will know that the one inside distributor was bad.

 

[NDIHC]

Its normal to have a spark when you open and close points with a screwdriver. The points are a path to ground for the coil. Opening and closing the points is just putting voltage to ground. That is what collapses the field in the coil, causing the high voltage spark from the coil.

 

[wore out]

I was thinking the same, that a big spark at the points indicates a bad condensor. This truck has an internal condensor and an external condensor. On the external condensor the lead is connected to the positive side of the coil.

I tried to check both condensors with my multimeter. On the Ohms setting at 2000K, the strongest setting on the meter, the meter showed gradually climbing readings, starting at about 4 and climbing to 17 or more. I believe this indicates it is good.

The internal condensor tested the opposite, the number would start about 16 and would gradually drop. I believe this indicates a bad condensor, but not positive.

I took the coil off and tested it by running it on a tractor. The tractor ran fine with the truck's coil so I'm thinking the coil is fine.

The external condenser on the powered primary coil terminal is for radio noise suppression and it's condition has NO effect on spark unless it shorts so badly it burns out the ignition switch or the primary ignition wiring.

Also, the suppression condenser MAY be of a significantly different value than the actual ignition condenser, so NOT a good idea to swap them except for a short duration test.

 

[olddavid]

The internal condensor tested the opposite, the number would start about 16 and would gradually drop. I believe this indicates a bad condensor, but not positive.

The condenser may have had a charge on it already and you were discharging it. The polarity of the meter leads can make a difference, here. It sounds to me like the capacitors may be ok.

 

[DRussell]

Or coil is shorted and can't make a spark

Took the coil off the truck and put it on a tractor. It ran the tractor just fine.

 

[JMOR]

Took the coil off the truck and put it on a tractor. It ran the tractor just fine.

Perhaps repeat that swap tractor /truck condensers?

 

[Duner Wi]

Please let us know the outcome when you get your truck running. I am curious.

 

[used red MN]

Hold the distributor end of the coil wire near the block. Do you get a spark from the distributor wire when you open/close the points? If you do the issue would seem to be with the distributor cap and or rotor.

What was the outcome on this test? A test light connected between the distributor wire terminal on the coil and ground should flash on and off while cranking. Should be off when points are closed and on when open. This would also answer another question I would have, is the distributor turning? This is likely a Chevy V8 which rarely have problems with cam and distributor drives. However. this was in the era that GM used plastic timing gear teeth. I think you had to get into the heavy truck for them to be equipped with the double roller chain setup.

 

[NDIHC]

If you open and close the points with a screwdriver, without touching any other metal, you should get a spark from the coil every time the points are opened. If you don't, try touching ground and the points at the same time with the screwdriver. Then remove the screwdriver. You should get a spark from the coil. If you do, that likely means the points aren't making contact when they close. Assuming wiring and power are good.

 

[504]

Drop in a HEI distributor and never worry about points again.

 

[olddavid]

Drop in a HEI distributor and never worry about points again.

Nothing wrong with points. Simpler.

 

[NDIHC]

Drop in a HEI distributor and never worry about points again.

That would be my vote. 1 wire for power and 1 wire for a tach if you want one. Simple to work on. If there was room at the firewall I'd have one in my 56 chevy, but no such luck.