Coil voltage fluctuates. Idle fluctuates correspondingly

[Bread]

The problem: The short of it is, I'm trying to figure out if the + lead from the ignition that feeds the coil should read a constand 12v while running, as well as when just switched on (not running). Mine reads 12v when switched on, and then fluctuates from 10v -12v while running. The idle corresponds by dipping up and down. Can't get it to smooth out.

Details: 1971 SII, 304. Holley Points distributer. I've replaced all plugs (Autolite 85, gapped to .032), 7mm wires, cap, and rotor. Cleaned and re-gapped points per service manual. Did the full Coil test as per the service manual, and some online instruction. Works perfectly, but replaced it anyways. I have not yet replaced the points set or condensor, but I purchased them today.

Background: The truck was running great until returning from a trip up to the pass for some skiing. Two days later, I started getting weak spark symptoms including a low idle (rough to accel as well), and black soot on the plugs. It smells like it's running very rich, but I never touched the carb. So now I'm looking at a weak spark condition. I pulled the coil wire and grounded it on the manifold. Produced a yellow spark, but not at all blue as I think it should be. This led me to replace the ignition components.

Current Status: After replacing all of the above items, I turned the idle screw out to get it up around 700rpm. Now it idles at this speed, but fluctuates. Put a tester on the ignition side of the coil. Reads 10v for a time, then picks up to 12v and the idle smooths right out. Cycle continues. My question: Is this reading normal? Or is this where I need to redirect my attention? I was going to run a jumper from the battery direct to the coil + to see if it would stabilize the idle, but I'm not sure if I should do that or not. I could put the new points/condensor in, but I don't want to throw a wrench into the current mix.

Only other thing I haven't checked is the ground wire from the dist. to the coil. It's looking quite frail and old. I'll go test it for continuity and resistance.

TIA

[dick ardrey]

Your GND could be made through the gears. Is your engine grounded to the firewall?
It is supposed to be! Also the large cable from manifold? to the - battery post. I do better when also placing a GND wire from the front fender supports to the battery - .[light flicker]
lazer

[Doc Stewart]

As Dick said and check your firewall connector for corrosion. Clean it with wire brush and put it back together using dielectric grease.

[Bread]

The only major GND I know of is from the Battery post to the d.s. cylinder head. Last night I pulled both ends, scrubbed them shiny, and reinstalled. Did the + side too.

I don't see any firewall ground locations. But there is a mess of wire crimps where the bulkhead connector used to be. Looks like grandpa cut the connector out and directly wired the two sides of the engine harness. The body is quite rusty, so if there is a firewall ground that I just can't see it may have worked loose upon my trip up to the mountain in sub-freezing temps.

Noticed some water in the fuel filter this morning (Fram G2). So I added a bottle of 'heet' (essentially alcohol) into the fuel tank and filled it up with some good Chevron gas. Drove it around for 15 min, but it still has a terribly rough idle. Tested the coil + again while running. Getting a fairly steady 12v, so I don't know what my problem was last night. Perhaps the voltage is supposed to slightly correlate with engine idle speed. and the fluctuating voltage on the + side of the coil is a result, rather than a cause of the surging idle.

Tested the - side of the coil with the engine running, getting readings of 9.5v. I'm under the impression that the - side of the coil gets an alternating on/off current from the distributer, so I suppose 9.5v is normal while the engine is running.

Using a very nice Fluke tester BTW.

[Bread]

Distributer trigger wire only has about .3 ohms resistance, no matter how I wiggle it. Seems to be working fine.

Still getting a good 12v at the + side of the coil with the engine running. Idle still surges. Timing seems to be right on from the book specifications. There aren't really any vaccum lines to speak of. Since I'm running a hydroboost system, I capped off that line. So I suppose I'll play with the carb some, and then may just need to look at the possibility of a worn cam or distributer gear.

[wheelerdealer]

12 volts at the coil seems a little high. Should be about 8-8.5 volts unless you have changed to an internal resistor coil and then 12 volts is good. As for the fluctuating voltage I would first look for a high resistance connection both in the above chassis wiring and chassis side wiring. As others said don't ovelook chassis side wiring otherwise called grounds.

[Bread]

Originally Posted by wheelerdealer

12 volts at the coil seems a little high. Should be about 8-8.5 volts unless you have changed to an internal resistor coil and then 12 volts is good. As for the fluctuating voltage I would first look for a high resistance connection both in the above chassis wiring and chassis side wiring. As others said don't ovelook chassis side wiring otherwise called grounds.

While running, on the (+) side it reads 12v, and on the (-) side it reads 9.5v. This is unusual? Would it be a bad idea to run a jumper from the battery directly to the (+) side of the coil to see if a good clean power source helps things?

[dick ardrey]

I am not completely up to speed on the resistor in the 12v lead but I know it is there to limit the current through the coil.
The POINTS [switch] the 12v through the coil to GND. The switch opens and closes at RPM to make A/C. The coil is a type of transformer and it will only work with A/C. The pulsating A/C voltage is transformed into a 50,000 volts A/C pulse that goes to GND through the spark plugs. Since the 12v's DC goes in one direction [to GND] the on/off pulse is still one way at 50,000v as the type of transformer makes it so.
When reading the point side of the coil , with 12v supplied, you will read the average of 12v on/off as well as the losses through the winding. 9v seems right for 12v in. When you drive the 12v will be higher ie: 14v? and the 50,000 will also be higher. A bad winding will show up as you describe BUT you say it has been replaced with NEW??
The points could also be worn so as to not tightly close the same way every time hence the varying RPM/POWER.
With known GOOD grounds and +12v, a new coil, and points, and cap, and wires you should not see what you say you see.
FYI: worn wires [dry rot] can short to the block and not give the fire to the plugs. This would also show up as a miss etc... This will show up worse if wet. A fix , is to spray the wires with a clear lacquer [paint] to seal them.
If you have bad wires the plug[s] will not fire well and be wet as if fuel is set rich.

lazer

[Bread]

Originally Posted by dick ardrey

I am not completely up to speed on the resistor in the 12v lead but I know it is there to limit the current through the coil.
The POINTS [switch] the 12v through the coil to GND. The switch opens and closes at RPM to make A/C. The coil is a type of transformer and it will only work with A/C. The pulsating A/C voltage is transformed into a 50,000 volts A/C pulse that goes to GND through the spark plugs. Since the 12v's DC goes in one direction [to GND] the on/off pulse is still one way at 50,000v as the type of transformer makes it so.
When reading the point side of the coil , with 12v supplied, you will read the average of 12v on/off as well as the losses through the winding. 9v seems right for 12v in. When you drive the 12v will be higher ie: 14v? and the 50,000 will also be higher. A bad winding will show up as you describe BUT you say it has been replaced with NEW??
The points could also be worn so as to not tightly close the same way every time hence the varying RPM/POWER.
With known GOOD grounds and +12v, a new coil, and points, and cap, and wires you should not see what you say you see.
FYI: worn wires [dry rot] can short to the block and not give the fire to the plugs. This would also show up as a miss etc... This will show up worse if wet. A fix , is to spray the wires with a clear lacquer [paint] to seal them.
If you have bad wires the plug[s] will not fire well and be wet as if fuel is set rich.

lazer

That's a lot of great information. Thank you. It does seem as if I have a ground problem somewhere. I did replace the plugs, wires, cap, and rotor initially. I then replaced the coil, but with no results so I put the original coil back in. The only components I haven't replaced are the points and condensor. I adjusted the gap, but don't have the correct tools to check the spring tension per the manuals specifications. I was quite frustrated after my last attempts and have since parked the truck. I'll replace the points and condensor tomorrow morning, and put the new coil back in for good measure. As a last resort I'll try running a jumper wire from the battery directly to the (+) side of the coil and see if that smooths it out. If it does, then I know where to look for an electrical problem. If not, then I'm on to carb or mechanical problems. It's an 37 year old engine, so it may just be worn out.

[TBAKPhi22]

I would really doubt its something internal with the engine... I had the same kind of thing happen with my buddies truck. Turned out the bushings for the mechanical advance in the distributor were gone, so the idle would fluctuate like you described (it was a Holley dist too) I would look into that first, before you start doing anything major!