Turn on AC... spark is screwy from then on

sreams

sreams

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My CS has been running great for several months now, ever since I put in the M30B35 and tuned the Weber DGEVs with it. Idle is butter smooth. I installed the Pertronix Ignitor kit (which I've used on several cars in the past), and that, along with setting the proper spark plug gap, has resulted in a smooth and even idle that is predictable. It has been 100% consistent until today.

I went to have my air conditioning system charged for the first time in years. I have a York compressor and an early aftermarket Frigette system (aftermarket condensor and evaporator). The system took the R12 fine and appears to have no leaks... although the York compressor is now sounding a bit noisy (didn't hear this when running it before the recharge)... so my plan in the near future is to recover the R12 and replace the compressor.

That said, the strangest thing has been happening today following the AC recharge. I was on the way home using the newly charged AC, and suddenly the idle became rough and dropped to about 300-400rpm. I turned off the AC, and the idle was still rough and far too low. I got home, and could clearly hear from the exhaust note that there was a seemingly random miss. I spent the next several hours trying other coils, trying other spark plug wires, testing voltages, etc... and nothing seemed to have any effect. I then started pulling plug wires too see if it was limited to particular cylinders. At this point, for no reason that I can comprehend, everything started working perfectly again. I then spent the next hour just getting my idle mixture and idle speed back to where it was (I had messed with it a bit before I identified the problem as a miss). Again, the car ran perfectly. A night and day improvement. I turned the AC on for a few seconds and then off... and immediately the problems started again. The bad miss had returned which resulted in a horribly rough idle and popping from the exhaust.

Frankly, unless there is some -very- funky wiring, I don't see how the AC system could have any effect on the ignition system. Anybody want to give it a best guess?

-Scott
 
Did these cars have an idle speed compensator function to cope with the extra load the AC puts on the engine ?? if so, could it be this? allthough not sure how this would work with a carb car??
 
The car has no idle speed compensator.

Also... The problem remains even after turning off the AC completely. So far it seems like waiting a few hours and then not using the AC again is the only solution.
 
I am really guessing but carb cars are open loop and have no way of factoring in the load.
If that is the case one would have to set the idle with the AC on/AC clutch engaged and live with a higher idle when the AC is off. Maybe the AC load is negligible on a 3 liter car, and somehow the recharge and compressor are producing a much higher load than they should.

On the electrical side the only wild possibility would be that the battery is weak and the lower idle under AC load results in lower voltage as the alternator is not up to normal RPMs.

I would be surprised if I am right, but after the voltage divider guess I need to start a guessing streak.
 
and/or your alternator is weak. What amperage is it? Yorks also have a big drag on idle rpm's due to their inefficiency.

Arde said:
I am really guessing but carb cars are open loop and have no way of factoring in the load.
If that is the case one would have to set the idle with the AC on/AC clutch engaged and live with a higher idle when the AC is off. Maybe the AC load is negligible on a 3 liter car, and somehow the recharge and compressor are producing a much higher load than they should.

On the electrical side the only wild possibility would be that the battery is weak and the lower idle under AC load results in lower voltage as the alternator is not up to normal RPMs.

I would be surprised if I am right, but after the voltage divider guess I need to start a guessing streak.
Click to expand...
 
If this was a case where the spark was negatively affected only with the compressor on, that would at least make some sense, especially if the alternator was weak.

The thing that is really confusing is that the car runs horribly well after the AC is turned completely off. It seems to recover after a few hours, and then will run perfectly until I turn the AC on again. Then I'm screwed for another several hours... With the AC completely turned off.

How could that possibly happen?
 
It may be the alternator after all. While this problem is going on and the engine is running, my voltmeter shows just over 12V at the battery. I'm used to seeing more than that. Perhaps the alternator is weak, and turning on the AC drops the RPM and loads it up enough that it fails for a while. Hmmmm.
 
your compressor may also be drawing more amps than normal, as could your aux fan due to age. check them all out.
 
mmmm

Check your belt.(s) for proper tension.

12v running is too low. But what's weird is that your reserve on the battery should keep you out of trouble for a while. So add that to the list.

Set your idle for 950-1050 if your tune is spot on.I.E. -you're not too rich.

Noisey compressor- first check the mounting. ( common problem) It's two parts with metalastic bushings that disentegrate. Check the tech section- Devider's fix. Grab the compressor and shake it or use a large screwdriver to pry it while looking at the mount for movement. It doesn't do it sitting there, but when it's on it hops around like a frog.

The spec section has the loads ( amp draws) for the a/c components.

Do the heater hose by pass for improved a/c.
 
with your B+ voltage so low I would run the AC without the Aux fan to see if that is the draw that is killing your drivability.
Voltage at battery w/o AC?
Voltage at battery with AC?
Voltage at all 3 terminals at the coil with key on ?
 
Thanks for all the good ideas. I will test the draw of the aux fan and the compressor. I'm leaning toward the compressor. I've turned the system on multiple times before the recharge and never encountered this before. I'm thinking that now that the compressor has R12 to pressurize, the extra work is exposing a fault.
 
Well, I haven't tested the draw of the fan and compressor yet, because now I can't get back to getting a consistent spark at all. There are random misses at all rpms. I tried another coil, I tried switching back to points. No luck.

I did discover that the B+ wire on my alternator was loose. Tightening it got me back to a solid 14V while running... but this fix has had no impact on my spark problem.

It's frustrating, because the problem started this morning and wouldn't go away for about 3 hours. Then, all of a sudden (and for no apparent reason), the problem completely disappeared for about an hour... then came back again suddenly when I tried to turn the AC on and then off just once more, and it hasn't gone away since. I've even tried disconnecting the positive leads to the compressor clutch and aux fan, thinking there might be an intermittent short there. No luck.
 
Charging system maybe putting out A.C. (should be D.C.) ?????? A.C. current will give you all types of strange problems
 
Measure

your voltage/ resistance at all the low voltage ignition side. Switch, still running a ceramic resistor? (Look for a crack on the back, that would definately do what you describe or lots of heat), voltage to coil, voltage to points/ pertronix. Check the black /red wire from the starter solenoid. Make sure you're not using both a resistance coil, resister, and resistance wire and you're voltage to the points is way below spec.

What year car? Resistance wire? IIRC that started in 74. ( Maybe late 73?)
 
Given my voltage/RPM suggestion did not drive me out of town tar'ed and feathered, I explore the mistery of the poor running afther the offending AC is off. The electric system itself has no memory with the exception of the battery. One way the overall system would have such memory is: load causes low RPMs, which causes less charge to battery, which, assuming weak battery, results in low voltage, which results in poor spark, which results in low RPMs and that is the feedback loop until enough charge builds eventually in the battery to raise the voltage.

The other possibility for memory is that AC load causes poor spark (via low RPMs) so the fuel is not entirely combusted and somehow gets the plugs wet. Wet plugs cause poor running even after AC load is off, which mutatis mutandis sustains poor RPMs until the plugs get drier or battery to coil voltage increases.

BTW, Does the AC blow cold? Maybe they charged it with penaut butter instead of R12?
 
My multimeter shows 3.7 ohms resistance between the positive and negative sides of the coil. While running, voltage at the positive is 14V, and 7-8V at the negative. I have a coil with an internal resistor. I do not have an external resistor and do not have a resistor wire (car is a 1972). Dwell is a solid 37 degrees with the Pertronix.

I don't have the wire from the starter solenoid to the coil connected. My understanding is that that wire is only meaningful when using an external resistor... in that it bypasses the resistor during cranking. I obviously cannot bypass the resistor that is built into the coil.

There is no resistance between the base of the Pertronix module and ground.

I've tried completely disconnecting the alternator in case it was a contributing factor. That changed nothing. The battery is strong and turns the starter over vigorously.

I also tried connecting a wire directly from the positive terminal of the battery to the positive terminal of the coil. No change.

61porsche said:
your voltage/ resistance at all the low voltage ignition side. Switch, still running a ceramic resistor? (Look for a crack on the back, that would definately do what you describe or lots of heat), voltage to coil, voltage to points/ pertronix. Check the black /red wire from the starter solenoid. Make sure you're not using both a resistance coil, resister, and resistance wire and you're voltage to the points is way below spec.

What year car? Resistance wire? IIRC that started in 74. ( Maybe late 73?)
Click to expand...
 
The resistance

seems OK. 14v/3.5=4 amps, which is waht a 4-6 should be. If you were running points, I would suggest the voltage be less going to the points. Measure resistance from each coil terminal to center high tension.

Try resistance through the rotor, cap, high tension wire (s) and wire terminals. Could be something was weak and the pertronix brought it to life.
 
A wild idea here -- Could the magnetic field generated when the ac clutch is engaged influence the electrical system? The Pertronix module perhaps?
 
Rotor
0 ohms

Cap
3 ohms

High Tension Wire from coil to cap
8560 ohms

Spark Plug wires:

One
5790 ohms

Two
5760 ohms

Three
5810 ohms

Four
5920 ohms

Five
5860 ohms

Six
5650 ohms

Wire terminals on rotor
0 ohms

61porsche said:
seems OK. 14v/3.5=4 amps, which is waht a 4-6 should be. If you were running points, I would suggest the voltage be less going to the points. Measure resistance from each coil terminal to center high tension.

Try resistance through the rotor, cap, high tension wire (s) and wire terminals. Could be something was weak and the pertronix brought it to life.
Click to expand...
 
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