Volt drop when in gear
Hi everyone,
    Been a while since i have been on here. i hope everyone is doing well.  So I got back to working on my 70 mach 1 and i am having an issue that is driving me nuts. I had a bad voltage regulator that was overcharging the battery, my Sun volt meter was up over 15-16v and bouncing all over the place. i verified that with my multi-meter, up over 15-16v and all over the place. OK so with that i am not 100% sure if this issue was happening before the VR failed and the battery was being over charged or not. But with the car running, my volt meter and multimeter on battery it is showing 14.3-14.5v charge at idle.  which i am ok with.  But when i put the car in drive or reverse or anything other than park or neutral the charging drops immediately and fluctuates from 12.4+/- to 13.1+/-.  

  I have replaced the VR with a electronic one, no fix, i had the alternator tested, it passed, i by-passed the VR also by jumping the A and F terminals, the charging went up to around 15v+/- which is ok.   i replaced the battery - no fix, replaced the starter solenoid - no fix.  Still when i put the car in reverse or drive the voltage drops.  put it back in park, volts go right back up to 14.4+/-. Also to test the alt i disconnected the neg battery cable, the motor continued to run BUT it picked up RPM as soon as i pulled the cable.  Is that normal?  I am suspecting some sort of wiring issue. i checked the ground to the Alternator, it is good. cleaned the neg battery cable to ground. Nothing is fixing this. So any help would be greatly appreciated.  Has anyone had this or a similar issue?

That's a tough one.  It almost sounds like the alternator is bad.  But you said it was tested.  These alternators were marginal in size at best.  I think most were only rated at 55 amps.  But at idle, it will not produce 55 amps.  I remember when I was younger it being common to see the headlights dim at idle then brighten back up when accelerating, and dim when the heater is turned on high with cars of this era.

Is the idle speed correct?

Have you checked for a drain on the battery simply to be certain there is none.  That is easy on these older cars.  Turn everything off.  Disconnect the positive or negative batter cable.  Connect a test light (not one with its own battery) to the disconnected battery cable, then touch the battery post the cable was removed from with the point on the test light.  If the test light illuminates, there is current flow through it and a drain on the battery.  If that happens and everything is turned off, disconnect the wiring to the alternator and see if the test light goes out.  If the light goes out, there are shorted diodes in the alternator.

The wiring in the car is old (unless you replaced it), maybe try running a temporary 10 ga. or heavier jumper from the battery terminal on the alternator directly to the positive battery post or the starter solenoid terminal the positive battery terminal connects to.  If the results are the same, that eliminates the original charging wire from the alternator to the starter solenoid being bad.  Hopefully that's it.

After all this, right now my suspicion is the alternator output is marginal and simply too low at idle speeds.  Maye normal for that alternator. Or maybe it has shorted diodes inside of it.

I installed a 75 amp original style alternator from Tuff Stuff and an electronic voltage regulator from NAPA on my 1969 Mach 1.  With those, I don't have any strange charging issues.  I also replaced the original charging wire from the alternator, even though the original still looked okay.

Just to be certain, make sure the wiring connector at the firewall near the master cylinder is clean and tight. I don't think that's the problem here. But I've seen that cause strange issues on other cars.
Another possibility is a worn V belt, worn pulley, a glazed belt or a combination of all three. Will let the alternator slip, and produce low voltage. Check the belt it should feel rubbery, and pliable. Not hard and slick or shiny. When properly adjusted, the belt should fit flush with the top edges of the pulley itself, if it's sucked down into the pulley. The pulley itself is probably worn as is the belt. Seeing as how you are reading around 12.4 volts and above I'd sure take a look as I've seen a bad belt and pulleys cause the symptoms you are describing. A good way to test for this. Turn on the heater on high, and the head lights. Then see if it drops, if it does you almost certainly have belt slippage. Raise the engine speed slowly and listen for a squeal. observe the voltage. if it struggles to get up over 12 volts. It's most likely the belt slipping. Hope it helps 

Thanks for the responses.  i will try the suggestions soon.  My idle speed in drive is 650+/-. is that too low?  In park/neutral is it around 800+ warm. Cold idle speed is 1100-1200 rpm.  I will check the belt too to make sure it is not bad.

My heater is not connected currently, so i can't turn it on.  the only thing i can do is turn on the lights, which all work and maybe drops it down a little from 14.5v, if at all, when i turn them on.  Hopefully it is something as simple at the idle speed being to low.  I will kick that up a bit and see if it helps.

Thanks again
800 RPM in park or neutral doesn't sound too low at all.  Is the motor stock?  I only ask because it seems 800 RPM down to 650 RPM in gear is quite a jump for a stock motor.  If the carb idle A/F mixture is adjusted too lean, the RPM in park or neutral will need to be higher than necessary to keep the engine idling when in gear.  If it is not stock, sometimes the cam is too aggressive for the torque convertor, thus higher idle speeds are needed to keep it idling when in gear.

If all is okay, at 650 PRM, the output of the stock alternator would be minimal.  To work around that, I suspect a better alternator would be needed.  Higher output than stock.

Many people install a modern Ford 3G small case alternator on these cars.  3G is simply third generation.  These are an internal regulator type alternator.  I believe the capacity of the lowest output small case 3G is 95 amps and they have good output a low RPM.  They look like this one.  https://www.summitracing.com/parts/wgl-7.../make/ford  But, you need to install a V-belt pulley. Some come with both a the serpentine belt and V-belt pulley so you can install what is needed.

There are a lot of original stock looking higher output alternators available that still use your existing wiring and external voltage regulator as well.
Yes it is a 100% stock 302 and C4 out of a 68 cougar.  I am not sure if i have the A/F mixture right but at idle i have 20" vacuum and a touch over 18" in gear. I also replaced the old points with a Pertronix I and a matching pertronix I coil about a year ago, and I was wondering if it was at all possible it somehow got damaged when the VR failed. So I bought a new set of points and condenser that i am going to put on this weekend (if it gets delivered today or tomorrow) to see if that helps anything.  My thinking is if the charging does not drop when car is put in gear with the points, then the Pertronix may have failed. If is still drops then the Pertonix is ok.  I am not sure if that has anything to do with it, but for $9 it is worth a shot.  Pray 
 I am also going to do continuity tests on the wires to see if there is any that are not good.  And lastly i am going to turn up the idle to around 700+ in gear to see if that helps.  Just need my daughter to sit in the car with the car in drive to help with that one.

I don't know how the Pertronix would affect the charging.  But, honestly, I am not a fan of those drop in points replacement systems.  They have a history of being unreliable and causing strange ignition system issues.  I think they are a "gimmicky" form of electronic ignition.  They don't address the other very common wear areas in an older Ford stock points distributors.  Also, they are many times installed using the original resistor wire Ford buried in the wiring harness that goes to the ignition coil from the ignition switch instead of a 12V switched power source.

If you want to switch to electronic ignition.  My thoughts are the least expensive and very reliable route would be to install a Ford Duraspark ignition system.  National Parts Depot sells the wiring harness, ignition module, and coil.  A distributor is still needed which can be obtained from any parts store for a mid to late 1970's Ford car or truck. For that matter, the wiring harness is the only item that is needed from National Parts Depot. All other parts can be obtained from parts stores.
ok so this weekend i had a chance to mess with the car. I noticed that the belt was a little loose (about 1/2 to 3/4" play) for the alternator. but it was all the way at the end of the adjustment bracket so the belt could not be tightened anymore.  It was a 41.5" belt, i went and got a 40.5" thinking i could tighten that one up more to see if that was the issue, which i was able to do and still have about an inch or so of room to tighten further if needed.  BUT -  No luck: The charging is still dropping to around 12.4v - 13.2v in drive.  So i turned up the idle, it is now around 900 in park and 700-750 in gear.  That helped, the charging now will not go below around 13.7v in gear.  But I am still not convinced that is the answer.  Anyway, next i am going to take the whole wiring apart and check for continuity from the battery to alternator, to starter solenoid, to the starter, all of it to see if there is anything messed up.

Can anyone tell me what your volts are in gear in comparison to in park please?  I want get a better idea of what is normal and what is not, as i have read many websites and some say, volts dropping is normal if the battery is charged and others say volts dropping is indeed an issue.  So i am not sure what is correct.  

Thanks again for the help

You are correct, turning up the idle speed is not the solution.  It sounds like what is there is working correct but the voltage output at 650 RPM is a little lower than normal.  With that said, I think you will need a better alternator if you want higher charging voltage at idle.

Who tested your alternator?  Are you certain it is okay?  How old is it and what amperage is it rated at?  In my area we have a shop that only rebuilds alternators, generators, and starters so I have a little more confidence in their testing as opposed to a parts store.

This won't really help you since my 69 Mach 1 is a 4sp car.  With the cam it has, it likes to idle at about 775-800 RPM.  At those idle speeds the charging voltage is 14.2V-14.3V with or without headlights and heater turned on.
It isn't good to run an alternator with no load, it can be damaged. If you are doing more driving than idling then it will probably be charging enough  Idea

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