So after having got my '67 D200 up to driving condition recently, I went ahead and moved on to the charging system.

Not long ago I replaced the old, tired alternator with a remanned unit that has worked well. The problem was that sometimes the truck would charge, and sometimes not. When it did, the ammeter in the cab would tend to jump around a bit, but not enough to be a major red flag or anything. I tried to figure out the charge / no charge condition for a while and I ultimately found that if I unhooked the ignition wire going to the voltage regulator from the ignition switch and hooked up a jumper from the battery to that terminal, the truck would charge just fine all the time.

Fast forward to this past December and the ammeter fluctuation seemed to be getting worse. Additionally, I found the truck wasn't charging more often than it was. Now, even if I hooked a jumper wire I wouldn't get anything out of the charging system. For the record, the regulator did appear to be a original unit and was in near pristine condition on the inside and outside. This now full time no charge condition prompted me to go ahead and spring for a new regulator, to which I installed shortly. After firing the engine up, I hooked an analog load tester up to the battery and just used it to monitor voltage instead of relying on the gauge in the cab. It was charging again, but had a rapid voltage fluctuation on the battery going back and forth from 12 to 14 volts as fast as the needle could move between them - the same as it was doing with the old regulator. I then unhooked the wire coming from the ignition switch in the cab and hooked up a wire directly to the battery. Just like that, it was now charging a smooth and steady 14.2 volts as long as I kept the wire attached.

That is where I am today. I don't want to hook the ignition switch back up to my new regulator again in fear of blowing that one too. I've tried doing some research on this, but haven't found any solutions to this kind of issue as everything I have seen just ended up being a bad voltage regulator. I do have a 67-68 service manual on these trucks and for "Excessive Ammeter Fluctuation" it says "High resistance in the field circuit to the alternator or an improperly set voltage regulator" which doesn't prove true in my case since the charging system works perfectly fine when I connect the regulator to the battery directly.

At this point, I'm pretty sure this just boils down to a bad ignition switch, though I can't say for certain.

What are your thoughts on this?

Thanks.

Your still running a mechanical regulator, some what the nature of the beast is it is an on/off type regulator.

There is an electronic version of that regulator that is smoother.



The above statement is irrelevant. It might work ok hooked to the battery but that is not how it is designed to work. It needs to be adjusted to work as designed, which is what the manual is telling you. I bet if you put a meter to the regulator power connection you'd see that the voltage is different than at the battery. Which is part of the issue, it should be within 0.1v and I bet it isn't.

Check the bulkhead & ignition switch connections

Well, I am not running a mechanical regulator anymore actually. The new one I bought is fully electronic and is not adjustable.

I know it's not designed to work hooked directly to the battery as I was only doing that to rule out the alternator and / or voltage regulator as the issue, which they aren't.

Since you mentioned it, I went out and tested the voltage at that wire. Battery voltage is 12.27 with ignition on (12.88v with ignition off), and the wire is at 12.20v, +/- 0.01v (ignition on).

Means nothing. Test it under running conditions when it is acting up. It is a very simple system.

Just forget it at this point. I was trying to have a discussion on this and gather opinions, but it seems it's quickly turned into an argument of who knows the most about how something works.

I would check your wiring . Back track from the ignition switch forward. You might find the problem. You could have a wire in the harness failing/worn that is giving you the fluctuation.
Another thing I would do is, run a wire from the battery lead of the alternator going directly to the positive terminal of the battery.

I will try those things. Thanks.

Just my .02 worth, make sure you have a good ground on the regulator, been there.

I'm assuming you fed the regulator ignition feed terminal directly from the battery for your test? Is this what you did and the problem was solved? If so, the circuit starting at the battery, going thru the fire wall, ignition switch, firewall again to the regulator has an issue. The ground side has to be good as well. This includes the regulator, alternator, and battery all seeing the same ground. But since the problem was solved without changing any grounds, they should be OK. The regulator uses these two circuits to determine field current. Field current adjusts alternator output. When you have what we refer to as voltage offset is where the issue can occur. All the points mentioned need to see very close to the same reading. All grounds the same. reading. All ignition circuits the same reading. Both within a couple of .10s under load. I solved a similar mystery while working at the FCA electrical lab a few years ago. Though the system was a little more complex. But the problem was ground offset. The PCM ground was good enough for the vehicle to run. But poor enough that the PCM thought the sense voltage being sent from the alternator was varying at the PCM itself. This caused the PCM to vary the field current output. Turned out heat shrink glue had wept between the ground terminal and the wire itself. This car had been at the dealer many times before we got it at CTC. It had a new PCM , alternator, wiring between the PCM and alternator. Nobody ever checked for offset.
Doug

Agreed....these connections as well as the ammeter circuit are typically the culprit. Thoroughly inspect these connections and the + and - wires on the back of the ammeter.

Anything that would affect the field current or the wiring from the alternator, through the ammeter, to the battery will give you those symptoms. I'd bet that it is on the output side of the alternator. If you have a poor connection in that part of the circuit the battery voltage will be unstable. When this happens the regulator starts chasing it and the entire system becomes unstable.

Just a comment, you can't intelligently work on something without know how it works and what it is supposed to do.

if the truck has the original wiring harness, there could be several, if not many places causing your problem.
check all the wires for brittleness, even if it means un-taping the harness to get to inspect the charging wires.
it would also be a good idea to pop the terminals from the plastic connectors to check the crimps and the wire condition at the crimps.
as has been already mentioned, grounds are very important to check, and in function of the circuit.
wiring problems can be a mystery at times, but mysteries can be solved by investigating everything carefully.

Good solid grounding is really needed to make any direct current circuit work well, not just the regulator

the terminal on the VR you are testing is sometimes called the "Voltage sense" terminal. Meaning that terminal looks at the available voltage when running ( ideally should be about 13.8-14.2) at about 1500- 2K RPM. up to 14.5 or so is acceptable.

The Voltage should be very close to what the battery AND output stud on the Alternator are. Wiggling things around while testing will sometimes reveal the issue. One can go so far as to hook up a V meter to the regulator placing it inside the truck so you can observe it while it is happening. NOTE: digital meters do not respond as fast as an older analog type and may "mask" the fluctuation.

The VR looks at this voltage and adjusts the field current to the alternator thereby increasing or decreasing the alternators output.

The VR, alternator battery etc all require good grounds and Voltage connections for the current to flow and function properly.
A flakey ground or connection will impede the current flow and cause the V to drop. the regulator SENSE's this and cranks up the Field current increasing the Alt's output.

I am hoping his simplified explanation will help you find the culprit. As most have said, the bulk head and ammeter connections are common problems but it could be grounds as well. Keep us posted

Update 7/23/2023: Turns out it was a simple connection issue at the fusebox, caused by rust. I must have dismissed this possibility before because I tested both sides of the fuse and it had power, but obviously it wasn't a consistent power source. After discovering this, I cleaned the fuse holder and all has been well and reliable again. I later cleaned the rest of the fuse holders since I started having intermittent brake lights too, and all the electrical works consistently now.

The truck charges good now, however is does not charge at idle which I figure must be related to that new voltage regulator since the alternator will charge at idle when the regulator is bypassed (only for testing - I wouldn't drive it like that).

I have a bad habit of overthinking simple problems, so that is all this was.

Thanks for the help and suggestions.

thanks for the update
And just an FYI: You can have voltage present but the corrosion will inhibit current flow. Both are required for the circuit to function properly. The flakey connections at the fuse box were likely drawing variable amounts of current causing the voltage to vary as well. This would result in the issue as the VR and Alternator were tying to respond to the changes. might be a good idea toc oat that FB contacts with a conductive coating to prevent further corrosion