Been having problems with my 77 D100. Battery keeps going dead. Charged the battery and had it tested and was told the battery is fine.Thought maybe the alternator had a bad diode. A replacement alternator was only $25 so I replaced it. Same problem. Read Ehrenberg's bulkhead connector fix and ran a wire with fusible link from the alternator to the battery stud on the starter relay. Same problem. Bypassed the ammeter, same problem. I'm thinking voltage regulator. When I ran the wire from the alternator to the starter relay, I disconnected the wire that runs from the alternator to the bulkhead connector. Is that going to be a problem? Everything seems to work fine other than the battery going dead after a day or so of use. I'm at a loss.

what does the voltage read at the sensing line at the vr and field lineS at the alt with the key on?

I would undo the bypass & rehook the alt black output line to the bulkhead. What Andrew said, with the key on, the blue wire at the alt field terminal and the blue wire at the reg should both be hot. the green wire from the other alt field terminal to the other (green wire) terminal at the reg should have continuity to each other in that green line. reg must have a good ground & I missed if you had replaced it or not. other than that all that is left is the alt (if the above check out we can test the alt). when we get it charging put the battery on a charger to get it back up as opposed to letting the alt charge it in the vehicle. holler back when you can. short version: bad alt (possibly brush related)/bad reg/reg ain't grounded/blue wire to reg and or alt is open/open in green wire from alt to reg.

Tomorrow I will do the recommended checks. I just replaced the volt reg this afternoon. The alt was replaced a few days ago and while I've seen many parts bad out of the box, I don't think that is the case. This is a pita as the truck is my daily driver. I should probably install a volt meter while I'm at it.

Alright lets see if we can speed this up, see if the blue wires are hot & if so, see if the green wire has continuity

If I am reading this correctly, you bypassed the voltage regulator & have the alternator going directly to the battery.

That's not good - it will actually cause you bigger problems - so put it back.

Go get a cheap voltmeter from Harbor Freight, Sears, or whatever store is in your area.

You should be getting about 12.7VDC at the battery when the van is off & about 13.5-14VDC when the van is running.

You need to check the voltage regulator on the firewall & the plug.

I bypassed the amp gauge, not the voltage regulator.With my battery charged, I'm getting 13.6 volts with the engine off. I did Richard Ehrenberg's recommendation of connecting the alternator directly to the battery stud on the starter relay.I have disconnected the black wire from the alternator to the bulkhead connector. Should I hook it back up in addition to the wire going to the starter relay? Every thing was working fine until a week ago when it left me sitting. When I got the truck, someone had put a 100 amp alternator on it. When it started giving me trouble, I couldn't afford a new 100 amp unit. I had all the brackets for the 60 amp unit, so that's what I put on it. I am thinking that the 100 amp unit may not have been bad since I'm having the same problem after replacing it. I have an after market amp gauge from Harbor freight. Not sure whether I should use it or just get a volt gauge. I just replaced the voltage regulator this afternoon and don't know if it's charging now or not. Guess a new gauge will tell me.

so you need a multi meter to run some tests. hopefully you have one, if not they should be under 10 bucks at most places.

So lets run through how this works to make sure we all understand what happened.

you had a 100 amp alt. Based on the year of the truck, it should be a dual field alt.
you replaced it with a 60 amp you had laying around.
Again, please let us know if it was a dual field alt as well? Just making sure.

This suddenly stopped charging as far as you know.

There are 3 wires to a dual field alt.
2 field wires and then the big post out of the back of the alt.

1 field wire goes back to the VR. This should be some value less than 12 volts with the key on.
1 field wire is battery voltage with key on.

the big alt wire you have said you wired directly to the starter relay, on the big stud there, which also has a wire directly back to the battery.

This should also read battery voltage, with or without the key on.

The original wire that used to go here, you have removed.
you still have power to the inside of the cab which means you still have power going in.
This is really independent of the charging system, other than the key on parts.

I would hook the black wire from alt back to the bulkhead & disconnect the bypass (from alt to battery or relay). this will let the ammeter read honest since you dont have a meter so we can easily tell when it we get it charging then you can reinstall the bypass if desired. do you have a 1157 dual filament taillight bulb & socket & pigtails handy in your parts pile that you could use to check if the blue wire ign1 circuit is hot at the alt and reg with key on? then when solved you can reinstall the 100 amp alt if desired (I hear it is a pretty trouble free unit). As said a ten dollar HF multimeter will suffice & if you buy something else, their coupons might let you have it for free

I have a multi-meter, what I don't have is the factory ammeter. It went gunny bag last year and I bypassed it so the truck would start. Both ammeter wires are now connected to one post. The alternator is new. I will be able to test for power at the field connection, voltage regulator and continuity at the other voltage regulator wire tomorrow and I will post what I find. If the voltage regulator took a dump, would that cause a no charge situation? I replaced it last night, but have not had time to test anything yet.

yes it would.
but it could also be no voltage at the sensing line or field line.
or bad ground at the vr as well.

ammeter bypassed is unrelated. we'll assume the new alt/new reg are functioning and that bolting on the new reg took care of it possibly being ungrounded. all that is left (if above are OK) is are they being fed (blue wire circuit) and green wire continuity between the two. with all that confirmed it MUST charge.

Been raining pretty steady for the past 3 days, so I haven't had the chance to test anything yet. When I installed the v reg, I ran a ground strap from one of the mounting bolts to the engine. Didn't want to take any chances on it not grounding. Looked at a wiring diagram and noticed that I have the field wires backwards from the schematic. Not sure if that will create a problem. I will swap them when I can get outside.

field wires switched on alt field terminals-no problem. blue/green switched on reg-big problem (if it full fields tho likely it would just be a no charge which is far better than full fielding), tho not likely on them cuz the reg triangle connector/pigtails are blue/green. holler back. EDIT blue goes to either alt field terminal and to the "top" reg triangle terminal with it facing up like a pyramid. green goes to the "side" reg terminal and the other alt field terminal.

This is where I'm at today, disconnected the jumper wire from the alternator output to the battery stud on the starter relay that I put on. Reconnected the original black wire to the output. With the key on I have 12.6 volts at the alternator and at the v reg, also have continuity from the other field wire connector to the v reg. When the truck is running, with the headlights and blower on, I have 12.5 volts at the battery. When I turn the lights and blower off, I get 12.9 at the battery. Any thoughts?

it still ain't charging unfortunately. the headlites/blower are draining the batt then when you shut em off it stops the drain & lets the batt voltage come back up a bit just because the drain from the batt has been stopped. EDIT normal charging voltage is gonna be at least in the 13's

That was my thought. Well, not sure what to do next. I suppose it's possible that the v reg I used could be bad as well. I pulled it from a parts car a couple of years ago because it looked new. I've never had an electronic one go bad on me. Funny thing is that my ecu went out at the same time.

Key on not running, do you have battery voltage at the blue wire on the alternator? If not you have a broken wire or poor connection in that circuit.

Engine running, use a jumper wire to ground the other terminal at the alternator (where the green wire would go). Measure voltage at the battery, should be quite high, 16 or better. Be quick about this as the overcharge can damage components.

This is called "full fielding" the alternator and tests to see if it capable of charging.

If you don't have overcharge at the battery, check the voltage at the alternator output stud "fat wire". If you have high volts there then the problem is bad wiring between the alt output and battery positive, including the bulkhead connection and ammeter.

Also won't hurt to check voltage between the alternator case and battery negative, this will show a ground problem.

Ran into this on my 89 d350. Had 21 volts at the alternator! 12 at the battery. Found a corroded connection in the charging wire.

What Ruderunner said: full field the alt for 5 seconds max, just long enough to take a voltage reading from alt batt stud to alt case. run it at a fast idle & have all lights/accessories off. Tho on the test I would pull both field leads & jump from batt positive post to one male alt field stud and as said ground the other male alt field stud.

Something I've been wondering, when doing Ehrenberg's recommendation of hooking the alt output directly to the bat stud of the starter relay, will that make my alt and v reg constantly live? The alt will constantly have voltage from the battery won't it? I know the v reg is switched with the key in the factory set up. I'm just curious. I'm thinking that my charging issues are the result of a wiring harness problem.