I have a D 7 F with a prestolite alternator. The ammeter works fine, and the machine has no accessories other than the starter. It's never shown to be charging much, if any. Once in a while I have had to charge the batteries. I put a new regulator on it about 25 hours ago, and the alternator worked fine until today. It indicated a charge rate of 25-30 amps after startup and tapered off some after running a while. Now it's gone back to the way it used to be, with just a little twitch from the ammeter. Is there a handy way to test the regulator? It has 2 wires and the case is grounded. The system is 24 volt. Can I connect an 1157 12 V tail light bulb or some other resistor in place of the regulator to see if the alternator is working?
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D-7 Alternator
- Thread starter TXGOAT
- Start date
The problem with an ammeter is that the battery will only take a big charge if it's a little low to start with. If the engine fires right up the low charge amps doesn't mean much. You want to use a voltmeter and see if you get about 26-28 volts running.
I'm pretty sure it quit working again. I can turn on the glow plugs and crank it with the fuel shut off to load the batteries, and then start it, and it still acts like it did before I replaced the regulator. When it's working, it's very obvious.
Yeah, I guess it is obvious, but I just like to know what's going on. You can give the alternator field the full 24 volts and it won't hurt anything for the 30 seconds it takes you to confirm that the voltage is going up, or the amperage is there. But why not use a voltmeter, with a little reading you can figure out specifically what is wrong with the alternator.
I wonder if there are any naysayers about giving the alternator field full voltage? There's a hole on the back of Delco alternators that you ground a terminal to the case to "bypass" the voltage regulator, but then there might be a resistor built into that circuit? If you bypass the regulator you have a positive feedback loop, the only thing keeping it from releasing the magic smoke is the battery's ability to absorb the full output of the alternator (and hopefully you do this at low idle
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)Shimmy1
Senior Member
Why not just check the voltage?? Your batteries are obviously good if you can crank it enough to check them. Just put a voltmeter on the positive terminal of alternator. If you have at least 26 volts it's charging. If you have 27 or better it's fine. One more thing, not sure why but some alternators I have found the engine has to get to high idle before they start charging.
I hooked a 12 V tail light bulb between the F terminal and the other (small) terminal that the 2 wires from the regulator connect to. The bulb lights up and the alternator appears to charge normally. At low engine speeds, the bulb dims and at higher speeds it gets bright.This suggests to me that the alternator is OK and the wiring from it to the battery is OK. The regulator is a little potted solid state thing. Does anyone know of a way to check it with a volt/ohmeter? I may have got hold of a defect regulator. The alternator ground seems to be secure and there isn't much wiring to go wrong. The regulator case is bolted to a clean bare surface on the alternator frame. The batteries are a matched set and seem to be in good shape. The regulator has a voltage adjustment screw on it, but since it worked fine out of the box I don't think it needs adjusting. The ammeter is 30-0-30 and the alternator will push it to about 28 A at high idle. With the engine off and the glow plugs on, the ammeter shows around - 30 A.