This one has me stumped. I have a 1937 Chrysler. It has been converted to 12v/Neg ground. It is currently running a 1965 GM alternator and has been for some time without issue. Yesterday, I took it out for a drive. Gone for several hours. Parked for a period of time and went to start it. Dead. Got a jump, started and drove home. Turned it off. Attempted to restart. Dead. Both time, it would turn a time or two (so not a starter issue), slowly....just like you would expect with a dying battery. So, I put a charger on the battery and let it fill. Got it back up and when to start it. Starts right up, no problem, as expected and as it has for the past 2 months (length of time I have had the car). So, I am thinking the alternator is not charging the battery. Check all the connections and everything is tip top. So, I start testing with a Multimeter... I put the meter on the battery posts and it comes in a 12.9v (remember, I charged it). I put the red for the meter on the red (to battery) post on the alternator and the black on the alternator housing. Same reading. Everything is hooked up and as per my understanding, this is in essence reading the voltage of the battery. Again, the reading was the same at the battery and when read though the alternator. So, I fire up the car and rev it up to around 1500rpm. When running, I place the meter on the battery as before, and then on the alternator as before and I get the same reading for both, 12.5v, less then when the car is not running. From my limited understanding, that tells me that the alternator is not charging/doing anything and is bad. Great! As I suspected.... I pull it off, take it to the auto parts store. Ask them to test it. They do....and say it is fine. I am floored. Well, I get a new one anyway (that one was dodgy looking) and hook it up. Do the same tests as mentioned and get the exact same results. I might add that the working amp meter in the car dash showed a discharge when the car was running and continues to do so. So, what am I missing? I did the test of the alternator as I understand it and both times I get information that seems to say it is not working. No, I did not take the brand new one down to be tested, but they did the old one and it was fine. What next??
A 1965 GM alternator has a separate voltage regulator. If the alternator is ok, the regulator maybe bad PS....in the later S-I alternator [think 1970s] the exciter circuit runs through a light bulb [shows up as "GEN light on the dash] and if that bulb is burned out, the alternator won't charge the battery...I'd check that bulb first off. Then the regulator
Thanks for your reply Rocky. Thing is, didn't the test I describe with the multi-meter directly on the alternator show that it was bad? I guess that is what throws me off. I did the test, got a result that seems to show a bad alternator....but the alternator is fine. Here is a video showing the sort of testing I did...just to help explain it.
The early alternators with the separate regulator get their field voltage from the regulator, so if the regulator is bad the test you did would have the same results whether your alternator was good OR bad. Are you certain the alternator is from a 1965 vehicle? Post a photo of the alternator where the plug-in connector is. That will tell us for sure that it is the older version. Have you located the regulator? It looks like this:
The alternators with the external regulator have a connector on the back labeled "R" and "F". The internally regulated ones have a connector labeled "1' and "2". Old and new versions pictured below:
Here is a picture of the alternator I took off the car. I replaced it with the same. It looks just like the one from your larger picture above.
Here is a picture of the voltage regulator. It is from a 1965 Chrysler. The system you see has been in place since June of last year....and has worked fine...until Sunday. The green wire that you see coming out of the alternator goes to the regulator as you can see. The black ground is attached as you can see. The red wire goes to that relay unit you see above the regulator.
I know I'll get shot down for this but I've always checked alternators by starting the engine, give it a blip to excite it and then take the earth lead off the battery. If it stalls, dead alternator. Keeps running, alternator good, battery knackered.
Check the ground to the alternator and regulator; it should read zero ohms at all points to the negative battery post. If that's good, then jumper the two connections on the regulator together while the motor is running. If voltage goes up, it's a bad regulator. No change, bad alternator or bad connection. Sounds like the regulator.... By the way, removing a battery terminal as an battery/alternator test with the motor running is a bad idea as it's very hard on the regulator.
Created a jumper to go from the 'F' on the alternator to the positive on the alternator, which by my understanding, bypasses the regulator. Before bypassing, checking the battery at the posts shows 12.62 volts. Hooked up the bypass and started the car and checked the battery. 12.3 volts. No difference from the tests before..... I am as confused as ever....
It has a Chrysler regulator and a GM alternator? I've never tried to mix 'n match but regulators pretty much do the same thing in they regulate the field, so I suppose it's workable. Putting 12 volts on the field winding should cause the alternator voltage to raise significantly. How much faith do you have in your new alternator, and the guys that tested the old one? As a postscript, I found it amusing that, in the YouTube diagnostic video you posted, it is very obvious at about the 46 second mark (when the guy puts the probe on the negative battery terminal) that the battery cable is loose, yet at the same point in the video he mentions having good solid battery cable connections. Video below....
The Jalopy Journal
