I agree with Erik K- check out your ground. Let me elaborate...
The basics of the starter are pretty simple. It requires only 3 connections.
1. B+ voltage from the battery- this is the big thick cable from the battery (+) terminal down to the big lug on the solenoid.
2. Ground connection to the battery- this is made from the metal case of the starter to the engine and then to the battery (-) terminal through the braided cable.
3. Start voltage from the key. This is the smaller red "control" wire that connects from the key to the solenoid. When you turn the key to "start", 12V is connected through the key to the solenoid. The solenoid energizes with a "click" and connects the B+ voltage to the starter and it should turn. So the small wire "controls" the starter.
Nothing else should have any effect on turning the motor with the starter. So troubleshooting the starter should focus on these three. Here's how to do it.
1. Connect your handy volt meter to the starter: (+) directly to the big lug on the solenoid and (-) clipped to the body of the starter somewhere. Should read 12-14VDC. If not, you have a problem either with the B+ cable or the ground braid. For the latter, take off the connection to the engine, clean it, and bolt it up tight!
2. Now turn the key to start. The voltage on the starter should drop a bit, down to 12V or so as a big slug of energy is drawn from the battery. If it doesn't maintain 12V, then you have a poor connection with B+ or the ground braid. Or the starter is shorted.
3. If the starter doesn't turn over in step 2 and you still have a good solid 12-14V, then check the red control wire- the one that comes from the key. Measure it's voltage- from the starter body to the red wire. Should be 12V when you turn the key to start. If not, you have a control problem, through the key.
4. Now, bypass the key. To do this, clip a jumper wire on the starter solenoid- remove the small red control wire and clip on a jumper. Make sure it doesn't short anything out. Now briefly touch the jumper wire to the (+) terminal of the battery and see if the starter turns over the motor. If it does, this confirms the starter is okay and that you have a problem with the red control wire or the key switch. If it doesn't turn over, you probably have a solenoid problem.
One other little thing...remember I said the starter makes it's return path from the metal case to the engine block? There have been instances where either the engine mounting point and/or the starter were painted by an enthusiastic restorer. The paint is a poor conductor and starting problems have resulted. This is rare, but it has happened.
Finally, I've been charging up batteries in vintage Saabs for 40 years and I've never had a problem with charging them in the car, with the cables all connected. Those alternators are pretty rugged, as are the separate voltage regulators. Now, modern alternators have internal voltage regulators with fancy electronics- THEY might be damaged by a charger.
Hope this helps. I've had starter problems before and had them in and out of a vehicle half a dozen times. Very frustrating. Let us know what you find!
eric in vermont
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