Re: Starter Gremlins - Bulletin Board

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Re: Starter Gremlins
Posted by eric in vermont [Email] (#2058) [Profile/Gallery] (more from eric in vermont) on Tue, 28 Nov 2017 15:43:07

In Reply to: Re: Starter Gremlins, Old Guy, Mon, 27 Nov 2017 21:14:50
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Hey Old Guy,

I just went out to the barn and measured the resistance between the starter second post and starter case, on 5 Saab starters that were on the shelf. The second post is the one that is normally open and when the solenoid control from the key is energized, the solenoid energizes and 12V B+ power from the battery is then connected to the second post and the starter spins.

So I tested 5 of them and ALL of them had about 0.5 ohms between the second post and the starter case. This is pretty close to a short, same as yours. Huh. That stopped me for a minute or two.

Then it dawned on me. It SHOULD be a very low resistance between the second post and case (chassis ground). That's the LOAD of the starter motor. Typically, a 4 cylinder starter will pull at least 100 Amps from the battery during starting and as much as 200 Amps. By Ohm's Law, that results in a load resistance range of the starter of 12V/100A = 0.12 ohms to 12V/200A = 0.06 ohms. That's why the ohmmeter test reads a short between the second post and case.

So I hope that explains that issue.

What does bother me and seems like the key to the problem is your measurement of 12 to 14V between chassis and engine block. That points to a bad ground between the battery (-) and the engine block. I know you said you got continuity with your meter, but that doesn't mean it can support starting current. What I mean is, your meter has a little battery in it that supplies a little current, a few milliamps maybe, when you measure continuity. That may work just fine and you get a short circuit. But it doesn't mean you can drive 100 or 200 Amps through it if the connection is tenuous. And since you measure a voltage across that point, it says that its NOT a short.

So I would double check the connection from the battery (-) to the engine block- take it apart, clean it, and torque it down. If there's no improvement, then take a jumper cable and connect only one wire of it, say black. Connect one black connection to the battery (-) and the other to the engine block, somewhere not painted. Then try starting the car.

All for now- hope that helps!!

eric in vermont

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