No Crank, No Start, Starter Draw Test

I recently had a 15 suburban with a no crank, no start, but had one click.  Scope showed 50amps when the starter click occurred.  Passed voltage drop test, failed bench test, new starter fixed it.  Today, another guy had a 15 cherokee in, with the same symptoms, and same test results.  The starter is under the intake, so no voltage drop test until he took the intake off.  Nearly 10v dropped on the power cable.  He tightened it up, and it was fixed.

In the case of a seized starter, I was expecting current to be off the charts, but it wasn't.  Maybe internal bad contact not allowing enough current through and it wasn't actually seize?  That brings me to the question.  If you can't access the starter, are getting solenoid engagement, it doesn't sound like the current test is conclusive.  Am I missing something here?  How are you guys testing in this situation without starter access?

Where did you have the amp clamp hooked up? I'm wondering if you might have seen the current from the solenoid pull in coil??

I was on battery negative. I thought maybe I was seeing the solenoid too, but it was a steady 50amps, not a kick. I also thought I was doing the mv to a conversion wrong, but I'm pretty sure I'm not.

With the clamp set to 1mv/100ma, I had .500v. That is 50a, right?

wrote:

With the clamp set to 1mv/100ma, I had .500v. That is 50a, right?

That´s right...

If the main contacts never close then the pull in and hold in windings will allow current flow thru them as long as you hold the key in the start position. These two currents may well sum up to as much as 50 amps, although this on the high end, often time I will see values around 30 to 40 amps.

In your diagnostics you can look out for the current in the solenoid circuits to drop after the initial contact when the pull in circuit should open and only the hold in circuit stays active as long as you keep the key in the start position. If the current drops slightly it indicates that the main contact indeed closes and allows at least some voltage to be present on the switched contact side.

If the solenoid isn't switching power to the motor the pull in coil will remain energized. The pull in coil grounds thru' the starter motor and drops out when the solenoid switches power to the motor. Current clamping the solenoid wire would be a good test for an inaccessible starter. Seeing what the two coils are doing will give lots of good information about the power circuit. Rule of thumb the hold in coil will draw less amps than the fuse. Both will draw more. Pull in may draw more by itself. So don't test for more than a couple seconds at a time. If both coils remain energized something will get HOT.

Other tests before going for the starter would be volt dropping the block to B-. That and poking the power feed as close to the starter as you can get to check for volt drop on the power side.

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Thanks for the replies. I'll chew on this for a bit.