I have a 88 bronco - when I turn the key to start it, it makes a humming/buzzing sound. I have tried a known good battery and that is not the problem. Any ideas?

Thanks

 

I think you should have your starter bench tested
It's easy to remove on most Broncos
Just a couple of bolts and you can get to it pretty easy
A lot of parts stores will test it for free

 

Don't they also have the fender-mounted starter solenoid?

Might want to try using a bump switch or jumper on it and see if it starts.

HTH

Matt

 

It really does no good to bench test a starter. It require very little power or voltage to spin on the bench but that doesn't mean it will start your car. If you have a DVOM, first check battery voltage while trying the starter to see if it maintains enough voltage. If it does try connecting one lead to the positive battery terminal and the other to the starter terminal and see if it reads any voltage when trying to start. If it does, then you have a voltage drop somewhere between the two. Try the same test between closer points to narrow it down. Full battery, no voltage drop means bad starter.