Voltage swings and starters


#1

Background: My starter was grinding when cold. So I put in a new (used) starter. Still grinding. Had the battery load tested before starter swap, test said everything OK.

New thing: At the posts the battery reads 12.4 volts. But at the starter solenoid and other places on the positive wire it reads anywhere from 3 to 9 volts. If I leave the multimeter on it the voltage will swing back and forth and all over the place between about 3 and 9 volts in the space of 5 seconds.

Any suggestions on where the voltage might be going? Bad ground? Could this be what’s causing the starter to stick? Related at least?

It’s a 2003 PT CRUISER non-turbo.

Thanks!


#2

That shows a major voltage drop between the battery and the starter.

Check all the connections between the two.

Teaster


#3

Is this voltage swing measured while the starter is cranking? If so go from +battery post to starter post to find the + voltage drop while the starter is engaged. Do the same from the - batter post to the starter case.

You could narrow down the voltage drop by going from each available connection to the next looking for the voltage drop. Fix the connection you find.


#4

I assume you checking the voltage on the battery lead that connects to the solenoid and if so that means either connection at the battery is bad or the cable has internal corrosion inside it and it needs to be replaced.


#5

12.4 volts is about normal for a charged battery with the engine off. So that’s ok. If you are measuring those other voltages with the engine off, there is definitely a problem somewhere. But my guess is you just are not making a good connection with the meter to whatever you are measuring including the ground side.

If these voltages are measuring during attempted cranking from the starter terminal to the starter case, well, a starter won’t crank with 3 volts. Some will crank with 9 volts though. My Corolla is spec’d to crank as long as there is at least 9.6 volts (at both the battery connection and the “start” connection during attempted cranking). For run of the mill econobox’s, generally a mechanic would want these voltages to measure above 10 volts minimum during attempted cranking before making a decision on the starter motor. More powerful engines might be different though. The shop manual would give the exact spec.


#6

These swings aren’t when cranking, engine is off. I’m going to get battery cables tomorrow. Today I noticed that numerous ground wires were broken, the ones from the hood to the body and both ones from the struts to the body were broken. Going to try and fix those tomorrow as well.


#7

With the engine off, no current is flowing in the starter circuit. So the voltage on the B+ connection of the starter (the thick wire) should be the same as the battery voltage. And the START connection of the starter (the thinner wire) should be close to zero volts. When you turn the key to “Start”, then the thin wire should be switched to something close to the battery voltage, but the battery voltage may be lower than 12.4 v at that point because of the current load the starter motor places on the battery. (If you disconnect the thin wire and just measure the voltage at the thin wire disconnected, then it should be nearly the same as the battery voltage with the key in “START”, but of course it won’t crank then. ) Like I mentioned, during cranking, the thick wire to the starter should measure above 10 or better 10.5 volts to insure good reliable cranking. If you don’t measure that, then there is a problem in either the battery, the connections, the ignition switch and any associated relays, or the wiring. (A shorted coil in the starter motor could cause this too.)

In practice it’s not so simple to measure these voltages during cranking. When I do it, I make up a set of long leads which I run from the starter motor into the passenger compartment, connected there to the meter, and I install some “T” adapters on the two starter terminals, so I can make a good connection by “T”-ing in using spade adapters to measure the voltages while I turn the key to START. I also put an adapter connected to the case of the starter motor for the ground connection to the meter.

Sounds like you are well on to a solution. Best of luck.