Ever have a "too much current" problem with a starter motor?

Maintenance/Repairs
1

Any explantion for how a “bad” starter motor can crank the engine, but use too many amps in the selenoid circuit?

I had this problem when I installed a rebuilt starter motor in an early 1990’s Toyota Corolla recently. It was one of those offshore rebuild starter motors. When I installed it, and turned the key to start, nothing. ARRG … I have to take it back, and it takes me about 2 hours to install it, and another 1 hour to remove it! It’s not like changing a lightbulb!

Anyway, first off, before removing it, I checked the voltage (that comes in the smaller cable from the ignition switch,not the one in the thick cable direct from the battery) right at the starter selenoid, and it was only 8.6 volts. At first I thought it was a weak battery, but the battery voltage was 11.8 volts under the same load at the battery. And the voltage at the ignition switch was 11.6 volts. It was loosing 3 volts in the wiring path from the ignition switch to the starter motor! Then I noticed the wiring harness was getting warm that held that wire. Oh Oh.

For the life of me I couldn’t figure out what was wrong. Unwilling to throw in the towel and remove the unit, I decided to completely bypass the ignition switch and the entire wiring harness, so I wired it up with my own switch I bough at Radio Shack and my own 16 AWG (the wire in the harness is 18 AWG) wires clipped direct from the battery to the starter selenoid. This is all from the front of the car. I just wanted to see if I could get it to turn and crank the engine. It worked, it cranked ok and everything, but the second time I tested it with this experimental configuration, it melted my 10 Amp Radio Shack switch! The switch was supposed to be able to handle 10 amps! Remember, this is the selenoid circuit, not the main power circuit from the battery.

So I then DID throw in the towel, and took it out, took the starter motor back to the auto parts place. ( I had a little dust-up with the parts place manager; he at first refused to refund my money. He wanted me to take another rebuilt one, but I’d lost trust in their rebuilt units and I just wanted my $$ back. We compromised on a voucher for future purchases.) Anyway, the parts place tested the problem starter motor I had returned to them, and they said it tested “bad”. So the one they sold me was indeed no good. (Why they didn’t test it before they sold it to me is another question.)

But here’s the mystery: If the rebuilt starter motor tested “bad”, and it indeed was “bad”, how could it have cranked the engine with the experimental heavy guage wiring ? Any explantion for how a “bad” starter motor can crank the engine, but use way too many amps in the selenoid circuit?

2

The solenoid and the starter are two different circuits. Once the contacts make on the solenoid, the battery voltage is fed directly to the coils of the starter motor through the 4 or 6 ga wires from the battery.

Now for the solenoid itself. It is in series with the ignition switch, safety switches and the wiring. The wires have resistance so the resistance of the wiring and the impedance of the coil in the solenoid form a voltage divider.

3

Now if this wiring has 1 ohm of resistance and the solenoid has 2 ohms of impedance, then a 12 volt battery would send 4 amps through this circuit. The wiring would drop 4 volts and the solenoid would drop 8 volts. 8 volts is what you would measure between the contact on the solenoid and ground.

4

Now if you jumper out all the ignition wiring with a heavy duty wire with 0.1 ohm of resistance, then the total circuit resistance would be 2.1 ohms. That would draw 5.71 amps. The solenoid would then drop 11.43 volts and wire would drop .57 volts.

5

It looks like your solenoid probably has about 1 ohm or less of impedance which would indicate that some of the coils were shorted.

6

I’d be curious if the contacts had been replaced in the solenoid, when they wear this kind of behavior occurs. It would be odd, they’re EASY to replace.

7

If you saw the place where they “rebuilt” that starter, you would understand…

8

Thanks to all for comments. The problem was definitely a bad rebuilt stater motor. The way I got the car back on the road, I took the original starter (the one that I was attempting to replace by the rebuilt unit) to a local auto-electric shop and had them fix it. When I installed it afterward, it worked on the first crank, and has worked ever since. The voltage drop in the solenoid circuit is less than 0.5 volts, much less than te 3 volts I measured with the bad rebuilt one.

I was looking in a pretty good Popular Mechanics book titled “Complete Car Care Manual”, and it explained that the stater on a typical car is energized in a two step process. In step one, the solenoid circuit is activated (by turning the key to “start”), which pulls in the solenoid. Pulling it in requires a lot of current, as something is being moved, so there’s briefly a fairly large current draw on the “pull-in coil” through the solenoid circuit; in step two — the key remains in “start” --step one completes, the solenoid is fully pulled in, solenoid contacts switch the full battery power to the motor through the heavy guage battery circuit, and the"pull-in coil" is de-energized so that as much current as possible is available to the battery circuit, and a second coil, called the “hold-in coil” is activated, which uses just enough current to hold the solenoid in, but not as much as was required to pull it in in step 1.

I’m thinking in my case, my “bad” rebuilt starter for some reason didn’t transition from step 1 to step 2, so the current draw remained high on the solenoid circuit, which prevented the starter to crank the engine as too much current was being used by the solenoid.

I was curious if anybody else had had this problem. It’s a toughie, as you can get the starter to work if you supply our own heavy AWG wires to it directly from the battery, but it pulls way too much current for the car’s wiring to handle.

9

I have replaced several starters because of low resistance (short) in the solenoid coil winding. Some Chrysler products have a fuse on this circuit.

10

The old trick was to use a screwdriver to short the large post on the starter (battery) to the small post (ignition). The screwdriver can handle all the current.

11

I had a Vega that once a month the starter solenoid wouldn’t respond so I would open the hood and touch the starter posts with a quarter to start the engine.

12

Thanks @Nevada_545. I was thinkin maybe I was the only one to ever have this problem! BTW, to avoid this problem in the future, I put a fuse in my selenoid circuit. I wish I had heard from you prior, as I could have avoided several hours of trying to figure out why it wasn’t working.

I’m sort of surprised rebuilt units aren’t tested for this problem in the first place. I wonder why?

13

I have an early 70’s Ford truck. I’ve had it since it was new. It still has the original starter motor, and it is as energetic as ever. One time, years ago, the solenoid failed though. But the solenoid isn’t part of the starter, it is a separate gadget up by the battery. Replacing the solenoid is about as simple of a thing that can be done, about the same time it would take to replace a windshield wiper. 5-10 minute job. I think this is a better scheme than packing the solenoid in with the starter, as it is usually the solenoid contacts that are the only thing that ever fails. I wonder why do they make it so difficult in modern cars?

14

@GeorgeSanJose: I suspect it has to do with the necessity for an ‘extra’ battery cable running to the starter, and the voltage drop, as well as underhood space on modern cars being at such a premium. There might be some safety issues in a crash as well, with the uninsulated terminals of the old-school Ford solenoids being more open to shorting and causing a fire.

15

Most owner’s manuals stress not turning the starter on for more than a specified time, for fear of overheating the starter motor. If you had a big brute of a battery, you could easily burn out the starter motor by cranking it excessively.