Electrical Short that goes away after restarting the vehicle

Question

2010 Fiat Ducato 2.3L Turbo Diesel

So I've got this really weird very intermittent short. Basically, every once in a while I'll stop at a stop light and notice the engine idling really high around 1200 rpm. System voltage drops to around 11 volts, and if I parallel my supplementary batteries the amperage meter shows a large draw. If I turn the vehicle off for a minute then restart it the problem usually goes away, sometimes might need to restart it twice.

The fact that whatever this is seems to be massive as indicated by the large system voltage drop, like around 100 amps of draw would make me think it should have blown some fuse pretty quick, but there aren't any blown fuses and it keeps happening very occasionally.

The fact it goes away after restarting the vehicle makes me think it's something operated by a relay. The only thing I can think of that is on a relay that would draw that much juice is the starter, since the solenoid is basically a relay. But the starter isn't active when I'm driving around. Also, I recently replaced the starter, and this was happening both before and after replacing it.

Any ideas where to start looking for something like this?

EDIT1

So I went out and started the truck and immediately had the voltage drop problem. RPMs went up to 1200, system voltage down to 12.5v on my dash USB charger. Paralleled my backup batteries (220 Amp Hours) for a minute and saw about 15 amps draw and saw a drop in RPMs basically proving that the high RPMs where due to low voltage. I think the alternator is rated at 70amps, so 70+15 is 85 amp draw, theoretically. Direct measurement at the battery gave me 12.3v.

No voltage on the control wire for the starter solenoid and the starter didn't seem to be engaged or powered. However when I measured voltage at the main hot wire on the starter solenoid I saw 14.4 volts! Really weird.

As a note, the alternator hot wire is connected to the starter solenoid hot post. Then there is a second wire on the same solenoid post going to the battery. Haven't seen that setup before.

I then tried the trick of stopping and starting to get it to go away, and after trying three times the starter stopped working. The solenoid closed, I see a 20 amp draw, 0.001 volts across the solenoid and 0.26v across the starter motor. So the solenoid is working fine and passing full current to the starter motor. It looks like the starter motor is shorted out. I then take out the starter to bench test it, see it working with a healthy 10.25v across the motor when activated, put it back on the engine and everything starts right up no problem and works fine.

Key questions:

1. How could there be 12.3v at the battery and 14.4v at the starter solenoid hot post when they are directly physically connected?
2. How could the starter motor behave and measure as if it were shorted, then immediately after pass a bench test and work fine?
3. The system voltage drop is still unexplained since the starter solenoid was clearly not active at the time.
4. How are all these things connected?

Basically, the alternator is directly connected to the starter solenoid. There is then a separate heavy wire that connects the starter solenoid to the battery. I would normally expect the alternator to be directly connected to the battery.

Since this last incident the system has been stuck around 14.4v all the time... Don't understand why it's over charging all the time.

Answer 1

Anything else other than the starter motor that is drawing 100A would be generating smoke in a very short time. In fact, the starter motor would get very hot and eventually burn out.

I was once driving home from work and noticed that some of the electrics in my car were not working. For example the stereo was dead. I just thought I would investigate once I got home. When I stopped at a junction I thought the engine sounded a little different.

When I got back, I found that the starter motor was burned out. The ignition switch had not moved back from the start position fully and the starter motor had stayed engaged. That also explained why several circuits were not working, due to the ignition being in the start position.

I would check that this is not happening to you. It would explain why stopping the engine and restarting fixes the issue.

Edit after your comment -

You could wire up a lamp in the cabin that will light up when there is power to the solenoid. That will identify if the starter is being powered up.

You could then move it to a connection before the starter relay and see if that is being power up.

Or disconnect the starter wire from the relay and move it to a separate switch and see if the fault goes. You will just have to isolate parts of the circuit until you find the fault.

I would have thought for power to get to the starter, it is either the ignition switch or the relay. A shorting wire wouldn’t add power. The fact that you turn the ignition off and on for the fault to go would make me think it is the ignition switch.

Answer 2

I think I had two separate car batteries somehow intermittently disconnect an internal cell, causing a large "voltage drop". The car would start fine one time, and then later it wouldn't, with the battery measuring with low voltage. Later, untouched, the battery voltage would come back up again, able to start the car.