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