Starter shorted?? - PeachParts Mercedes-Benz Forum

siredmund · #1 06-29-2008, 01:31 AM

1985 300D turbo diesel has been starting and running fine (tho with very slight hesitation of starter kicking in after turning key). Drove 5 miles, went into a store for 5 or 10 minutes, and when i came out , battery was very dead, about 3 volts, and taking + battery terminal off, measured almost dead short from + terminal to ground.

Took off alternator plug and no difference. Starter seemed a bit hotter than the surrounding metal. Hit it w/ a hammer, and waited a while, and the resistance increased to about 3 K , kind of gradually, almost like when you put an ohm meter on a large capacitor.

Got a ride home, came back with a fresh battery a few hours later, and it started right up. Now, about 4 hrs later, with the engine luke warm, the resistance still acts like a giant capacitor is being charged, and levels off at about 3 K ohms. Could that 3K be from the clock, the only thing i can't shut off w/o pulling a fuse?

I searched the threads for about an hour, and found nothing like this. Would love to hear if anyone has an idea about what might be going on.

sir edmund.

(small fleet of various decaying VW's & Mercedes)

Jeremy5848 · #2 06-29-2008, 01:56 AM

The clock does indeed have a couple of fairly large electrolytic caps (100 microfarads or so) in it. Your short circuit could be caused by the starter solenoid getting stuck in the "engaged" position due to some mechanical failure, dirt, corrosion, rust, etc. Sounds like a new one might be in your future.

Jeremy

siredmund · #3 06-29-2008, 11:53 PM

Thanx Jeremy,

With the car cool, i found that the low resistance was on the red wires that apparently go to the light switch and either the ignition switch and/or the glow plug relay (no thanks to my bogus Haynes manual that has electrical diagrams mostly for UK models, and doesn't cover my year, even tho the cover says it does.)

The yellowish wire from the starter measured about 1 meg to ground, with the red wires removed.

I started it up today with no problem, and ran it about 20 minutes, and the battery charged to about 13.9 volts, and no short evident when i shut it off.

There are a couple of plastic boxes that are part of an aftermarket security device with a rats nest of wires, i am removing that, possibly those gizmos malfunctioned somehow. Either that, or the starter shorts when it gets hot, which it didn't do idling for 20 minutes.

The battery has been really good, so it's hard to imagine how a short could have dragged it down so quickly without burning up a wire unless it is an intermittent short in the starter itself, but why would that have not fried the thing so that it would never work again?

I remain baffled.

ed

compress ignite · #4 06-30-2008, 01:03 AM

Usually lots of dust buildup from 20+ years of brushes,Etc.

I had the Bosch on the '84 SD taken apart and cleaned when it decided it
didn't like HOT temperatures any more.'Ran it for another 100K + after that.

You want an OLD Style Alternator/Starter shop...They'll take it apart and
bead blast the end caps if needed...and replace the brushes ...assuming
the windings and rotor are still good.

barry123400 · #5 06-30-2008, 01:34 AM

More likely your glow plug relay was sticking on. Monitor the glow plugs as relay might be switching back on sometimes. Thats what I might suspect first. Remember when you turn off the key switch if the contacts close up in the relay the glow plugs can stay activated. Just my opinion.

siredmund · #6 06-30-2008, 02:02 AM

Thank you both for these great suggestions. It is always a good to find someone who can actually repair something rather than just replace it with mass (re)manufactured crap.

I'm wondering if glow plugs stuck on could have depleted the battery so thoroughly in just 8 or 10 minutes. Would the pre-glow light have stayed on if the glow plugs stayed on?

ed

barry123400 · #7 06-30-2008, 04:42 PM

No the glow plug light may not stay on with a defective glow plug relay.The best way to eliminate the possibility is to unplug the glow wiring harness plug after the car is started and just plug it back in before a restart.

psfred · #8 06-30-2008, 11:53 PM

The glow plugs draw 60A or more, less when fully heated or if some are burned out. This is enough to pull the battery flat with the engine running at highway speed if you have the headlights on -- the relay failed in my 85 Volvo TD, did exactly that. I was looking for a place on the side of the road where I could sit with no lights (the Volvo has an electric shutoff) when I hit a bump and the chunk of something shorting the relay fell out and the headlights came back on.

The relay is fed off the main red wire, too -- it's the power buss.

Peter

siredmund · #9 07-01-2008, 12:57 AM

Removed the aftermarket security system (it has about 30 wires) and took the car on a 30 mile drive today and could not replicate the problem. Stopped 4 or 5 times to check the battery voltage with the engine running, and the resistance with the batt. disconnected.

No short, and the battery voltage stayed at about 13.

Not sure if the removal of the security stuff fixed it, or it was just an intermittent fluke that will pop up again. Will check the resistance to ground on the glo plug relay if it does. Those glo plugs are such a p.i.t.a. to change, hate to be burning them up while rolling.

Thanks again for all the ideas.

ed