Burned Fusible Links after Starter Run-on Explained and Illustrated

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amphi

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Starter run-on is usually the result of a stuck starter relay or stuck starter solenoid.

The act of disconnecting the front battery (negative) ground wire, during a starter run-on, can cause the fusible links to fry. This is due to reversed-polarity feedback current flowing from the rear battery thru the driver’s side alternator or any other directly connected 12v device.

Normally 24v current flows from both batteries thru the starter to ground and back. (See illustration)

Disconnecting the front battery (negative) ground cable will make ANY connected 24v load lose its normal ground path. This load can be a run-on starter, manual 24v glow plug system, switched-on 24v radio, or a slaved 24v device such as a pump. These connected loads will now see 12v from the rear battery and seek a ground path thru the trucks 12v system. (See Illustration)

Under this circumstance the entire 12v system now sees 12v at a reversed polarity from the rear battery. The result is reversed polarity current flowing thru any directly connected 12v device. This can be damaging to any connected 12v devices that cannot tolerate a reverse polarity - like the driver’s side case grounded alternator (Gen 1), CB radio etc. (See Illustration)

The end result is usually burned fusible links thereby supposedly protecting the devices from heavy reverse current flow.

Good clean ground connections are essential in this dual voltage system.

This is one reason why a single battery disconnect switch at the front battery negative ground is NOT recommended for the CUCV dual voltage system.
 

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cpf240

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Would a disconnect between the 24v buss bar and the cable to the starter do the trick? Sure, the starter solenoid would still be engaged via the stuck starter relay, but the starter would not be running away anymore, yes?
 

doghead

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Safely, in what situation?

For maintenance, yes(but I would not recommend it).

For an emergency when you experiance starter run-on, not really unless you have one on each negative cables and throw them at exactly the same time.

Just replace the under dash starter relay with the one I recommend and you'll be fine.
 

Hammer

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I did the relay swap first thing. Not really worried about that part.
I like the idea of having the disconnect to keep the batteries from draining when it sits for longer periods of time.
And it does make maintenance a lot easier, but that isn't that big of a deal really.
 
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cpf240

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Safely, in what situation?

For maintenance, yes(but I would not recommend it).

For an emergency when you experiance starter run-on, not really unless you have one on each negative cables and throw them at exactly the same time.

Just replace the under dash starter relay with the one I recommend and you'll be fine.
Why couldn't the disconnect switch be placed between the 24v block and the hot cable to the starter? Or a disconnect in between the starter relay and the starter solenoid? Could even be dash-mounted.

The starter relay and solenoid would still be engaged, depending, but I don't think the starter will be turning anymore. Might give enough time to pull the starter relay and save the rest of the harness.

Of course, the simple answer is to do the DH relay mod...
 

doghead

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Why couldn't the disconnect switch be placed between the 24V block and the hot cable to the starter?
Yeah, this would function to stop a starter run-on, but it costs less to change the under dash relay and solve the real problem.

At some point, practicality and cost need to be factored into the equation.


If your experiencing battery drain, then you should fix the problem. (not in this thread/sticky)
 
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Humvee1538

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Yeah, this would function to stop a starter run-on, but it costs less to change the under dash relay and solve the real problem.

At some point, practicality and cost need to be factored into the equation.


If your experiencing battery drain, then you should fix the problem. (not in this thread/sticky)
Yes EVERYONE do the converstion. I purchased the relay at NAPA and was going to change it in a couple of days on my M1009. The next day the starter relay hung up...I quickly pulled the cable from battery "one" but melted a couple of wires and fried the #1 Alternator. When it happened I had the tools ready but it really happens FAST (or slow motion in your head while you smell the wires melting, see smoke come out under the dash and the starter whinning while you pop the hood and try to disconnect the battery).
 

jvlmed

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There was a previous forum that stated to stop the starter run on during the event you simply have to pull out the relay under the dash. This situation happened to me in my 85 M1028. I had read the forum about starter run on, purchased a replacement relay (didn't install immediately though)and then it happened. I pulled out the relay and the starter stopped as predicted in the forum without frying any other components. I had located the relay prior to the event making it easier to find. Thank you to these forums I was able to solve the emergency. This situation also makes me nervous lending my truck to anyone that is not aware of the potential issue. I looked at the update and will be installing soon.
 
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FMJ

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DH, I thought I read somewhere here that the 1986 model year had this issue corrected? Is this true, or not?
 

doghead

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They used an updated relay, but not one that is the same as the one I selected(which by design, is not likely to stick on ever).

No way to know if yours was updated really, either.
 

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Keith_J

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Safely, in what situation?

For maintenance, yes(but I would not recommend it).

For an emergency when you experiance starter run-on, not really unless you have one on each negative cables and throw them at exactly the same time.

Just replace the under dash starter relay with the one I recommend and you'll be fine.
Just had starter run-on WITH your modification. First thing I did is pull that relay yet the starter CONTINUED to run on. Must have been the Bendix drive solenoid on the starter which stuck/welded in the closed position.
 

doghead

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Yes, We have mentioned before that it can happen for other reasons(than the under dash relay).

As long as you have power to a device, there will always be a possibility of failure.

Taking reasonable efforts to protect or avoid a failure of any kind is a matter of practicality and compromise.

Your issue had nothing to do with the underdash relay, and that is the point of my comment quoted, my modification, my discussion quoted.
 
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doghead

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Just had starter run-on WITH your modification. First thing I did is pull that relay yet the starter CONTINUED to run on. Must have been the Bendix drive solenoid on the starter which stuck/welded in the closed position.

Looking forward to reading your thread on this, and helping you determine the actual cause.


edit, here's Keith's thread explaining his run-on cause by a broken starter(nothing to do with the relay mod).
http://www.steelsoldiers.com/showthread.php?122850-Broken-starter-nose-cone
 
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underdog

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Just had starter run-on WITH your modification. First thing I did is pull that relay yet the starter CONTINUED to run on. Must have been the Bendix drive solenoid on the starter which stuck/welded in the closed position.
Reads as a starter problem.
I did this modification after repeated failures of the stock relay.
It works good and I have had no further problems from that.

Hesitant to post in this thread as it is a good sticky.

Others have so here goes.
My key start part has gone bad, it is sticky.
I guess that is the ignition switch; But could be anything with this old gal of ours.
When I go to start it I have to real careful to manually turn the key back to run as soon as the engine fires.
It is hard on the starter so I am wiring a push button momentary switch to the relay.
Key on push the button and hopefully the old gal will make it to her final resting place.
 

Keith_J

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Yes, I thought the 1031 had a factory starter since it only had 9009 miles when recovered in 2012. It was missing the pinion retainer which caused it to go over center and stick.
 
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