Testing 12v Resistor

[Milton]

Hello all,
Is there a way to test the 12v resistor for the glow plugs with out replacing the glow plugs first?
I have searched but couldn't come up with anything.
Thanks

 

[watkinssr]

not easily.

With the glow plugs not connected, you should read 24 volts at the output of the resistor. But that isn't goint to tell you what it is doing under load.

 

[2deuce]

I just recently bypassed my resistor. I think it takes 1 or 2 seconds longer to crank the engine before it starts when the engine is cold and this is summer weather. The benefit is glow plugs don't fail from getting too many volts through the resistor. With the resistor working your voltage goes up as each glow plug fails. When 1 fails you can tell because it takes longer engine cranking sending more voltage to the remaining glow plugs for the additional time. That causes more to burn out in increasingly rapid order 2 out even more voltage.. the cascade effect. Your whole set will burn out fast. With the resistor bypassed each glow plug always receives 12 volts if one burns out, it doesn't affect the voltage to the remaining GP's except you will have to crank longer using them longer.

 

[Milton]

Thanks,
I understand the cascade of failing glow plugs, I now have 8 dead ones. When I tested voltage going to one of gp's I got 25 volts. To properly bypass the resistor should I go straight to the front battery or just do the "temporary bypass" and call it good?

 

[2deuce]

I took 12 volts from the terminal next to the glow plug relay. I disconnected the line at the bus bar that goes to the resistor and the feed from the resistor to the glow plug relay. Didn't cut any wires in case I wanted to reattach them. Then made up a short wire to connect the 12 volt terminal to the GP relay where I took off the wire off from the resistor. You probably can't use the slave cable port to start your vehicle if your batterys are dead. But if you keep both ends of the wires from the resistor tucked behind it you can take them out and reattach them if you need to use a slave. Disconnect your batterys before you start the work.

I once wasted a new set of glow plugs because one of the clips didn't make a good connection on the plug. Shouldn't ever happen again with that resistor disconnected.

 

[ida34]

Check the troubleshooting section of the-20. IIRC you should have 3 ohms resistance with both sides disconnected from their connections.

 

[Milton]

Thanks guys, I'll check the resistance before I replace the plugs.

 

[Shop Rat Fab]

Disclaimer: This is the method I used to test my glow plugs and resistor. I ended up replacing all my glow plugs with Wellman brand and restested, truck starts fine and the resistor works fine. Use a voltmeter to test the resistor, on the drivers side of the resistor that goes to the glow plugs you will have 24-28 volts then engage the glow plug relay, the voltage should drop to 12-14 if all glow plugs are working correctly and the resistor is working, if the voltage is between 14-24 you have burned out glow plugs. Bypassing will resistor is not a good idea.

 

[doghead]

Bypassing will resistor is not a good idea.

Please explain your thoughts.

I see no negative to bypassing the resistors, other than if you experience a dead front battery and only have a slave cable to get it started(in this case, I would simply reconnect to the stock setup).

In my opinion, having a system design that allows one failure, to cause 8 other items to fail, is a compromise that we hobbyists, don't need. We do not need to use a NATO cable in combat situations.

 

[that1028guy]

The resistor drops voltage but not amps. So you have the amps of both battery working to heat up the glow plugs (faster starts) as opposed to the frt battery amps only. And why do you all think you wouldnt be able to use the slave to jump start with the 12 volt mod? The way its wired up, it feeds 24 volys to the buss bar which feed both batterys. So you will have 12 and 24 volts. Correct?

 

[2deuce]

I don't think that the glow plugs will use any more amps from 2 batterys than from one if the batterys are good and your truck starts as it should. If you have another problem that is using alot of amps and running down the batterys 2 will keep the supply going longer. If the resistor is letting more than 12v through it, it probably will shorten cranking time especially in cold weather but may also shorten GP life. I say bypass that resistor. A cold morning and a bad connection at one of your glow plugs can waste them all with that resistor not bypassed.

For what its worth...I once let my Isuzu idle my whole 8hr shift at work cause my glow plugs were out and I wanted to get home. I no longer have it cause the body rusted out but that engine would have out lasted a couple more bodys.

 

[amphi]

The way its wired up, it feeds 24 volys to the buss bar which feed both batterys. So you will have 12 and 24 volts. Correct?

Maybe !!

Two 12v batteries in series, receiving 24v charging current from any source, drop voltage just like two resistors/loads in series. The amount dropped depends on the relative charge level of each battery. If they are balanced, by being equally charged/discharged, each will drop/see 12v. Any 12v load connected to one of these batteries will see this voltage.

However, an unbalanced condition is most likely on a dead truck.

Slave starting the CUCV

If the rear battery is completely shorted:
No voltage will be dropped resulting in the front battery and the entire 12v truck circuit seeing 24v. Not a good thing

If the rear battery is open:
All voltage will be dropped and the front battery and 12v load will see nothing.

Anything in between these two extremes is possible. This is why the TM -10 clearly states:

"If one battery is missing, DO NOT attempt to slave start."

"Wait 3 to 5 minutes after hooking up slave cable to "dead" truck, before attempting to start it. Damage to the truck’s electrical system may result if the truck is started sooner."

This wait time is an attempt to allow the batteries to receive charging current form the source/slave and become somewhat balanced allowing the 12v circuit to see a voltage close to the designed 12 volts.

IMHO, the 12v resistor bypass mod is a good thing and should be done.

Be very careful when slave starting the dual voltage CUCV -- follow the TM.

Sorry for the highjack - just wanted to add this info.

 

[Hammer]

Bypass the resistor!

Amphi is dead on. If both batteries are ok, just really low on juice, run the slave for a few minutes to help balance them, then you can still start the system, even with the resistor bypassed.

If you have a shorted battery, it could very well fry all the glow plugs in short order!

Heck, the Army bypassed the resistor in my m1009. Took me a while to figure out what was 'wrong' with the wiring. I guess even old dogs learn new tricks, and even the Army bypassed it.