Frying GP switches

[RSchanz]

Truck wouldn't start this morning and after gassing it a little while trying to start I got white smoke. Engine cranks but doesn't start. Disconnected batteries both are good. Took GP switch off and tested continuity and it was fried. I've now fried two GP switches since I got the truck 6 months ago, the original one that was on it and then the new one I put in. Both were ignition switches from Napa (https://www.napaonline.com/en/p/NW_786101?cid=paidsearch_shopping_dcoe_google) so I woudn't rule out crappy manufacturing but what else could be doing this??

I'm worried that there must be some kind of electrical issue.


Luckily I had another switch in the truck so I installed that and it started fine.

 

[rhkcommander]

Gp switch?

These come with automated gp controllers. Is the manual switch installed correctly? They should have minimal load by triggering the gp relay. That switch can't handle a direct load to gps.

Very high chance you have shorts somewhere if its the relay instead. The old cabling can crack to and from the relay, on the glow plug connectors, etc

 

[Farmer Rock]

There should be an inline fuse.I always install the fuses if they aren't there already.My 250 already had manual GPs when I bought it,but no inline fuse.Also,how many amps are the switches rated for?





Rock

 

[RSchanz]

Gp switch?

These come with automated gp controllers. Is the manual switch installed correctly? They should have minimal load by triggering the gp relay. That switch can't handle a direct load to gps.

Very high chance you have shorts somewhere if its the relay instead. The old cabling can crack to and from the relay, on the glow plug connectors, etc

The gp controller was bypassed by the previous owner. After replacing the switch it started fine so I'm guessing all the gps are good. I did test all of them like 4 months ago and they were good.

There should be an inline fuse.I always install the fuses if they aren't there already.My 250 already had manual GPs when I bought it,but no inline fuse.Also,how many amps are the switches rated for?

There aren't inline fuses and I tried to add them last go around and I couldn't find a fuse that would work with the same gauge wire at napa so I just said screw it...

Those linked Napa switches are 15amp and the one I just installed today is 60amp

 

[rhkcommander]

If its installed right woulsnt even see 1a. If wrong, its about 8x15A

 

[RSchanz]

If its installed right woulsnt even see 1a. If wrong, its about 8x15A

I didn't install the bypass but I'm guessing there is a how to/sticky on doing so? Seems like maybe reading that and tracing the steps might be a good idea.

 

[rhkcommander]

There is a sticky here somewhere, but usually its done one of two ways:

Switch hot or switch ground. From memory:

I did ground method, saves on wiring, safer, etc.
The relay should have two big fat posts, one from battery, one to the plugs. It should have two little posts, one ground and one switch post. Both are labeled I believe, an upside down tee, and an S. Anywho you make a jumper cable from the big battery post to the little S post, then run a wire from the ground post into the frame/battery negative to test it, then if it works run it to the cab, to the switch.

You shouldn't hear any clicking during this process, unless you are touching the ground wire to test it - if you do hear it with the 12v jumper only someone replaced your relay with the wrong one. Undo the jumper ASAP.

Then assuming all is well run a little wire to any grounded point in the cab.



Switch hot is similar. Same posts. No jumper,s. Little ground to ground. Switch post goes to the switch, then you need a positive to the switch, most run another wire from the battery, or jumper off the key on power.

If your relay doesn't have 2 big and 2 little you'll want the second option.


Then if you're using motorcraft beru GPS its about 8 Mississippi sometimes more. Bosch were 5 but most here are diehard beru fans because they can take the abuse of the gp controller, and rarely bulge/break off



While you're at it, make another jumper for the starter relay with a switch inline. You'll thank me later but you can crank the engine while under the hood. One big ring terminal, a switch, and a little ring. 12v battery post to the switch/signal/sense post. Great for solo diagnostics under the hood.

 

[IDIBRONCO]

Switch hot or switch ground. From memory:

I did ground method, saves on wiring, safer, etc.
The relay should have two big fat posts, one from battery, one to the plugs. It should have two little posts, one ground and one switch post. Both are labeled I believe, an upside down tee, and an S. Anywho you make a jumper cable from the big battery post to the little S post, then run a wire from the ground post into the frame/battery negative to test it, then if it works run it to the cab, to the switch.

You shouldn't hear any clicking during this process, unless you are touching the ground wire to test it - if you do hear it with the 12v jumper only someone replaced your relay with the wrong one. Undo the jumper ASAP.

Then assuming all is well run a little wire to any grounded point in the cab.



Switch hot is similar. Same posts. No jumper,s. Little ground to ground. Switch post goes to the switch, then you need a positive to the switch, most run another wire from the battery, or jumper off the key on power.

This only applies to the 7.3 style controllers. I'm going to assume that his 86 still has the 6.9 style. To do a manual control on the 6.9 style, you just use the button/switch to put power to the small terminal on the glow plug relay which is on the passenger's side lower fender. It's mounted horizontally behind and lower than your starter solenoid.

 

[The_Josh_Bear]

If its installed right woulsnt even see 1a. If wrong, its about 8x15A

Yep, and it's even hotter than that... overloads my 200A meter.

 

[RSchanz]

I did find a sticky on a 6.9l so I'm going to trace these instructions:

https://www.oilburners.net/threads/how-to-manual-glow-plug-switch.36169/#post-404408

 

[ifrythings]

If its installed right woulsnt even see 1a. If wrong, its about 8x15A

Yep, and it's even hotter than that... overloads my 200A meter.

Each plug draws around 30-50A when cold and depending on battery voltage, I’ve tested a lot of glowplugs for these trucks and the average resistance is 0.25ohm per plug. 8 plugs in parallel is 0.03ohm, battery drop with plugs on is about 10V, 10/0.03=330A this drops fast as the plugs heat up but is still a large current to deal with.

 

[rhkcommander]

This only applies to the 7.3 style controllers. I'm going to assume that his 86 still has the 6.9 style. To do a manual control on the 6.9 style, you just use the button/switch to put power to the small terminal on the glow plug relay which is on the passenger's side lower fender. It's mounted horizontally behind and lower than your starter solenoid.

Thanks for that. My 87 6.9 was using the newer system so I spaced that. The relay looks about the same to me.

 

[RSchanz]

It looks like both of the wires from either end of the GP switch go to the starter solenoid. Would that even be possible? The two newer looking red wires - one on the post on the left with a handful of wires including the orange one and then one red wire on the post on the right.

No in-line fuse anywhere and not connected to the fuse box.

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[rhkcommander]

It looks like both of the wires from either end of the GP switch go to the starter solenoid. Would that even be possible? The two newer looking red wires - one on the post on the left with a handful of wires including the orange one and then one red wire on the post on the right.

No in-line fuse anywhere and not connected to the fuse box.

I could see one tapping into the starter relay post for hot, but not both. Maybe someone tried wiring up the glowplugs to come on during cranking? Or some weird backup relay...

Trace the wires and find out. It's not hard to start fresh but thats odd if true.