Dead Truck

[mattmo]

Hello!

Hoping to get some help diagnosing a problem.

While driving to work this morning I turned on the heater, as soon as I did everything, including the motor, died. Battery is still good as auxiliary lights still work. I got out and unplugged the battery and plugged it back in. Still nothing. It's as if that battery isn't connected. Almost froze to death waiting for a tow, learned a lesson not carrying emergency winter gear in Kansas. Is there some kind of main fuse that could of gone out? Why would turning the heater on cause this to happen?

Thank you,

[85lebaront2]

What has probably happened is the fusible links that feed everything on these trucks have failed. On the right inner fender you will see a group of fusible links attached to the starter relay where the positive battery cable goes. These go into the alternator harness, and re-emerge to go through the firewall under the heater or AC casing underhood.

Look for one or more that are burned/melted in that area. Go here on this site for the EVTM (Electrical and Vacuum Troubleshooting Manual) for your truck. Look in the charge and power distribution (Gasoline Engines) for the diagrams. Good luck with it.

[Gary Lewis]

Bummer! It was 27 degrees here this morn, and we are ~250 miles south of you. Bet that was COLD!

There's a fusible link that has probably blown. I've pasted the pages from the 1985 EVTM below, but you will want to get to the whole EVTM over time, and it is at Documentation/Electrical/EVTM/1985 EVTM. And these pages come from Charge & Power Distribution - Gasoline Engines.

In the second diagram you can see that AC/Heater uses Fuse 9, and that circuit goes through the ignition switch to Circuit 31, which is a yellow wire. Then go to the top diagram and you can follow that wire to Fuse Link I, which attaches to the starter relay.

HOWEVER, I included the pages for a truck with no ammeter, meaning one with a warning light. If you have an ammeter then you need pages 17 & 18, and it is Fuse Link M that is probably blown. And it connects to the shunt for the ammeter, so is probably near the can or round ball below the jack.

A fuse link is a smaller wire than the rest of the wiring and serves as a slow-blow fuse. It has special insulation on it that won't catch fire. But when it blows you can usually tell by pulling on it and since the wire is burned through internally the insulation stretches.

So that's where I'd start - the fuse links.

[mattmo]

What has probably happened is the fusible links that feed everything on these trucks have failed. On the right inner fender you will see a group of fusible links attached to the starter relay where the positive battery cable goes. These go into the alternator harness, and re-emerge to go through the firewall under the heater or AC casing underhood.

Look for one or more that are burned/melted in that area. Go here on this site for the EVTM (Electrical and Vacuum Troubleshooting Manual) for your truck. Look in the charge and power distribution (Gasoline Engines) for the diagrams. Good luck with it.

Thank you for the direction, I will start with the fusible links.

[mattmo]

Bummer! It was 27 degrees here this morn, and we are ~250 miles south of you. Bet that was COLD!

There's a fusible link that has probably blown. I've pasted the pages from the 1985 EVTM below, but you will want to get to the whole EVTM over time, and it is at Documentation/Electrical/EVTM/1985 EVTM. And these pages come from Charge & Power Distribution - Gasoline Engines.

In the second diagram you can see that AC/Heater uses Fuse 9, and that circuit goes through the ignition switch to Circuit 31, which is a yellow wire. Then go to the top diagram and you can follow that wire to Fuse Link I, which attaches to the starter relay.

HOWEVER, I included the pages for a truck with no ammeter, meaning one with a warning light. If you have an ammeter then you need pages 17 & 18, and it is Fuse Link M that is probably blown. And it connects to the shunt for the ammeter, so is probably near the can or round ball below the jack.

A fuse link is a smaller wire than the rest of the wiring and serves as a slow-blow fuse. It has special insulation on it that won't catch fire. But when it blows you can usually tell by pulling on it and since the wire is burned through internally the insulation stretches.

So that's where I'd start - the fuse links.

Thank you Gary. I appreciate the time you put into organizing this information. This is a great place for me to start.

[mattmo]

Bummer! It was 27 degrees here this morn, and we are ~250 miles south of you. Bet that was COLD!

There's a fusible link that has probably blown. I've pasted the pages from the 1985 EVTM below, but you will want to get to the whole EVTM over time, and it is at Documentation/Electrical/EVTM/1985 EVTM. And these pages come from Charge & Power Distribution - Gasoline Engines.

In the second diagram you can see that AC/Heater uses Fuse 9, and that circuit goes through the ignition switch to Circuit 31, which is a yellow wire. Then go to the top diagram and you can follow that wire to Fuse Link I, which attaches to the starter relay.

HOWEVER, I included the pages for a truck with no ammeter, meaning one with a warning light. If you have an ammeter then you need pages 17 & 18, and it is Fuse Link M that is probably blown. And it connects to the shunt for the ammeter, so is probably near the can or round ball below the jack.

A fuse link is a smaller wire than the rest of the wiring and serves as a slow-blow fuse. It has special insulation on it that won't catch fire. But when it blows you can usually tell by pulling on it and since the wire is burned through internally the insulation stretches.

So that's where I'd start - the fuse links.

Thanks for the help guys. It ended up being the Fusible link.

Gary, you were right on the money with it being Link I.

I replaced link I and the truck fired right up.

Thanks again!

 

[Gary Lewis]

Thanks for the help guys. It ended up being the Fusible link.

Gary, you were right on the money with it being Link I.

I replaced link I and the truck fired right up.

Thanks again!

Excellent! Glad you got it going. But, did it just fail or did you have a short somewhere else?

[mattmo]

Excellent! Glad you got it going. But, did it just fail or did you have a short somewhere else?

It appears to have completely failed. When I tugged on the wire it fell apart. This is somewhat worrisome. I'm curious as to what would cause a fusible link to fail. And if possible maybe I should replace the fusible link with a normal fuse?

[Gary Lewis]

It appears to have completely failed. When I tugged on the wire it fell apart. This is somewhat worrisome. I'm curious as to what would cause a fusible link to fail. And if possible maybe I should replace the fusible link with a normal fuse?

Getting very hot causes them to fail. So I'd guess something pulled a lot of current.

As for a fuse, there's been a lot of debate about that. A fuse usually goes much more quickly than a fuse link. The latter can get very hot before failing, whereas even a slow-blow fuse goes much more quickly.

I'd inspect the wiring as best you can, but you know that it happened when you turned on the heat, so that's where I'd start.

[Danny G]

Getting very hot causes them to fail. So I'd guess something pulled a lot of current.

As for a fuse, there's been a lot of debate about that. A fuse usually goes much more quickly than a fuse link. The latter can get very hot before failing, whereas even a slow-blow fuse goes much more quickly.

I'd inspect the wiring as best you can, but you know that it happened when you turned on the heat, so that's where I'd start.

read this a little late, I could have sent you one... i have a ton lol.