81 GMC dumptruck

[tekno62]

in Dec my dad passed away

Im now working on his heavy equipment so that we can sell it, his 1981 dump truck has a problem with the charging system. the alternator is working as I get 14V out but its not reaching the battery's. I was wondering if someone here would have an idea where the fuse/breaker would be that is interrupting the flow of power. the truck starts just fine. so its just the charging
also I looked this up on autocheck and got this info - is the truck a C7d042?
Im asking so I know what to look up later

VIN: 1GDM7D1Y0BV58XXXX
Year: 1981
Make: GMC
Model: Topkick C Series / C7D042
Style / Body: Conventional Cab
Engine: 10.4L V8 Diesel

Thanks

[tekno62]

in testing I found this round thing and would like to know what it is - the power goes in but not out

http://imgur.com/3UekTcW

[panhandler62]

Hopefully someone will have a better answer than me....

It's clearly a relay. One lead to is should be a signal path to activate it. Start the truck and see if you have voltage at that lug.

If not; jump a live wire to it and see if you then have voltage at the heavy lug where you currently do not have voltage.

If it works then you are missing signal (fuse or damaged wire) if not, then it's your culprit.

[VetteVet]

My condolences for your father. I lost my mom in 2012.

A google search for your number revealed several hits on the GMC trucks of that model. here is one for an 83.

.......http://www.interschola.com/displayit...items_id=27986


Here is a picture and I see a relay like yours on the upper left with a large red cable that could be what you're missing. You can see it a lot better in the link.



I'm confused by your post, that the truck starts OK but the battery isn't getting power, yet the alternator is putting out 14.4 volts. If the batteries weren't charging then you should see low batteries and the alternator couldn't sense the battery voltage and it should default to 16 volts.

What you are describing is a relay like Ford uses for their starters but it can be used to transfer higher voltages using a low voltage control switch.
The two large terminals on the side are for the bigger wires or cables and the smaller terminals are uses to connect the larger terminals on the sides.

The 74 and later vehicles run the alternator output wire, the large one on the back to the starter solenoid where it connects to the battery cable for the positive side of the battery. It may have a fusible link in it that has blown which would not let the batteries get a charge.

There should also be two smaller wires on that same connector which feed charging voltage to the cab for lights and ignition as well as other loads.

The relay on the firewall might be for a hydraulic pump for the dump cylinders or a winch. If someone has wired the charging system through it then it will be hard to sort it out.

[tekno62]

thanks - I'll be back to my mom on the 31st. I live 2 hours away so only get to work on the truck on the weekends Im off. I'll let you all know what I find then

Originally Posted by VetteVet

My condolences for your father. I lost my mom in 2012.

A google search for your number revealed several hits on the GMC trucks of that model. here is one for an 83.

.......http://www.interschola.com/displayit...items_id=27986


Here is a picture and I see a relay like yours on the upper left with a large red cable that could be what you're missing. You can see it a lot better in the link.

Attachment 1255747

I'm confused by your post, that the truck starts OK but the battery isn't getting power, yet the alternator is putting out 14.4 volts. If the batteries weren't charging then you should see low batteries and the alternator couldn't sense the battery voltage and it should default to 16 volts.

What you are describing is a relay like Ford uses for their starters but it can be used to transfer higher voltages using a low voltage control switch.
The two large terminals on the side are for the bigger wires or cables and the smaller terminals are uses to connect the larger terminals on the sides.

The 74 and later vehicles run the alternator output wire, the large one on the back to the starter solenoid where it connects to the battery cable for the positive side of the battery. It may have a fusible link in it that has blown which would not let the batteries get a charge.

There should also be two smaller wires on that same connector which feed charging voltage to the cab for lights and ignition as well as other loads.

The relay on the firewall might be for a hydraulic pump for the dump cylinders or a winch. If someone has wired the charging system through it then it will be hard to sort it out.