Some questions about a 1988 GMC C7 .

lastcowboy32 · May 20, 2021

roundhouse said: ↑

The tires may be bias ply, which gets flat spots when they sit.

the two speed axles have some issues with the shift arm on the diff
They usually have a air cylinder to flip the lever on the rear axle.
Check the switch on the shifter in the cab to make sure you have air coming to it and that it’s opening and getting air to the two speed cylinder on the back axle.
Once you’re getting air to the axle , if it still doesn’t work then disconnect the air cylinder and try to move the shift arm by hand , might need to get one wheel off the ground when you do this so you can move the wheel a little while you’re trying to flip the lever.

I dunno why they didn’t just remove the sides of the dump body. A dumping flat bed that you can install sideboards on is pretty handy.

we used to have a 70s model ten wheel version of this , with a gasoline 427 with two four barrel carburetors on it.
Click to expand...

So, it looks as though it has a 12V Electric transducer/actuator (solenoid? motor?) to do the axle shift. So... looks like it could be a wiring issue? Bad motor? I looked around the internet a little bit so see if I could find this actuator on ebay or parts sites, just to get a feel for if they are available new... and such.

lastcowboy32 · May 20, 2021

roundhouse said: ↑

The tires may be bias ply, which gets flat spots when they sit.

the two speed axles have some issues with the shift arm on the diff
They usually have a air cylinder to flip the lever on the rear axle.
Check the switch on the shifter in the cab to make sure you have air coming to it and that it’s opening and getting air to the two speed cylinder on the back axle.
Once you’re getting air to the axle , if it still doesn’t work then disconnect the air cylinder and try to move the shift arm by hand , might need to get one wheel off the ground when you do this so you can move the wheel a little while you’re trying to flip the lever.

I dunno why they didn’t just remove the sides of the dump body. A dumping flat bed that you can install sideboards on is pretty handy.

we used to have a 70s model ten wheel version of this , with a gasoline 427 with two four barrel carburetors on it.
Click to expand...

I also took a picture of the model/serial tag on the axle... maybe I can use this information to find the type of actuator for the axle shift???

lastcowboy32 · May 20, 2021

This looks like the correct axle shift actuator. I didn't pull the trigger on buying it... need to see if it's even getting power, or if the switch is switching it first...

BoxCarKidd · May 20, 2021

lastcowboy32 said: ↑

Some pictures of the rear rim.
Click to expand...

So yes Daytons. You can loosen the nuts on them and tap the wedges with a hammer. Make sure they are all loose and then remove the nuts and the rim. Reposition them and reinstall. Make sure they run true. Check all the threads, spacer and such.

BoxCarKidd · May 20, 2021

lastcowboy32 said: ↑

So...

Here is a video of the old thing running. Right after we put the diesel tank back on (cleaned, de-rusted, primed and painted, along with its mounting brackets and straps)... along with new rubber sections of fuel line and a fuel filter.

Luckily... the only trick we did was to fill the new filter with fuel... no other special fuel system bleeding. Better to be lucky than good. Cleaning up the tank and filter (the filter hadn't been changed in years, by the looks of it)... cleaned up the emissions a ton. When it was driven into the yard, by the person that I bought it from, it smoked like a forest fire...

Anyway. Here are some remaining "quirks"...

First some info.

Detroit 8.2L diesel

GVWR is 27028lbs
Was originally a dump.
About half of the dump truck mechanicals are there... but it's converted to a flatbed stake, at the moment. (although... whoever converted it didn't think about clearance for the rear tires; when you drop the bed down without the rail that the dump bed pivots on, on top of the chassis frame rails...)

9 x 20 tires, fronts say that they are rated at 80PSI single or 70PSI dual. (They came to me with about 40PSI... I have them at 70PSI at the moment).

I'm still learning about how to manage the duals. The extension on the driver's side, inside dual appears to have been knocked out of place. It's stuck back out of position, instead of poking through the outside rim. I bought a couple of parts for the inflation gun on the compressor in my shop... I haven't put them together and tried to get at that extension yet. I'm curious about this, because I road tested it (it's registered and insured... I have the 10 day temporary inspection to work with)... and it acts as if a tire has either a flat spot from being parked... or it's out of balance... or underinflated. Hard to describe... but... I'm familiar with tractors; if you park a tractor overnight on a cold night... the tires take a set; and the whole machine does a little "limp" or rocking action from side to side... until the tire warms up and gets back to round. This got MUCH better when I adjusted the pressure on the fronts... I'm wondering if there is a little bit of problem left with the rears.

About that road test. It seems that somebody disengaged the axle shift. It's always in low axle, regardless of what I do with the button on the shifting lever. I can probably just follow the linkage for that and check to see where it goes.

The brakes work great, and the pressure system appears to work fine. I'm an engineer and a farmer, not a diesel mechanic or truck driver; so I haven't done a "leakdown test"... yet... but it holds. I will do a leakdown test soon...

It actually holds "too well", because the parking brakes don't activate, even when I pull the button to engage. There must be a solenoid somewhere... or a valve.... any hints where to look would be appreciated.

The dashboard is a random blinking lights show, no fuel gauge (the sending unit is fine and connected, we checked that, when we did the tank and ohmed it out...)

I was looking for a fuse panel to start pulling dashboard fuses; but I didn't find a panel... what I found is a system of terminal blocks on the engine side of the firewall just forward of the steering wheel. I'm wondering if this vehicle uses inline fuses?

Any hints on where I could find a general wiring diagram? I'm an electronics engineer, and I have a pretty well stocked "lab" in my garage with soldering and terminating tools/equipment... I don't mind re-making wires or even harnesses... I just need some sort of general reference...

I apologize in advance if this novice-type of questioning isn't welcome here where real diesel mechanics work... I'm not trying to take work away from y'all... this is going to be a farm truck... I just want it safe and functional, as much as possible.

Thanks!
Click to expand...

I already tried to do this once but I will go again. Then burned out for the day.

Log in or Sign up

Forums

Find Trucking Jobs

Some questions about a 1988 GMC C7 .

lastcowboy32 Bobtail Member

Attached Files:

2021-05-20 07.47.55.jpg

Trucking Jobs in 30 seconds

Wait, employers can't reach you!

lastcowboy32 Bobtail Member

Attached Files:

2021-05-20 07.58.16.jpg

lastcowboy32 Bobtail Member

Attached Files:

EatonAxleshift.JPG

BoxCarKidd Road Train Member

BoxCarKidd Road Train Member

Attached Files:

KIMG0219.jpg

Trucking Jobs in 30 seconds

Wait, employers can't reach you!

Log in or Sign up

Useful Searches

Find Trucking Jobs

Some questions about a 1988 GMC C7 .

lastcowboy32 Bobtail Member

Attached Files:

2021-05-20 07.47.55.jpg

Wait, employers can't reach you!

lastcowboy32 Bobtail Member

Attached Files:

2021-05-20 07.58.16.jpg

lastcowboy32 Bobtail Member

Attached Files:

EatonAxleshift.JPG

BoxCarKidd Road Train Member

BoxCarKidd Road Train Member

Attached Files:

KIMG0219.jpg

Wait, employers can't reach you!