New to me: 1978 GMC High Sierra c25
 

Hey everyone,

I havent been on this forum in ages. I had once started a project on a 68 c10 but had to sell it due to lack of space. Anyways last night I picked up a 1978 GMC c25.

What i know from the seller is that its a Camper Special (cannot confirm), 350 small block (rebuilt 3k ago) 4 speed auto trans and was painted/bodywork a few years back. I picked it up from an older gentleman in Ontario at a farm. The interior needs quite a bit of work but other than that its pretty nice.

Overall its in decent shape. This morning i started it and it fired right up and i tried putting in gear (reverse, D, L) but it wouldnt move. Only at extremely high revving did it bother to roll a bit. Not sure what that is. Perhaps you guys can chime in on what to look for.

Re: New to me: 1978 GMC High Sierra c25
 

Do you know what trans? My best guess is 700R4, which I don't think was around until 82, so someone swapped it in. It's a little on the light duty side for a 3/4 ton. Did it drive fine when you bought it? Have you checked transmission fluid level? Is it possible you have a drum that's a little seized or something (see if you can roll it easily by hand)?

I have those same years on mine ('73 C20 CS), they flip in to make it not quite so outrageously wide.

Does it still have the build tag with all the RPO codes? If so, you can confirm Camper Special based on that. Or, the badge behind the door might be tent shaped, which means CS. Your rear end appears to be a 14FF, if you post a picture of your spring pack I can look at it vs mine to confirm CS.

Re: New to me: 1978 GMC High Sierra c25
 

Why not check the fluid?

Re: New to me: 1978 GMC High Sierra c25
 

Thanks for the reply. Ill check out what trans it is sometime this week. By the time I get home its usually dark. The battery died on me today so I couldn't crank it but I did put it in neutral and it rolled back so it might not be seized brakes.

When I bought it, I drove it very little as it was buried in snow and as soon as i moved forward it got stuck. We ended up pulling it out using the sellers truck and he drove it up the street. I had CAA (canadian AAA) tow it to my place. they towed it by lifting the rear as its 2wd. So i dont think they could have damaged it... I hope the tranny isnt gone otherwise it'll eat into my budget for other mods.

Re: New to me: 1978 GMC High Sierra c25
 

Seized brakes (or similar) wasn't something I thought was very likely, just a possibility.

You can tell tranny by pan shape. I know you said 4 speed but TH350 has a rectangular pan with a corner cut off, TH400 is oddly shaped, 700R4/4L60E is rectangular, 4L80E has one square end and then uneven corners removed on the other end. If you search "TH400 pan" on google image search you can get images.

Re: New to me: 1978 GMC High Sierra c25
 

so the one thing i hadnt checked yet was the trans fluid. I went to check now. When i pulled the stick it has barely any. I wiped it off and checked again and it was basically nothing. So I guess its the trans fluid. I'll keep you guys posted. I hope this solves it. Any idea as to how much I should add? As stated earlier, I was told its a turbo 400 trans.

Re: New to me: 1978 GMC High Sierra c25
 

for reference, i went out and took a pic of the trans. I believe thats a TH400 pan. How much fluid does a bone dry th400 take? any specific type?

Re: New to me: 1978 GMC High Sierra c25
 

I went and bought trans fluid. Poured it in. Long behold it runs!

Backed it out of the driveway to find a nice pool of fluid on my recently sealed driveway :( Just my luck.


I tried locating the leak but theres no dripping at this point. It seems like it lost all of the fluid. Any suggestions? Would it be worth even fixing this one?

Re: New to me: 1978 GMC High Sierra c25
 

Was it idling when you checked it? First place I would check is the dipstick tube seal or pan gasket

Re: New to me: 1978 GMC High Sierra c25
 

it was idling when i checked. What do you recommend that i run through it while testing? is there something else i can run through it during problem solving stage or should i continue to waste more ATF? I'm just frustrated as I spent $30 on the jug of fluid and it ended up all on my driveway :whine:

Re: New to me: 1978 GMC High Sierra c25
 

I would clean the trans really good with brake clean. Let it sit a couple of days and let the torque converter drain down. It will fill the pan and you should be able to see where it's leaking.