Been forever since I been on this lovely board. Anyways I now am the owner of a lovely GMC truck, it is a 1978 K20 frame/running gear/engine with a 1981 C1500 Diesel cab and box. The truck has a 350 SBC bolted to a TH350. On the 12 (Which was Tuesday) I changed the filter and fluid in the trans. I used Type F fluid, as it is what everyone I know puts in there TH350's in there trucks. The level is just a tad high, as I added 6 quarts with the filter change. I also fixed the vacuum line for the modulator. It had a gash in it and I repaired it, by using some stiff emissions line and putting in it (About 7-8 inches worth) I now have this lovely problem where the truck wants to go up to 4200 rpm before it decides that second gear is the now needed gear. The shift is firm, and its not slipping. You can feel it pull. Now if I drop it into L1 then shift into L2 at 2500 rpms or so, it shifts them, nicely, and does the same from L2 to Drive. I am thinking about buying a modulator, and a long length of line tomorrow since i get paid (And I work at autozone so discount!) and replacing the modulator and the line. But I wanted to ask you guys if there is anything I may be missing? Is there something more obvious I am missing? I dont wanna put money in a modulator and line if it wont fix it! Again it only does it from first to second, second to third is at about 3200 rpms, and thats fine with me! Thanks in advance guys!
I had the same thing on a OT '77 Burb, 454 th400. The vacuum line came undone, make sure the rubber that connects the metal vacuum line at both ends is not crack/split. Once I put new rbber on both ends, trans shifted like it should. Dave
If the governor was hung wouldnt it cause it to shift late from second to third also? I guess I may try new vac line and modulator, and check the governor. Anyone ever took the governor out of a 4WD truck? Very hard?
On 350's when the governor hung up they would not do a 2-1 downshift on coastdown. Sounds more like the vacuum supply to the modulator or the modulator/valve. Most of the governor problems we had in the 70's were too much clearance in the bore and late no/upshifts especially hot. We had a bushing for the governor bore kit back then. Hoop
Governor is easy to check, just pull the cover off, and there it is. Yank it and see if the gear is worn. Don't worry you won't get it back in right, there is no wrong. Make sure it's flaps move freely, and the gear is not worn. That's all there is to it. Cosmo
I've had plenty of bad modulators. One even allowed fluid to get sucked into the engine, running really rich. Best way is short vacuum rubber with steel line as far as possible. that's what she said.
Most of the problems with the Diesels shifting late was either the vacuum pump or more often the vacuum regulator that attaches to the injector pump that line goes to the modulator. Although type F fluid will make your shift firmer it may deteriorate the glue that bonds the friction material to the clutch plates Type F fluid was designed for Fords back when they used asbestos friction material not the organic friction materials used from the 70's to present. I would strongly recommend using Dexron fluid in a GM product. Also the modulator for a diesel runs at lower vacuum then a gas about 15 inches max tee a vacuum gauge into your line and observe the vaccum it should be about 15 at idle and drop of to probably 3 inches doing 4o mph also the regulator is adjustable if they are still available? Regards, Brian
The OP said he had a "1978 K20 frame/running gear/engine with a 1981 C1500 Diesel cab", I'm not sure if he has a diesel 350. If so the Vacuum Regulator would be a prime suspect. I will check to see what the output should be. Otherwise you need to be sure the modulator is seeing a clear unrestricted signal from the manifold that changes instantly with throttle blips. jm2c Hoop
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