1984 C1500 transmission question

[FoolishMortal]

Hi, All. I'm new here and used the Search, but I couldn't find anything that specifically answers my question. I'll try to give the pertinent details in the hope that someone can give me some direction.

The Truck
1984 GMC C1500 High Sierra, 305ci V8, 4-speed auto, RWD, 92k miles

The Sad Story
I use my truck to commute 12 miles to work and to haul my motorcycles. On the way back from dropping off a bike at the mechanic's shop, cruising at 55mph, I felt the transmission "give way" and the engine revved while the road speed slowed down.

I drove the truck all the way to my auto mechanic's shop (30 miles) at 45mph, but he doesn't do tranny work, though he mentioned that the fluid level is fine. I seem to have lost 3rd gear and overdrive, though Reverse, 1st and 2nd are okay.

The speed it can handle keeps slipping down. I've driven it very, very little, but now it slips like crazy at anything over 35mph and I don't dare drive it anymore. I don't hear grinding or clunking, just slippage and over-revving. Nothing seems to be leaking.

The Question
Could it be that there is just a valve or shift solenoid issue? Maybe some internal seal has given way?

I'm no mechanical genius, but I have a neighbor and some friends who would help me drop the tranny and tear into it. I'm happy to buy some tools if I have a shot at pulling it apart and replacing some minor bits.

I want to be able to give this truck to my son in 15 years, but if it needs a new tranny, I'll have to junk it - the truck just isn't worth the money and I'll buy something newer. I would appreciate any insight.

Thanks,
Brandon

[chengny]

Be sure that the vacuum tubing leading from the transmission to the very back of the intake manifold is intact. The way I have seen it most often is a piece of rubber hose about 2 inches long that connects to a nipple which is screwed into the manifold just forward of the distributor shaft penetration. Above the rubber coupling (probably a vibration isolating device) the line changes to shaped steel tubing that is bent in a hairpin curve. It then squeezes down between the fire wall and terminates at the front right (passenger) side of the transmission (again with a rubber coupling).

This is from memory so don't quote me.

I am speaking from experience though. My brother-in-law once drove my truck for over 100 miles without ever getting into 3rd gear. I, like you, thought the transmission was toast. But a quick inspection revealed that the rubber had parted and the tranny was not being supplied with the necessay vacuum. I dont even think I renewed the rubber - just removed the broken off end piece and re-connected it to the manifold. Truck shifted as normal after that.

[PAPA BEAR]

sounds like 3-4 clutches gave out probly due to atorn seal or busted piston both are common. if you can rebuild an engine you can rebuild the transmission is there a transmission shop close by

[FoolishMortal]

There are a few shops nearby, but I don't know how much I trust them - I'm sure you know how that goes. And no, I can't rebuild an engine...I told you, I'm a mechanical idiot.

chengny, I'll check that tubing when I get home. You can't imagine how stoked I would be if that's the answer!

[hotrodpc]

Don't junk the truck. If in no hurry, look around in junk yards for another trans. To give my honest opinion since I have built many trans back in those days, that is about the time the 4 Speed Autos were just coming out. They are weak and have several flaws if it is the original trans. Get the right converter, and convert it to THM350 3 speed auto. They are dime a dozen and very durable trans. Should be a direct bolt in, linkage, mount and all should be all in the same place. The only thing I question because I do not recall, you might have to change the driveshaft yoke. Probably time for a new U joint anyway. You might lose a mile or 2 of highway fuel mileage, but it sounds as if you do not drive it on the highway anyway, but you will have a much more durable trans that will likley last the 15 years until you give it to your son. The 350 will also be cheaper and easier to find. For strength, get the THM350, not the THM350C. The differance is, the "C" is for lock-up converter. They both look identical, except the C has an electric solenoid on the driver side, top of trans over the pan close to the bell housing. Also keep in mind, the Convertes are not interchanable. The C converter will NOT work on the non lockup model so be sure to ask whoever you are buying from if the converter is the right converter for the trans.

This has nothing to do with vacuum. No need to waste your time there. The vacuum line is for noting more than a vacuum modulator, working with the governor to decide shift points in the transmission and has nothing to do with fluid pressures or clutch drums.

The sad truth is, IMO, the trans needs a full rebuild. To get into its operation and how it works, and what happened if you want to waste your time reading... You have several clutch drums in the trans. A band or 2 which holds clutch drums from spinng, and oneway sprag gears that can either let a drum ride free or spin one direction. Each combination of clutch drums being compressed by fluid pressure, and depending on which drums are active, decide which gear you are in. In this case, your low and reverse clutch drum is in good shape, your intermediate clutch drum is likley OK, etc. My guess without looking, is your direct drive clutch drum is now not operational. Being that you mentioned it just gave way all of a sudden, no warning of slipping at all, just gave way, the clutch drum itself is in good shape, but it is not functional. I'd be willing to bet you the truck, that a sprag or other hard part just shattered and went to a fist full of metal pieces which are to big to make it the pan although if you take the pan off, you will think you struck gold. No, its not worth anything. It is more like shiney brass and bronze shavings. Being all the shaving and metal are now ran thoughout the trans and converter, it needs a full rebuild. Your clutches are over 20 years old anyway. Age killed the clutches more than mileage did. So there is the short lesson. You have broken hard parts in the trans. I would just keep the old truck, and convert to the cheaper, more plentiful, stronger THM350 and it will be working in 15 years for your son to have. JMO.

[FoolishMortal]

What an awesome reply - thanks for taking the time!

I'll re-read it a couple of times, wrap my brain around it and speak with some knowledgeable folks, then see what I can find out about a swap.

[maverick25204]

i would go with hotrod, go to junk yard find a good 350 not 350c, like he said the c sucks un less u do alot of highway tow, it keeps the tranny from burnning up but soon was truned into the 400 trans

take the old trans and use it as a tool to help you learn how to do a tune-up or rebuild go to part store and get haynes teckbook #10360 its the mannual that shows you how to do overhauls on trannys

[hotrodpc]

Actually Mav, you are wrong. Of course you never know who told you such info, so don't take it personal, but because I do know for 100% sure, I'd rather educate people so they don't make mistakes and acquire the wrong good stuff.

First Wrong is the 350C used for towing. Quite the opposite. The 350C uses a tad weaker pump assembly and certainly a weaker torque converter. It is still a good unit but it usually more expensive and it sounds as if this guy is wanting to get the truck running as cheap as possible as a secondary vehicle.

Second Wrong is the THM 400 has been out for a very long time too like the THM 350. It was not derived from the 350C. The 400 is usally for Heavy Duty applications like 3/4 and 1 ton trucks, and big blocks as far back as the 70's while 1/2 tons, and passenger cars got the THM350 unless it was a big block. Like today, you have the more common 4L60E and 4L80E for the heavier duty appications unless you get Duramax or 8.1 then you get the Beefy Allison. The 400 is just as about as bullet proof trans as money can buy with a few simple modifications. Racers love the THM400. I used to be a racer too, but would not use the 400. The THM350 has a better first gear, lighter weight, robs much less horsepower, and is dime a dozen where you will pay a pretty penny for a 400 these days. Steet/Strip racers can usually get a few years hotrodding out of a 400. Granted, I had to rebuild the 350 about once a year, and broke a few, but it was a learning experiece of their weakness, like breaking the input shaft in the direct drive drum. Simple fix, weld the shaft to the drum, never broke another one. Thrashed a wedge sprag, replaced it with the later model roller sprag, never broke another. I've even had a THM350 explode all over the street, but for a daily driver street application with a small block or V6 they are hard to beat for the money and their durability. Who cares about breaking and rebuilding trans when you can pull it yourself, rebuild it and reinstall it in less than 3 hours for, at that time $40 for the kit. The 350 is a real nice trans for racing when you throw away a few srpings and check balls, drill a couple holes in the valve body plate, weld the input shaft and replace any wedge sprags with roller sprags. There are also some aftermarket beefier Sun Gears and optional planetary gear sets to change gear ratios too. Fun Stuff, and those are the facts my friend. There may be some that disagree with me, but they will have to prove it to me.

[FoolishMortal]

Good deals?

(3) Chevrolet 700R4 Transmissions - 700r4 tranny x3
700R4 Tranny - 700r4 @ $300
Transmission TH350 - TH350, rebuilt @$325
Chevy 305 motor, 700r4 tranny, 350ph tranny - 700r4 @ $250
Rebuilt Turbo 400 - rebuilt TH400, Turbo-hydromatic @ $450 (can I even use it?)

I'm so lost...

[chengny]

Don't over analyze this. Check the easy/cheap fixes first. I clipped a service bulletin and a system description from alldata - not specifically about your truck but the fundamentals still apply:

A/T - Harsh Shift/3-4 Shift Cycling
GMC NUMBER:
90-7A-23


GROUP:
7A Automatic Transmission
DATE:
December, 1989

CORPORATE
NUMBER:
967102


SUBJECT:
HARSH SHIFT AND/OR 3-4 SHIFT CYCLING
MODELS:
ALL 1980-1990 MEDIUM TRUCKS EQUIPPED WITH ALLISON AT-500 OR MT-600 SERIES, TRANSMISSIONS AND CARBURETED 350, 366 AND 427 GASOLINE ENGINES (RPO'S LF5, L86 AND L43) WITH VACUUM MODULATOR (VIN CODES A, B AND E)

Some of the subject vehicles may experience a harsh shift and/or 3-4 shift cycling due to low vacuum. To correct these conditions, a new "Service Only" vacuum modulator kit has been released.
PARTS INFORMATION:
Part Number Description29051199 Vacuum Modulator Kit Kit Includes:29051199 Vacuum Modulator8658110 O-RingUse the modulator kit only after appropriate troubleshooting has been completed. Verify the carburetor is performing properly and the vacuum lines are in good condition (no leaks or splits) prior to installing the new kit. The new vacuum modulator kit is purple in color and can be purchased through Allison Transmission Division Parts Distribution System. For vehicles repaired under warranty use:


More about the vacuum modulator:

Description
The engine vacuum signal is provided by the vacuum modulator which consists of an evacuated bellows, a diaphragm and a spring. These are so arranged that the bellows and spring apply force that acts on the modulator valve so that it increases modulator pressure. Engine vacuum and a spring oppose the bellows and spring to control modulator pressure.

To reduce the effect of altitude on shift points, the effective area of the diaphragm is different than that of the bellows. Atmospheric pressure acts on the resulting differential area to reduce modulator pressure.

Operation
A vacuum modulator is used to sense engine torque input to the transmission automatically. The vacuum modulator transmits this signal to the pressure regulator, which controls line pressure, so that all torque requirements of the transmission are met and proper shift spacing is obtained at all throttle openings.

Vacuum Modulator Valve
Provides modulator pressure that senses engine torque and vehicle speed. The vacuum modulator speed is used to vary the shift points according to throttle opening and to raise line pressure proportional to engine torque.

2-3 Modulator Valve
Senses modulator pressure to apply a variable force that tends to hold the 2-3 shift valve downshifted.

3-2 Valve
Shuts off modulator pressure from acting on shift valves after direct clutch has been applied. This allows fairly heavy throttle operation in third speed without downshifting. In third speed detent pressure or modulator pressure above 105 psi can be directed to the shift valves to provide downshift forces.

Bottom line - if the modulating valve is not seeing the appropriate vacuum, which communicates the torque/load on the engine to the transmission, it will not shift as designed. Check it out, what have you got to lose?

Even if you get lucky and the fix is an easy one - do yourself a favor and change the hydraulic fluid and filter in the transmission. It is not the entire capacity - there is a lot in the torque converter - but it is better than nothing. Take a good look at the used oil. Check for bits of metal. Most of these units have a magnet in the sump that will hold on to any ferrous material and brass (shiny specks) will float around in the oil and fetch up on the filter. If you see little or none of this stuff, you can be fairly certain that your transmission is good for another 100K miles.

I am certainly no expert on transmissions but as I said in an earlier post - I had the same symptoms and it was only a detached vacuum line.