Sudden TH350 failure

[ukhky17]

Hey everyone... I know that this topic has been rehearsed several times before, but I wanted to get some feedback on my situation. Thanks in advance for helping out.

I bought a 83 C1500 SWB about a year ago as a fixer upper. Transmission seemed to be the most reliable and lowest priority for maintenance. After a year of driving it pretty often with no trouble to be seen, I took it out last week to find 3rd gear was slipping. Checked levels and was just a bit low. Not knowing when the trans was serviced I thought to drop the pan, change the filter and add fresh fluid in hopes of solving or discovering a problem. Other than some sludge settling at the bottom of the pan and some dirty fluid there wasnt any type of metal shavings.

Contrary to fixing the problem, I couldnt get into 3rd gear at all. R/1st/2nd felt fine and shifted nicely. Cleaned the governor and replaced the vacuum modulater, still no 3rd gear. Is there anything that comes to mind before I start thinking about a rebuild/replacement? Why such a sudden decline without any noticeable issues?

 

[85 CA SIERRA 1500]

Hi Mike,
Experienced the exact same thing in a F-150 recently, I know ,different trans and brand of truck, but still, complete sudden failure of high gear, still had rev., low-1 and low-2. The clutches and bands for high burnt out and there was not enough friction to drive the transmission output. The reason for failure is up for speculation, possibly old dirty/contaminated fluid, a weak servo not providing enough pressure on the bands and allowing slippage that went un-noticed until there was wear on the clutch material to the point that it would no longer grab, could be either of or a combination. Point being, the failure seemed to be instant, as in while driving down the road.
Best regards and luck fixing your trans,
Tracey>>>>>>>>>>>>>>>>>>

 

[Jason Deckard]

If I remember right, third gear is the forward clutch drum. You very well may have a blown lip seal that is preventing the clutch from engaging. Your governor is also the key. If it mics out less than .799 then your third gear won't engage. I can help you more if you need it. Let me know.

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[yevgenievich]

It would probably be possible is that the contaminated fluid actually what was still letting it work with burned up clutch. Once clean fluid is in, it might not have enough clutch material to even slip. But, a good question if it is engaging 3rd and nothing is there or if it is not even trying to shift to 3rd

 

[Matt69olds]

LOk, some basic transmission diagnostics is needed first. Look over the charts below, you can see that to get third gear, the forward clutch, direct clutch, and the low roller clutch are active. For reverse, the low:reverse clutch and the direct clutch are applied. Since the direct clutch is used in both gears, and if the trans has a good solid reverse, and the no third is a sudden failure, I’d be willing to bet the clutches are fine. Your issue is most likely valve body or something related.

Is the 1-2 shift normal? Is it delayed more than when 3rd gear worked? Does the trans downshift as expected? Look over the lockdown cable, is it melted, or linked? Make sure the cable will pull out of the casing freely, and snap back without binding. If the cable is being held in the lockdown position, the trans won’t upshift until much higher speeds.

Before tearing into the trans, take the truck out for a test drive and make some hard WOT tests. Obviously, don’t overrev the engine, but see if it will eventually make a 2-3 shift.

Another place to look is the govenor. It’s behind a round cover on the pass side of the case, towards the back. Be thankful this is a truck, those covers are a pain to remove on a car (no floorboard clearance!) make sure the weights move freely, make sure the bore in the case that the govenor rotates in isn’t gouged or worn.

The th350 is a pretty durable trans.

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