Author Topic: Th350c problem  (Read 1757 times)

Offline AlphaLimaRomeo

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Th350c problem
« on: August 25, 2019, 12:11:14 PM »
I have an 86 c10 Silverado long bed with the original 305 and th350c (torque converter lock out).

So I finally got it to daily driver status for a few months. The transmission had been shifting fine through out all gears.
One day on my way home I noticed that after I had stopped and shut the truck off for a few minutes that the temp gauge maxed out. As I was going to pull over it started going down and leveled out at just below half way (where it normally sits). Since I didn't see any steam or evidence of over heating I drove it home and watched the gauge closely. I stopped again in town and once again the temperature gauge was topped out when I turned it on and went back down once I started driving. I figured maybe my gauge or thermostat was off so I let it sit a while, checked the coolant (which was good) and drove it down the block. When I went to stop at the stop sign it started bucking like a standard that  was underpowered. I slipped it into neutral and back into drive and it went into first like normal. When I took off I had no third gear. It would run through first and second normal but felt like it was declutching instead of dropping into third. I tried manually shifting it with the same results. It also had a very heavy engine break until it dropped back into second. When I stopped, I checked the fluid level which was good but it was black. There was no solid matter on the stick.

Later on I took it down the block again to see if playing with the shifter would get it into third but it didn't. So I got the bright idea to try to force it into third. I took off and ran through first and second and about the time it would normally shift I punched the gas. It revved high and popped pretty hard into third. Now it's shifting all three gears fine again except if you take off slow, first has a slight hesitation and third feels a bit weak. The temp gauge did go up but not nearly as much and once it started shifting again it went back down. I haven't driven it since because of the bad fluid. Also though out all of this, reverse has been fine.

I don't have much knowledge on transmissions other than what I've read. I was thinking about dropping the pan and valve body to see if I had a sticky valve. I have read about the vacuum modulator and governor causing issue like this as well. What do you guys think? Also would that much heat have damaged the internals or did it just burn the fluid?
 
Thanks
1986 Chevy Silverado C10 >LWB!<
USMC

Offline zieg85

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Re: Th350c problem
« Reply #1 on: August 25, 2019, 02:32:28 PM »
The lockup function is not dis-engaging which may have other things going on.  I had a 90 Cutlass with that same scenario and I unplugged it and it worked fine for the rest of the time I had it.
 
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
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Offline JohnnyPopper

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Re: Th350c problem
« Reply #2 on: August 25, 2019, 04:06:00 PM »
Couple of thoughts:

How long ago did you last service the trans; fluid and filter?

Since you still have reverse, the R/3 Clutch pack appears to be intact.

Did you check the vacuum to the modulator? I had that experience driving away from a shop and having the trans act very weird. They had knocked it off.

A clogged trans filter can cause overheating. Was it whining during this episode?

Extremely hot trans oil will cause a drop in pressure that can affect shifting into 3rd.

A good service is in order, filter fluid and flushing out the cooling lines, which you can do yourself.

After that, if you can find a shop that flushes the trans while it is running, either by insertion into the cooling lines, or by extraction through the fill pipe, you will get the last black oil out of the torque converter.

Sorry I can't comment on the lockup mechanism, never owned one.

BTW, pls let us know what happens, We love to hear how wrong we were and actually learn new thing!

1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline AlphaLimaRomeo

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Re: Th350c problem
« Reply #3 on: August 25, 2019, 07:39:51 PM »
I haven't ever had the pan off. I'll try to get it dropped and see if there is any metal in the pan. If not I'll go ahead and service it and see what happens. While I'm down there I'll check out the modulator and vacuum lines to make sure they are in good shape. It doesn't whine.
If I let it sit for for a while will all of the fluid from the converter drain back into the pan? I've always been wary of flushing older transmissions.

I'll also check out the lock out harness while I'm down there. When I got it a few years ago the harness was messed up so I replaced it. I'll make sure it's still in good order and repair it if not.

Also how do I flush out the cooling lines?

Thanks for the replies.
« Last Edit: August 25, 2019, 08:07:28 PM by AlphaLimaRomeo »
1986 Chevy Silverado C10 >LWB!<
USMC

Offline JohnnyPopper

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Re: Th350c problem
« Reply #4 on: August 26, 2019, 02:34:59 PM »
Don't be surprised to see some metal in the pan, normal wear and tear, unless you see big chunks.... not good.

The converter is like a vertical doughnut, so there is always some oil in it. Best if you can flush it out because the black oil will dilute into the new oil.

Last time I changed the oil in my th350, I used an nylon tube connected to liquid extractor, down the fill tube, pulled it out one quart at a time. Refilled, ran it for a bit, than repeated till the fluid was bright red. I had changed the filter last year, so knew it was good.

Good that it's not whining, if it were, it would indicate that the filter is plugged and the front pump is starving for oil and grinding on itself.

I used a air conditioning cooling line flush, its a solvent that worked perfect, just have to blow the lines out with compressed air after the liquid runs clear.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline AlphaLimaRomeo

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Re: Th350c problem
« Reply #5 on: September 01, 2019, 02:26:17 AM »
So about how much transmission fluid does it take to do the flush and refill process?
1986 Chevy Silverado C10 >LWB!<
USMC

Offline JohnnyPopper

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Re: Th350c problem
« Reply #6 on: September 01, 2019, 12:50:47 PM »
I usually buy two gallons, you'll have some left unless it take more to flush the 'black' oil out.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline AlphaLimaRomeo

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Re: Th350c problem
« Reply #7 on: September 01, 2019, 06:16:16 PM »
And that counts for refilling the transmission after the flush? I thought it held more than that. I may have read it wrong though.
1986 Chevy Silverado C10 >LWB!<
USMC

Offline Stewart G Griffin

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Re: Th350c problem
« Reply #8 on: September 02, 2019, 11:27:58 AM »
First check to see if the vacuum modulator is connected.

My first engine swap with the truck, i could not advance  beyond 2nd gear.   It turned out either i forgot to hook the modulator up or the hose fell off.   You may have to secure it with a hose clamp.