CUCV: Tranny Woes - 1st gear winds up PAST 35mph!!!

[MuddWeiser]

1985 Chevrolet CUCV (M1009). Military Version of a GM Full Size Blazer.

THIS IS A DIESEL SO HAS NO ENGINE VACCUUM. (does this mean my tranny has electronic shift controls?)

Something horrible has happened to my THM-400 Transmission over this past Memorial Day Weekend.

I took a 3hr trip up to the mountains. On the way there I noticed strange drivetrain behavior. Motor is fine, but for whatever reason at a cruising speed (say 55mph) I could feel little jerks or jolts almost like the car had some DRAG to it - (like friction in the driveline somewhere slowing it down but then the RPM's of the motor were winding/revving so fast - it was fighting forward - all happening in fraction of a second it was like the car couldn't make up if it was slowing down or speeding up) This went on for pretty much the entire drive down.

The next ride is when I really noticed something wrong. I gave it gas and it wound up strong but didn't stop - FIRST GEAR WOUND ALL THE WAY UP TO 35mph before SLAMMING into 2nd gear - which then wound all the way up to 47-50mph before it finally drops down to 3rd gear - which also doesn't sound quite right.


As it currently sits you HAVE to floor it past 35mph or it wont shift out of 1st gear - then I have to floor it past 50mph or it won't go out of 2nd (into 3rd)

WHAT COULD POSSIBLY BE GOING ON HERE? It was working fine and very reliable not more than 4 or 5 days ago... DOESN'T LEAK A DRIP OF FLUID.

???

 

[Ambulancechser]

vac pump...

No,the thm-400 does not have electronic shift controls.I think you (and the rest of us)can be happy it dosn't.
what your 6.2 does have is a vacum pump.it looks a-lot like a distributor that someone took the cap off of and installed a disk like affair in its place,complete with vacum line coming off.inside the disk is a diaphragm,driven by a shaft.
this diaphragm could be leaking or failed completely,and not generating enough vacume.the lines,however,lead to the side of the IP(i think) and a line goes to the vacume servo on the side of the trans.
Since your shifting changed so radically in so short a time,i'd check these lines first.A break here or a leak could easily cause your symptoms.
The trans line is on the side of the transmission near the front passenger side.it leads upward to the vac pump.check all the vac lines first before going crazy changing parts.
I believe auto parts stores still carry the vac pumps.bring a fat wallet or find a junkyard part that works.

 

[gasser]

Mudd, I sent you a PM earlier today and posted to your other question ... maybe you didn't get it? Here it is ...

----OTHER POST -----
I think the trans vacuum unit that is mounted to the left of the IP will most likely needs to be adjusted. With that being said, make sure your vacuum hoses are good ... ones that run from the vacuum modulator on the trans and from the vacuum pump on the backside of the block (on a gas small block, that is where the distributor would be).


---- FROM PM ----

Tranmission may need some adjustment at the vacuum module .... its connected to the IP on the left-hand side. There are two bolts that you loosen, and the vacuum unit can be adjusted to give you better shifting points. I never had to mess with mine, so i can't tell you exactly what to do, but try moving it one way, and take it for a spin .....

Hope this helps,

-graham

 

[mangus580]

I agree with AC, that if it happened suddenly, you lost a line somewhere. the other thing that could cause this, would be if the kickdown switch were to stick... you can search the forum for pics that I posted on the swtich location.