Turbo 400 doesn't upshift

Rat Rods Rule

Help Support Rat Rods Rule:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

AZRodDave

Well-known member
Joined
Sep 28, 2014
Messages
1,017
Location
Phoenix, AZ
I was told it was rebuilt 3000 miles ago. It is in the 1990 GMC Suburban I just got from a friend and it doesn't want to upshift. He said it was always like that with a sly giggle. Tried looking up info online, but nothing really specific other than check fluids and other general info, then "take it to a trans shop." Uh, no. I figure some of you knowledgeable fellows would know why? Thanks in advance. :)
 
1. Check to make sure the vacuum hose is on the modulator and not leaking.
2. Dirt in the governor. Pop the round side cover off pull the governor out and spray it down with brake clean. Check to see if it moves freely, look in the tranny to make sure nothing is wrapped around the governor shaft.
3. Bounce the person who rebuilt it. 3000 ?
 
Thanks Iron! Your #1 was the ticket. The vacuum line had come off the trans because it was too short and soaked in oil. Cut a longer one, slapped it in there, and BAM !!! Like a new truck! Okay, like a new OLD truck. Good times! :) :D Woohoo!
 
Last edited:
So, yesterday it was shifting fine and reversing, just a little slow to engage. TODAY, it will not back up. I took the vacuum line off a 'T' that it shared with the power brakes and it is full of 6 fresh quarts of GM fluid. ARGH !!! Trans shop wants $800 to R&R and rebuild.
 
if your kick down cable isnt hooked up it wont shift either ,, but you may have other problems I know on a metric trans the reverse is the first thing to go ,, that means bans or clutches .
 
Last edited:
You may just have a busted diaphragm in the modulator. IIRC, they are held in by one bolt or screw in, pretty easy to replace.
 
Ok....

So, yesterday it was shifting fine and reversing, just a little slow to engage. TODAY, it will not back up. I took the vacuum line off a 'T' that it shared with the power brakes and it is full of 6 fresh quarts of GM fluid. ARGH !!! Trans shop wants $800 to R&R and rebuild.

Bad modulator....sucking fluid right into the motor/brake booster...might still be salvageable.....replace the modulator and refill it.....then see if moves....
Booster could be trashed from the fluid, but drain it and see if you can salvage it....lines should be separate, booster should have a large line directly to intake vacuum, like off the big port on the carb or intake....the modulator line should have small 1/4 inch line from intake...normally not tied together....Hope it helps..
 
and Bama I thought that too ,, but I thought I read it had fluid in the line if that was the case Id say yes ,, but I read it wrong , he said the trans was full of fluid . I thought that would be a cheaper fix , just to see if it works
 
400 trans hold about 12 quarts with the converter...

so if he only has six quarts in it...if memory serves me...6 aint enough...
 
Yeah....might have overstated that a bit....

it would take about 8 or 9 to fill torque and all up

Think the actual total is 10....6 in the trans and 4 in the converter and cooling lines...you got Me...LOL.:D..if he has fluid and the trans doesn't go into reverse he's either got direct clutch seal or disc issues and or the low reverse is out...if it doesn't move at all then the either the pump puked or the converter tangs sheared off...or it's just low on fluid...but I could have sworn after reading that post several times that the booster was full of fluid....guess AZ will have to clarify....
 
Haha, okay guys, sorry. I was at my folks' doing my nightly cook and clean duties.

This thing has a drain plug on it (aftermarket?) and the pan is stamped Hydramatic GMC. I drained it and got 4 quarts out of it, and assume that the converter does not drain, and that it holds its fluid(?). I can try putting more in. Like I said, it seemed fine yesterday. I can also change the modulator, but no idea what or where the booster is. There is only one vacuum line going into the trans, no cooler. I had better check again, but almost had the thing all the way out last week and didn't see any other vacuum lines to it. Just speedo and the modulator and a reverse light connector...
 
you say you don't run cooler lines, are the line connectors plugged off, or do you have small "loop" of cooler line connecting the two ports????

the cooler circuit, when it returns fluid to the transmission directs the freshly cooled oil
to the bearings that support ALL of the rotating parts, with out that supply of cooled oil
to the rotating assembly you may have destroyed the small sealing rings that direct the fluid to the different circuits.

also the main shaft has a restriction busing in an internal passage. a lot of people remove it to increase oil flow to some of the clutch packs, if it is removed then there is a small hole in the valve body that must be tapped to 1/4" coarse threads, this hole is a controlled leak with the restriction in the shaft, but when the restriction is removed you need to plug the hole, or it will smoke the direct clutches in a matter of minutes. Then no forward or reverse.

even on a light weight car I would run a very small cooler for the trans.
 
I will look again, but didn't see any cooler lines connected or any fittings. I bought the truck from a Mexican fellow who is apparently a worse backyard hack than I am! It has been quite an adventure fixing some of the shortcuts that were taken when he put it together (like the horribly "welded" exhaust). Had I done my due diligence and inspected closer, or taken a REAL mechanic with me to look at it, I probably would have passed on it. Oh well. Ya pays your money and takes your chances. Sometimes it works out, and sometimes you just inherit someone else's neglect!
 
AZ...

on the passenger side of the trans should be two cooler line fittings...might be that they plugged the fittings with pipe plugs or something...either way, that 400 runs pretty hot...it needs those cooler lines badly or it will overheat quickly...as do all auto transmissions....

Now as for this booster....did you say the brake booster was full of fluid? 4 quarts in the pan? Is that an estimate or how did you determine that? Normally you'll get about 5 or 6 qts out of the pan when you drop it...some could drain back out of converter but normally if you do a drain refill it's about 6 qts...

There is only one vacuum line running to the trans...for the Modulator...there is no "booster" on the trans....just the vacuum line to the mod, the electrical plug for the detent (kick down) and the speedo cable...are you sure it's a 400? Does the pan have two large dimples in it and is it an odd shape...like not square...?

I guess I'm just confused....pics would help but honestly, a couple of quarts down might make it slip or engage slowly, but normally would not make it totally inop...Was the fluid dark? Also, did the fluid smell burned? I compare the smell of burned trans fluid to burnt bologna...but that's me...guess we need a bit more info...
 
.Was the fluid dark? Also, did the fluid smell burned? I compare the smell of burned trans fluid to burnt bologna...but that's me...guess we need a bit more info...

So every time you suspect a burnt tranny you run in the house and burn a peice of balogna then run back to the driveway?

Sorry,I couldn`t help myself.The image popped into my mind,I started cracking up and couldn`t get rid of it.[P
 

Latest posts

Back
Top