Trans Slipping in 3rd

[Bobert]

*********************Correction on page 2***********************

My transmission is slipping in 3rd gear when it gets warm. It will drive normally for the first few miles then it will slip only in 3rd if I give it any throttle. It will also have a really soft shift from 1st to 2nd when it gets warm.

It was rebuilt 8 months ago and not my original trans.

I put in a new vaccum modulator on the side of the trans but that did not change anything. The fluid is normal and walmart dexron 3.

Could this be a vaccum problem? or internal?

Also, does this trans have and electric kickdown switch?
What should I do next?

Thanks,
Robert

 

[Coal Cracker]

Yes it does have an electric kickdown switch.

And sounds like it has to come back out and get torn down again, sounds like the rebuilder didn't measure clutch pack thickness, or governor bushing/shaft is worn or valve body is leaking.

check on the rear of trans, there is a dome shaped cup before the tailshaft on the side, this is the governor housing, check that the spring did not pop off housing, cousing it to liftt out of it's seat. I f it did it will change the governor pressure.


DO NOT GIVE IT MORE GAS IN THIRD AND MAKE IT SLIP! you'll burn it up totally, and make matters worse

 

[CUCV85]

Yes it does have an electric kickdown switch.

And sounds like it has to come back out and get torn down again, sounds like the rebuilder didn't measure clutch pack thickness, or governor bushing/shaft is worn or valve body is leaking.

check on the rear of trans, there is a dome shaped cup before the tailshaft on the side, this is the governor housing, check that the spring did not pop off housing, cousing it to liftt out of it's seat. I f it did it will change the governor pressure.


DO NOT GIVE IT MORE GAS IN THIRD AND MAKE IT SLIP! you'll burn it up totally, and make matters worse

Coal Cracker is correct, it does have an electric kick down switch located in the cab under excelerator pedal, don't worry about that right now. Pull the vacuum hose off the side of the transmission at the Transmission Modulator Valve and see if there's suction. I hope it's your Vacuum and not the Transmission.

 

[POOR WHITE BOY]

Sounds like internal problem with cluthes

 

[Coal Cracker]

Coal Cracker is correct, it does have an electric kick down switch located in the cab under excelerator pedal, don't worry about that right now. Pull the vacuum hose off the side of the transmission at the Transmission Modulator Valve and see if there's suction. I hope it's your Vacuum and not the Transmission.

You could check vacuum, but no vacuum at modulator will affect all gears. When it warms up, does all shifting change? or just third slipping?

Also was there a shift kit put in it? Transgo, B&M ?

Sometimes a misinstallation of the shift kit can affect valve body function, but as a 400 rebuilder, it shure sounds like third clutches, they may have put new clutch steels in and didn't measure, most of the time on 400's I leave the old steels in and rough up with sandpaper, and I get the proper clutch pack thickness.

 

[Bobert]

Thanks for sharing your expertise guys.

I checked the vaccum line and it has plenty of vaccum.

It has normal power in 1st and 2nd and 3rd is the only gear it is slipping in.
No shift kit, I went to the cheapest place in town and got a rebuilt one and put it in myself.

I was afraid it was internal. Maybe I should try a 700r4 next.

Thanks for the help guys.

 

[Coal Cracker]

Thanks for sharing your expertise guys.

I checked the vaccum line and it has plenty of vaccum.

It has normal power in 1st and 2nd and 3rd is the only gear it is slipping in.
No shift kit, I went to the cheapest place in town and got a rebuilt one and put it in myself.

I was afraid it was internal. Maybe I should try a 700r4 next.

Thanks for the help guys.

A th400 is much more robust than a 700r4, a 700 r4 is a good trans, but needs some updating to even come near the beefyness of a 400.

Talk to another trans guy in your neighborhood, he just might be able to go in and fix third.

 

[Bobert]

Here is an update.

Finally got it to the shop and they think it needs a vaccum pump. They say it has 9 in. and gets worse when it's warm. I didn't think this would affect it when it was in gear, but it would explain the soft shifts when it's warm.

Does this sound right to you guys? Any advice on vaccum pumps? Or are they all the same.

Thanks again

 

[SmokeyDod]

NO. You actually don't even have to have the vaccum pump working at all. You can unplug the rubber hose at the Mod. valve on side of your transmission and your trans will work fine-- IF you don't mind very hard shift changes and staying in 1st & 2nd gear longer than normal. My son has been running his this way for 3 yrs + and no problems and he pulls some heavy equip at times. I to have taken my loose for a week or so just for kicks.

Bad vac pump will certainly NOT make it slip in any gear much less than just 3rd gear. Its like Coal Cracker said, its inside trans. either gov. spring or poss valve body or in clutches.

If you or anyone elso takes their trans to a shop, I would insist that they rebuild yours. The reason say this is that (all ones I have ever seen 15+) the outer housing is a Heavy Duty one (stamped on top of bell housing area) "HD" This is much sturdier housing than whats it most civy vehicles. These are made to accept the alum casting flywheel cover that you bolt on and it not the thin sheet metal cover. The HD handles stress better when offroad or heavy pulling. Most likely they gave you a standard already rebuilt trans on their shelf and will rebuild yours later and someone else will get it. This is just what I have experienced.

 

[Bobert]

Thanks for the info, this shop never gives me straight answers. I asked them why weak vaccum would make it slip after shifting and he just said vaccum controls everything. The first time, I asked them to rebuild my transmission, when I picked it up I told them it was not mine and did not have the hd mark but they said there is no difference and I believed them. If anyone reading this is in Iowa, don't go to all drive transmission.

I have since picked up another cucv th400 and I might swap it in after the governor spring and such.

Thanks again guys, I would trust the info on this site over a shop sny day.

 

[Bobert]

Update:

After putting a new vacuum pump in, I realize that it is not slipping in 3rd but shifting down to 2nd.

At highway speeds it will want to drop to second with any gas but when you let off it shifts back. It also does it when going slow not and is more noticeable now that my shift points are lower.

After it sits for 2 hours, it drives like normal for 5 miles then 3rd gear is useless.

Is this thing junk? I didn't see anything wrong with the governor, still could be. What are the odds that my clutches are bad? Thanks for your patience guys.

 

[notsosanestan]

the placement of the vacuum pump changes the shift points and how hard im pretty sure also the vacuum lines could be in the wrong place and causing all this to happen

 

[Bobert]

Good point, I will have to check those.