Turbo 400 shifting??

[Yukon6.2]

Hi All
I'm looking for some quick things to check for my shifting problems.
The shift from 2nd to 3rd has changed,slow driving around 30mph/50km the trany wouln't shift from 2nd to 3rd.You can make it shift but the rpms need to be close to red line,so slow driving is in 2nd,it will stay in 3rd slowing down to 30/50.Trany dosn't seem to be sliping,oil is dirty so a filter/oil change is in order,probly weld a bung in the pan for a temp gage,unless there is a better place for getting a reading.
Any brand of filter that is better than rest?
Also witch line on the rad is the return to trans.
Oh fluid is at proper level
Thanks Thomas

[Robyn]

Sounds like a vacuum issue with the modulator.
The diesels use a vacuum control valve on the IP to control the vacuum to the modulator.

The vaquum pump caqn also be the culprit, Be sure you have 26 inchs of steady suck at the pump and then be sure the modulator is getting good high vacuum when you have light throttle and that it drops off nicely with heavier throttle application.

Good luck and let us know

Robyn

[Yukon6.2]

Hi
Should it be pulling a vacuum at idle? I checked mine and it had nothing,so i thought i would change it with one from my parts motor,no luck.It pulled nothing either.Can you buy just the pot that bolts to the drive?I checked at the napa 100 miles from here,they didn't have any in stock,and the one with the output drive was cheeper than the one for the J motor.
Thanks Thomas

[Robyn]

Well I would replace it with the correct one for the application.

You need to see 26"hg at the pump when the engine is running (Idle or at speed)
This is also your oil pump drive so be careful when you replace it to get things all back in.
I have seen some oil pump drive rods fall off to the side and take a little messing with to get them back in the drive hex. Just be careful and watch your oil pressure when you first restart to be sure its ok before hurring out to check your vacuum.

Make sure your lines are hooked up correctly to the vacuum control valve.
Once its all hooked up right you should have high vacuum at the modulator with the engine at idle (light throttle) and the vacuum will drop off as the throttle is advanced.

If you dont have the specs and the gauge to set the valve you may just have to mess with the settings to get a reasonable shift point.


DONT go too soft or you can mess things up.

You dont want the sucker to be mushy shifting but by the same token you dont want it to be doing a 1-2 shift at 40 under light throttle that tears the dash off either.

1-2 under light throttle should be about 15 MPH and under hard throttle maybe 30 mph
A stock 400 is not a hard shifting box but the shifts should be firm with no mushyness or slip during the shift. Better to be slightly high than low on the vacuum.

Setting the valve by the WAG method is not very scientific but you can also used the original bolt marks to get it back close.

If you are using a new valve just measure the location of the bolt marks and place a mark on the new valve with a little felt pen and it will get you in the ball park, then fine tune it.

If you get the vacuum lines mixed up its no big deal, it just wont work.

Main thing is to have high vacuum at low throttle and low vacuum at high throttle. (at the modulator)

You can also do your testing with a hand vacuum pump to get things adjusted.



Good luck and keep us posted

Robyn

[Low_Bridge]

With the input help on the brakes the 22' Champion G30 is my daily driver. Brakes have been free! I'm getting 12.9 mpg around town ... I don't feel that's too bad but she likes to linger on the shifts...... I Changed the Tranny filter & fluid yesterday for the 30k service & she was as clean as a whistle.

I'm posting on this thread due to the late shifting. I'm going to review these steps as well as here:

http://www.thedieselpageforums.com/tdpforum/showthread.php?t=7304&highlight=shift+point+400

I'll report back to my success. ;)