I am back, this time with a new vehicle. I traded my old van for a DOT truck : 1986 Chevy 30 1-ton, 6.2L, th-400 with np-205, dana 60's, and a freakin construction crane.

When i first test drove it, i could not get it over 45 on the freeway. I stopped and turned it on and off a few times checking thigs out, and then ended up driving it home for 2 hours - got it over 75 a few times without any hills. Seemed to go just fine, although underpowered compared to my 1990 van with 6.5 heads. And very crappy throttle response, almost makes it slower when i floor it.

Then I drove it next day around town. It was fine at first, but then would not start. Bad BAT connectors that needed cleaning. We jumped it and tightened the squeekig belts. Dash gage showed around 13V, so ALT was prolly working.

And then it would not shift out of first gear. Still have reverse.

I traced where the VAC line went from TH400, and it came to a black Y near IP with a line going to air cleaner. The line from the air cleaner was sucking air, but the line coming from the pump did not appear to have any vacuum or air movement. I drove with the Y adapter disconnected and it still dont shift.

The speedometer is ultra jumpy and i suspect other shorts. I ve read around and found posts regarding other newer trannies having problems with VSS and ALT and then getting stuck in LIMP mode or something. I dont know how different this all is on my 86 turbo 400 though.

Any ideas?

Well in 86 all you have is govener and throttle valve pressures opposing each other in the valve body to create a shift.. Govener pressure is linear to road speed, the faster you go the higher it goes.. Throttle valve pressure is just the cable running up to the throttle body or injection pump probably in this case.. Basically the two pressure oppose each other on certain valves in the valve body.. When these pressures are the same they deadhead each other and the valve doesnt move.. If road speed increases eventually it will overcome the throttle valve pressure and shift to the next gear.


The speedo being jumpy is caused by the speedo cable being dry and binding up in the casing..

it has to have vacuum to shift up.

in my old van (1990 6.2) the entire vacuum line to TH400 was disconnected - and it still shifted - exactly at 25 and 45 mph irregardless of rpm's... this one does not shift out of first gear at all... and it was fine up untill today...

- is there a way to adjust the governer pressure and throttle valve pressure??? where are the governer and throttle valve located?

- how can i test the vacuum pump?

You need to make sure there are no vacuum leaks and that the vacuum regulator valve is hooked up correctly.

The VRV is mounted on the injection pump on the end of the throttle shaft. There should be a vacuum hose that goes to it from the vacuum pump. There should be another hose from the VRV that goes to the transmission.

I would eliminate all of the other vacuum hoses and plug them. That way you can be assured that there are no vacuum leaks...

I've got to see some pictures of that crane! I want one... ):h It'd be handy for lifting stuff...

First chech if your pump pull -10inch water pressure at least (-27 normal)
#2 on the pic .I suspect it dont work at all
then connect the pump directly to you transmission and try it.
It should shifthttp://i68.photobucket.com/albums/i29/tigman_photos/Vegetal/location2.jpg

- is there a way to adjust the governer pressure and throttle valve pressure??? where are the governer and throttle valve located?


The throttle valve on the TH400 is what the whole vacuum system does. I can say that it isn't doing anything if the vacuum system is messed up.


The governor is under a round cover on the side of the rear of the transmission. I believe the TH400 has a clip that holds the cover in place. If you were to remove that cover you would see the governor assembly. You could reach up in there and grab it with two fingers and pull it out. Once it was out you would see that there are two weights and two springs in there. That is how you adjust the governor... You buy a "governor adjustment kit" which has a bunch of different weights and a bunch of different springs. You change the weights and springs until it shifts like it should...

That said, the governor adjustment is a one-time deal. If you are messing around with it on a tranny that worked good before, there is something else wrong... Trust me... Been there, done that.


There could be a problem with the governor, though. If the valve is stuck it will never shift. What you might want to do is remove the governor assembly and inspect it...

What you are looking for is a stuck valve. Once you get the governor out you will see that the governor shaft is hollow with a few little holes in it. Inside the shaft there is a valve that is actuated by the governor weights.

If you hold the governor with the shaft pointed upwards and the weights pointed down and move the weights in and out you should see the valve move up and down. If you don't, the valve is stuck and that would be the cause of the problem.


Hope this helps! :)

hey, thanx guys!
i really appreciate all the info!

i will start with testing the vacuum system, once i get that set up propa i may check the governor valve too, good idea

Hey man,

I will go with High Sierra on this one. I had this problem. It's your governor. The cap is where he says it is but it has 4 bolts and a fluid seal. Take it off and i bet the plastic teeth are sheared.

Transmission parts USA can sent you another one for $15 and your system will be good as new.

The vacuum system hardly ever fails unless the lines get corrorded.

Luck, G

all right,

1. first i adjusted the VRV all the way back, it was adjusted quite far forward before... test drove - nothing new

2. Went and got a brand new vacuum / pressure tester for $20

3. Got a reading from VP fluctuating 25-28", good

4. Got a reading out of VRV around 20-21", a bit high, but good for now

5. Got the same 21" rading out of tube at the VM, no leaks then

6. My vacuum modulator is adjustable, so i turned counter clockwise a couple turns. Test drove - not a change. Gradually turned it out prolly up to 10 turns total, the screw was almost sticking out, still no shift.

7. Looked on rear passenger side of tranny, past the vacuum modulator, there was a 2" square-round cover with 4 bolts. Pulled out the governor. The valve seems to be moving ok, although I dont know if it seals or opens completely. The white gear teeth are indeed sheared, no big chuncks or anything, but I saw thin strips of plastic that came off of pretty much every tooth in the gear. I dont know how much wear there is supposed to be, but I will look into replacing the thing.

If this does not do it, i guess i ll replace the modulator too

Thanks again guys

The white gear teeth are indeed sheared, no big chuncks or anything, but I saw thin strips of plastic that came off of pretty much every tooth in the gear. I dont know how much wear there is supposed to be, but I will look into replacing the thing.


Sounds like the gear is shot. It really shouldn't show any wear. If it is falling apart it most likely does not engage anymore. Replace that and the problem will most likely be solved.

Good point, Grizzly127... I did not mention the governor drive gear. The plastic gears are indeed very prone to failure.

I had the same problem with my 86' M1008. The govenor gear was striped out. I went to the local tranny shop and purchased 1 for around 15 to 20 bucks. It took care of the problem.