TH350 Shift points - The 1947 - Present Chevrolet & GMC Truck Message Board Network

1985K10 · 10-06-2003, 10:07 PM

My dad just bought an 84K30 for plowing. It has a 6.2L diesel, and a TH350. We took it for a ride up the road and it takes a while to shift to second gear, when it does it shifts quick and hard. After that it doesnt seem to shift to third, or ds to first when we slow down or stop. The speed-o doesnt work so i cant say at what speed it shifts to second. What could be the problem?

Mike76251 · 10-06-2003, 11:58 PM

There should be a round cover on the side of the trans......pop that off and clean what you find inside.

1985K10 · 10-07-2003, 12:00 AM

Do I have to drain the fluid to do that?

Mike76251 · 10-07-2003, 12:08 AM

I don't think so.

1985K10 · 10-07-2003, 04:48 PM

Same thing to fix it if it was a TH400, because I looked quick the first time, I just looked again and the tranny is really a TH400!

arveetek · 10-08-2003, 12:46 PM

I was just getting ready to say that you probably had a TH400. The 350 was never offered by GM behind the 6.2L diesel. However people do swap them in on occasion.

The 400 uses vacuum to shift. On the diesel, there is no natural vacuum present, so a vacuum pump is used. The vacuum pump is located at the rear of the engine in the same place a distributor would be on a gas engine. It's a large, round thing with a vacuum hose fitting on the top. There should be one or more hoses attached to the pump.

The first I'd do is to check the vacuum pump. Start the engine, and remove the hoses going to the pump. Put your finger over the suction inlet of the pump, and see if it's got good suction. If not, you need to repalce the pump in order for the tranny to shift right.

If the pump feels like it has good suction, check all the hoses for cracks, etc. Trace the line going down to the tranny, and make sure it's plugged into the vacuum modulator.

The modulator could also be bad.

The last thing to look at is the VRV, vacuum regulator valve, mounted on the side of the injection pump. It's basically a throttle position sensor for the tranny. If everything else looks okay, this might need to be looked at.

Good luck!

Casey

10-06-2003, 10:07 PM	#1
1985K10 Registered User Join Date: Oct 2003 Location: CT Posts: 315	TH350 Shift points My dad just bought an 84K30 for plowing. It has a 6.2L diesel, and a TH350. We took it for a ride up the road and it takes a while to shift to second gear, when it does it shifts quick and hard. After that it doesnt seem to shift to third, or ds to first when we slow down or stop. The speed-o doesnt work so i cant say at what speed it shifts to second. What could be the problem? __________________ Anthony 07 Tahoe LT 70 Chevelle 450HP 383-TH400 77 GMC Royal Sierra~Resto in progress~

10-07-2003, 12:00 AM	#3
1985K10 Registered User Join Date: Oct 2003 Location: CT Posts: 315	Do I have to drain the fluid to do that? __________________ Anthony 07 Tahoe LT 70 Chevelle 450HP 383-TH400 77 GMC Royal Sierra~Resto in progress~

10-07-2003, 04:48 PM	#5
1985K10 Registered User Join Date: Oct 2003 Location: CT Posts: 315	Same thing to fix it if it was a TH400, because I looked quick the first time, I just looked again and the tranny is really a TH400! __________________ Anthony 07 Tahoe LT 70 Chevelle 450HP 383-TH400 77 GMC Royal Sierra~Resto in progress~

10-08-2003, 12:46 PM	#6
arveetek Ultimate Diesel Dude Join Date: Oct 2002 Location: Southwest Missouri Posts: 328	I was just getting ready to say that you probably had a TH400. The 350 was never offered by GM behind the 6.2L diesel. However people do swap them in on occasion. The 400 uses vacuum to shift. On the diesel, there is no natural vacuum present, so a vacuum pump is used. The vacuum pump is located at the rear of the engine in the same place a distributor would be on a gas engine. It's a large, round thing with a vacuum hose fitting on the top. There should be one or more hoses attached to the pump. The first I'd do is to check the vacuum pump. Start the engine, and remove the hoses going to the pump. Put your finger over the suction inlet of the pump, and see if it's got good suction. If not, you need to repalce the pump in order for the tranny to shift right. If the pump feels like it has good suction, check all the hoses for cracks, etc. Trace the line going down to the tranny, and make sure it's plugged into the vacuum modulator. The modulator could also be bad. The last thing to look at is the VRV, vacuum regulator valve, mounted on the side of the injection pump. It's basically a throttle position sensor for the tranny. If everything else looks okay, this might need to be looked at. Good luck! Casey __________________ Project truck: '81 C/20 converted to 6.2L TURBO DIESEL bored .040", gear drive, 6.5L injectors/pump, custom pistons, custom 4" exhaust, 700R4, 4.10's Daily Driver: '95 K1500 Tahoe, 6.5L TURBO DIESEL, NP241, 4L80E, 3.42's

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10-06-2003, 11:58 PM	#2
Mike76251 Senior Member Join Date: Mar 2003 Posts: 2,499	There should be a round cover on the side of the trans......pop that off and clean what you find inside.

10-07-2003, 12:08 AM	#4
Mike76251 Senior Member Join Date: Mar 2003 Posts: 2,499	I don't think so.



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