TH400 Shift Oddity - The 1947 - Present Chevrolet & GMC Truck Message Board Network

tclaremont · 10-21-2007, 03:02 PM

Put the truck in Drive and nothing happens. Run the revs up and the truck does not move.

Put the truck in 2nd, and it shudders and wants to go.

Put the truck in 1st and it moves around as expected.

Unplug vacuum line to the shift modulator, and drive works again, but I suspect (have not confirmed) that it will not upshift to second or third this way.

Trans fulid looks clean and at the correct level once warmed up and in park.

Second question, there is a wire with a male plug on the end hanging right next to the shift modulator. It starts up high in the engine coparmtent but It is not connected to anything down underneath, and I do not see anytwhere for it to hook up.

Am I right to suspect a bad modulator? If so, does it just screw into the side of the tranny as I suspect? It has been forever since I worked on a 400 trans, but the last one I DID work on had the same symptoms and solution (modulator)!

ChevLoRay · 10-21-2007, 03:49 PM

The wire you wrote of is for the electric kick-down that TH400's have. There should be a plug-in receptacle for that plug if you still have the TH400 gearbox.

I assume you checked the fluid level in the transmission and that it is okay.

A dirty transmission filter can make operation difficult, especially when the engine/transmission is cold.

Usually, modulators develop a leak in the diaphragm and will suck the transmission fluid out of the transmission, into the intake and cause engine smoke. But, check the vacuum hoses and be sure they are not broken or leaking.

tclaremont · 10-21-2007, 03:58 PM

I bought a new modulator and replaced it in a matter of five minutes. It took me longer to find the half inch hand wrench than it did to remove the old one and push in the new one.

However, the problem remains. Same symptoms with the new modulator.

The fluid level reads pretty high with the truck not running and the transmission cold. When the truck is warmed up and in park, the fluid level reads perfectly.

The vacuum line that goes into the modulator...where does that go on the other end? Into the carb? I should probably check to ensure that the line is properly connected up there.

tclaremont · 10-22-2007, 07:39 AM

Anyone???

Should I replace the trans filter as a matter of course, or do the symptoms indicate a trip to the shop?

Longhorn Man · 10-22-2007, 03:14 PM

You can do a fluid and filter, but most likely you'll end up pulling the tranny and taking it to a shop.
Don't bother looking for the other end of the vac line, or hooking up the kick down wire, neither will cause the trouble you have.

10-21-2007, 03:02 PM	#1
tclaremont Registered User Join Date: Aug 2007 Location: Rochester, NY USA Posts: 70	TH400 Shift Oddity Put the truck in Drive and nothing happens. Run the revs up and the truck does not move. Put the truck in 2nd, and it shudders and wants to go. Put the truck in 1st and it moves around as expected. Unplug vacuum line to the shift modulator, and drive works again, but I suspect (have not confirmed) that it will not upshift to second or third this way. Trans fulid looks clean and at the correct level once warmed up and in park. Second question, there is a wire with a male plug on the end hanging right next to the shift modulator. It starts up high in the engine coparmtent but It is not connected to anything down underneath, and I do not see anytwhere for it to hook up. Am I right to suspect a bad modulator? If so, does it just screw into the side of the tranny as I suspect? It has been forever since I worked on a 400 trans, but the last one I DID work on had the same symptoms and solution (modulator)!

10-21-2007, 03:49 PM	#2
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Re: TH400 Shift Oddity The wire you wrote of is for the electric kick-down that TH400's have. There should be a plug-in receptacle for that plug if you still have the TH400 gearbox. I assume you checked the fluid level in the transmission and that it is okay. A dirty transmission filter can make operation difficult, especially when the engine/transmission is cold. Usually, modulators develop a leak in the diaphragm and will suck the transmission fluid out of the transmission, into the intake and cause engine smoke. But, check the vacuum hoses and be sure they are not broken or leaking. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

10-21-2007, 03:58 PM	#3
tclaremont Registered User Join Date: Aug 2007 Location: Rochester, NY USA Posts: 70	Re: TH400 Shift Oddity I bought a new modulator and replaced it in a matter of five minutes. It took me longer to find the half inch hand wrench than it did to remove the old one and push in the new one. However, the problem remains. Same symptoms with the new modulator. The fluid level reads pretty high with the truck not running and the transmission cold. When the truck is warmed up and in park, the fluid level reads perfectly. The vacuum line that goes into the modulator...where does that go on the other end? Into the carb? I should probably check to ensure that the line is properly connected up there.

10-22-2007, 07:39 AM	#4
tclaremont Registered User Join Date: Aug 2007 Location: Rochester, NY USA Posts: 70	Re: TH400 Shift Oddity Anyone??? Should I replace the trans filter as a matter of course, or do the symptoms indicate a trip to the shop?

10-22-2007, 03:14 PM	#5
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: TH400 Shift Oddity You can do a fluid and filter, but most likely you'll end up pulling the tranny and taking it to a shop. Don't bother looking for the other end of the vac line, or hooking up the kick down wire, neither will cause the trouble you have.



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