TH400 skips 2nd gears goes right for 3rd - The 1947 - Present Chevrolet & GMC Truck Message Board Network

yellow400ex05 · 02-12-2014, 08:14 PM

Alright I just bought a '84 C10 with a TH400 and a BBC and for some reason 2nd gear works fine when shifted manually but but accelerating in drive it'll go from 1st to 3rd.

what would cause this issue?

geezer#99 · 02-12-2014, 09:35 PM

You need to adjust your vacuum modulator. Pull the vac hose off and use a small flat screwdriver and turn the screw out (it's inside the hole) 6 turns to start. If it gets better then turn it out 6 more.

**rs74** · 02-12-2014, 11:08 PM

Quote:

Originally Posted by geezer#99

You need to adjust your vacuum modulator. Pull the vac hose off and use a small flat screwdriver and turn the screw out (it's inside the hole) 6 turns to start. If it gets better then turn it out 6 more.

Does this apply if the transmission shifts really fast. I will just start to get rolling and mine shifts to second and third comes early as well with my 400 turbo.

geezer#99 · 02-13-2014, 12:16 AM

Quote:

Originally Posted by rs74

Does this apply if the transmission shifts really fast. I will just start to get rolling and mine shifts to second and third comes early as well with my 400 turbo.

Yes it does. You need to fine tune it. Try for going into 2nd at 15mph and to third at 25 to 30 mph.

yellow400ex05 · 02-13-2014, 12:24 AM

Quote:

Originally Posted by rs74

Does this apply if the transmission shifts really fast. I will just start to get rolling and mine shifts to second and third comes early as well with my 400 turbo.

i almost feel like mine does that, i cant tell, but if it does its going immediately from 2nd to 3rd. making this truck feel very slow. i'll get a new vacuum modulator and vacuum line and have it adjusted, hopefully that fixes it, because it'll downshift if your already in 3rd and manually drop it to 2nd still, so everything seems to be okay except for that.

Ozzy2013 · 02-13-2014, 12:31 AM

Make sure to check your governor it uses the vacuum modulator to control the rate that the centrifical weights in the governor open they could be sticking and not swinging out like they should .Its an easy fix!Hope this helps you !Oz

yellow400ex05 · 02-13-2014, 12:37 AM

Quote:

Originally Posted by Ozzy2013

Make sure to check your governor it uses the vacuum modulator to control the rate that the centrifical weights in the governor open they could be sticking and not swinging out like they should .Its an easy fix!Hope this helps you !Oz

where is the governor you speak of? how do you go about checking it? I'm not familiar with these transmissions

geezer#99 · 02-13-2014, 12:42 AM

Thanks Ozzy.
That's the second half of the cure. I always try adjusting the modulator first.
Don't go running out and getting a new modulator Yellow400. Try adjusting yours first. Easy to do. Pop the hose off and rotate the screw. You might need to go a couple dozen turns to get it right or only 6 turns. Go easy. Test and adjust some more.
If you find no change then it's governor time.

Also if any adjustment on the modulator does nothing at all then it's possible the po installed a new one but left out the internal pin. I've seen that before.

yellow400ex05 · 02-13-2014, 01:11 AM

Quote:

Originally Posted by geezer#99

Thanks Ozzy.
That's the second half of the cure. I always try adjusting the modulator first.
Don't go running out and getting a new modulator Yellow400. Try adjusting yours first. Easy to do. Pop the hose off and rotate the screw. You might need to go a couple dozen turns to get it right or only 6 turns. Go easy. Test and adjust some more.
If you find no change then it's governor time.

Also if any adjustment on the modulator does nothing at all then it's possible the po installed a new one but left out the internal pin. I've seen that before.

alright thanks, is there any specific way to turn it, left or right? what will it do?

sorry for the questions just want to know what the cause and affect will be before i go and mess with it!

geezer#99 · 02-13-2014, 01:53 AM

Turn the screw clockwise. Remember where you start. Count the turns. Go six and see if that changes anything. If it does the shifts will be longer between gears and firmer. You can go to far and make the shifts too hard. Best to be conservative.

Ozzy2013 · 02-13-2014, 03:32 AM

It is under the modulator cover on the right side of the trans its behind the Big round Cover.

02-12-2014, 08:14 PM	#1
yellow400ex05 Registered User Join Date: Aug 2011 Location: confusion, USA Posts: 206	TH400 skips 2nd gears goes right for 3rd Alright I just bought a '84 C10 with a TH400 and a BBC and for some reason 2nd gear works fine when shifted manually but but accelerating in drive it'll go from 1st to 3rd. what would cause this issue?

02-12-2014, 09:35 PM	#2
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,732	Re: TH400 skips 2nd gears goes right for 3rd You need to adjust your vacuum modulator. Pull the vac hose off and use a small flat screwdriver and turn the screw out (it's inside the hole) 6 turns to start. If it gets better then turn it out 6 more.

02-13-2014, 12:31 AM	#6
Ozzy2013 Registered User Join Date: Oct 2013 Location: Elkton Posts: 693	Re: TH400 skips 2nd gears goes right for 3rd Make sure to check your governor it uses the vacuum modulator to control the rate that the centrifical weights in the governor open they could be sticking and not swinging out like they should .Its an easy fix!Hope this helps you !Oz

02-13-2014, 12:42 AM	#8
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,732	Re: TH400 skips 2nd gears goes right for 3rd Thanks Ozzy. That's the second half of the cure. I always try adjusting the modulator first. Don't go running out and getting a new modulator Yellow400. Try adjusting yours first. Easy to do. Pop the hose off and rotate the screw. You might need to go a couple dozen turns to get it right or only 6 turns. Go easy. Test and adjust some more. If you find no change then it's governor time. Also if any adjustment on the modulator does nothing at all then it's possible the po installed a new one but left out the internal pin. I've seen that before.

02-13-2014, 01:53 AM	#10
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,732	Re: TH400 skips 2nd gears goes right for 3rd Turn the screw clockwise. Remember where you start. Count the turns. Go six and see if that changes anything. If it does the shifts will be longer between gears and firmer. You can go to far and make the shifts too hard. Best to be conservative.

02-13-2014, 03:32 AM	#11
Ozzy2013 Registered User Join Date: Oct 2013 Location: Elkton Posts: 693	Re: TH400 skips 2nd gears goes right for 3rd It is under the modulator cover on the right side of the trans its behind the Big round Cover.



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