diesel engine to gas conversion with T-400 trans - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-15-2009, 05:52 AM
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diesel engine to gas conversion with T-400 trans

My question is. I have a th400 in my 1ton gmc that was originally mated to a 6.2 diesel. The 6.2 is long since been replaced but the trans is still set up to run off vaccum like a diesel. It has no kick down. Its shifts real early no matter how hard I run it unless I manually run it through the gears. It won't kickdown going up hill unless I am going really slow. How can I convert it to run properly behind my 406? Do I need to disconnect the vaccum hose and get a kick down and micro switch for a th400 or can I adjust the vaccum modulator? Thanks in advance guy.

Phil

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Old 04-15-2009, 09:27 AM
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The micro switch will cause it to kickdown. Just get one, wire it up, (polarity doesn't matter - it just gives 12v+ to the terminal on the tranny when you floor it.) That will solve the no kickdown issue. The vacuum modulator needs to be replaced with one from a gas vehicle which should fix the early shift issue.
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Old 04-15-2009, 09:51 AM
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Thank you for the info. Hope it takes care of it can't wait to see how hard it'll shift when it works properly. I already replaced the valve body thinking that's was the problem. Just got in a hurry when I put the 383 in didn't realize it didn't have a kick down. Then when I was pulling out the stroker I was under it remover starter and headers I looked up at the modulater and it hit I bet it was never convetered from diesel to gas. Actually it was the header that hit but anyways thanks 4 the help.
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Old 04-15-2009, 01:06 PM
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hi there,

when the electric kickdown solenoid is connected to a kickdown switch it will no longer shift so early

what year truck we talking here/

if old body style 91 or older then you could pull a stock switch from the accelerator pedal assembly of any th400 truck 73-up and run the single wire from it to the solenoid spade on the trans
and run power to the switch to the fuse panel ign source same as what factory did.

doing the same in a 78 right now converting from the original th350 to a nice 3L80HD from an '87

good luck

Last edited by fast68; 04-19-2009 at 06:26 PM.
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Old 04-15-2009, 09:04 PM
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the t-400 trans was set up to shift early with the diesel engines... that would be the governor and valve body too.

a modified governor would help for later shifts and that vacuum modulator as mentioned
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Old 04-15-2009, 11:38 PM
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i already changed out the valve body with an aftermarket 1 bout 2 months ago. so that should have changed the diesel governor then right. so all i have to do now is the vaccum modulator and the electric kickdown correct. the valve body i put in it allows me to downshift at any speed but still maintain full automatic when in drive. i was thinking about going to a full manual if i cant figure it out. but dont really wanna go that route coz i dont plan on leaving that configuration for more than a few months until i got the money together to get all the parts to finish the 6cyl cummins diesel swap with a ford zf5 trans.
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Old 04-16-2009, 02:31 AM
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The Governor isn't in the ValveBody. It's behind the round cover w/clip on the outside, towards the rear of trans.
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Old 04-16-2009, 04:59 AM
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its like a 2inch round or so?
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Old 04-16-2009, 05:57 AM
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Quote:
Originally Posted by RedBush1
its like a 2inch round or so?
Yes, Sorry, I was thinking of a TH350 that has the clip holding the cover.
The 400 Gov cover has 4 bolts holding it on. Right/Passenger side rear of trans.
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Old 04-16-2009, 03:04 PM
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so do i just go to the parts store and get a goverenor for a truck with a gas motor and swap it out?
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Old 04-17-2009, 05:36 PM
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I'm surprised no one has mentioned the lower stall speed of the diesel torque converter so far.

I have seen switches mounted both to the accelerator pedal assembly inside the vehicle and on the throttle linkage at the carb/TBI end too.
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Old 04-18-2009, 06:04 AM
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Quote:
Originally Posted by RedBush1
so do i just go to the parts store and get a goverenor for a truck with a gas motor and swap it out?
You can get them from a trans shop or order.
http://www.makcotransmissionparts.com/3L80-400hp.html

M&M's right about the Deisel torque converter. It will have a very low stall.
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Old 04-18-2009, 06:46 AM
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i already put a 2000 stall converter in when i put in the 383.
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Old 04-19-2009, 07:44 PM
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i just bought a b&m 20248governor recal and a summit g110 adjustable modulator will that fix my problem
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Old 04-19-2009, 08:06 PM
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diesel lower stall speed is why i ditched the converter from my th350c that was behind a 350 oldsmobile diesel in the 81 pickup

in the trucks the switch is mounted to the pedal assembly under the dash since 1971

70 and older were carb linkage mounted

73-up style trucks:


74 camaro one:

Last edited by fast68; 04-19-2009 at 08:15 PM.
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