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Another TH400 question
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Picture of benebob
posted
So ours has an issue. Doesn't like to shift until warmed up. It will do a hard shift (and holds pretty long) out of first and simply won't go into 3rd. You can get it into 3rd if you shift manually from 2nd to 3rd at a higher rpm but you then need to keep on the gas otherwise it slips into a false neutral. Fluid has been changed 3 times since we got it 14k ago. The last two were after our summer trip when I drained and filled, then drove 100 miles and drained and filled. The fluid looks great. I was amazed how nice it looked after 12k with lots of mountains and Southwestern heat. There was very little clutch material in the bottom either. We no longer have a good Tranny shop locally, just chains that wanna rebuild everything every time. The tranny might have 30k on it. Previous owner had shifting issues as well. I've adjusted the linkage, replaced some linkage bushings etc. and nodda. Also replaced the vac line going to the modulator as it was a little swollen from life. I'm wondering if a different modulator or governor might solve it. The modulator on the tranny is a Black and Red Strip. Don't know if that is what she should have or not. Once warmed up everything is just fine but with cooler weather coming I just don't like the idea of even longer warm ups for the tranny before hitting big roads. Any thoughts or recommendations.
 
Posts: 145 | Location: Lancaster, PA | Member Since: 09-06-2016Reply With QuoteReport This Post
Supporting Member of Barthmobile.com 7/17
Picture of Steve VW
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Sounds like it could be a vacuum modulator problem. If they are not working they will prevent the upshift. When the throttle is open wide at low engine rpm, the manifold vacuum is low. The trans modulator holds off the upshift until the engine "catches up" and begins to pull more vacuum. If the modulator is disconnected or broken the trans will not upshift properly, thinking it is still in hard acceleration mode.

Be sure the vac line is getting good vacuum signal. The modulators are easy to change, some replacements are adjustable also.

There is also a wide open throttle switch mounted on the throttle pedal. When the switch is closed by flooring the pedal it will call for a downshift while floored. Be sure the switch is not stuck or shorted. The wire runs to a connector on the driver's side of the trans. Mechanic


8607-3346-33TFPOB------9708-M0037-37MM-01
86 Regal SE 33 Tag axle--"98" Monarch 37
Chev P3(7) 454TBI--------Cummins 8.3 300 hp
400 hp fuel injected-------6 spd Allison, Spartan MM
 
Posts: 3217 | Location: Kalkaska, MI | Member Since: 02-04-2011Reply With QuoteReport This Post
Picture of benebob
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Any ideas what the difference in color strips on them is Steve? I'm guessing that dictates the shift points etc. I'm just wondering if they put the wrong one in when it was rebuilt. Vac seems pretty strong down there but I didn't put a gauge on it. If it was the switch though wouldn't it have issues all the time and not want to downshift when flooring it? It functions fine once it is warmed up.
 
Posts: 145 | Location: Lancaster, PA | Member Since: 09-06-2016Reply With QuoteReport This Post
Supporting Member of Barthmobile.com 7/17
Picture of Steve VW
posted Hide Post
good point on that switch.

I'm not sure what the stripes are for on that modulator, probably to identify various ones they use.

A generic modulator should work, some are adjustable for shift point. hmm


8607-3346-33TFPOB------9708-M0037-37MM-01
86 Regal SE 33 Tag axle--"98" Monarch 37
Chev P3(7) 454TBI--------Cummins 8.3 300 hp
400 hp fuel injected-------6 spd Allison, Spartan MM
 
Posts: 3217 | Location: Kalkaska, MI | Member Since: 02-04-2011Reply With QuoteReport This Post
Picture of benebob
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Everything I have read says there is a difference and to replace with the like but darned if I can find what the difference is. Just hate buying the same only to find it is wrong as well. The adjustability is for shift points only I think.
 
Posts: 145 | Location: Lancaster, PA | Member Since: 09-06-2016Reply With QuoteReport This Post
Supporting Member of Barthmobile.com 7/17
Picture of Steve VW
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This one will probably work fine. My replacement trans and convertor are TCI.

https://www.summitracing.com/p...-350001/applications


8607-3346-33TFPOB------9708-M0037-37MM-01
86 Regal SE 33 Tag axle--"98" Monarch 37
Chev P3(7) 454TBI--------Cummins 8.3 300 hp
400 hp fuel injected-------6 spd Allison, Spartan MM
 
Posts: 3217 | Location: Kalkaska, MI | Member Since: 02-04-2011Reply With QuoteReport This Post
Picture of benebob
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So what kind of pressure should I be getting at idle? My gauge at a warm up idle (around 1000rpm) is reading 17.5 (a little on the lower side. Can't find any leaks or nasty hoses. It does have the banks and from what I've read that lowers it a bit I'm guessing from the more open exhaust??? I also did find that the black and red strip modulator is for lower vac applications. I'm cheap and don't just want to throw parts at it that may or may not be working. Oh I did put a pump on the modulator today to to verify that it isn't leaking
 
Posts: 145 | Location: Lancaster, PA | Member Since: 09-06-2016Reply With QuoteReport This Post
Supporting Member of Barthmobile.com 7/17
Picture of Steve VW
posted Hide Post
I'm not sure what the exact numbers will be..

The transmission normally has no way to tell what the load is and will shift based on rpm and internal pressures in the trans. The vacuum modulator is used to sense when the throttle is open wide, ie when vacuum is low. The modulator overrides the normal shift points. If vacuum is low it will trigger a downshift from the higher gear and keep it in the lower gear until rpms get high enough and vacuum is higher.

For upshift it will delay the shift until it senses lower WOT (wide open trottle) vacuum going to higher vacuum, for downshift it senses the vacuum going from higher part throttle to lower vacuum WOT. The actual high and low numbers will vary but the transition point in between is where the modulator does its thing. That transition point is adjustable by changing the preload spring tension on the diaphragm in the modulator.

The adjustment is a simple preload screw. If you remove the vacuum hose you can adjust the internal spring with an Allen wrench.

The adjustment will affect both upshift rpm and downshift rpm, depending on vacuum and throttle position. One way it will downshift with less throttle and stay in the lower gear to a higher rpm. Other way downshift requires more throttle and it will upshift at a lower rpm.


8607-3346-33TFPOB------9708-M0037-37MM-01
86 Regal SE 33 Tag axle--"98" Monarch 37
Chev P3(7) 454TBI--------Cummins 8.3 300 hp
400 hp fuel injected-------6 spd Allison, Spartan MM
 
Posts: 3217 | Location: Kalkaska, MI | Member Since: 02-04-2011Reply With QuoteReport This Post
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