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Discussion Starter · #1 ·
Hi y'all!

I'm posting this thread for the purpose of getting feedback from folks who've used the TH400/475 [Pontiac] trans.

I have doubts that it is enough tranny for the job because of the number of dovebid.com stepvans described as having blown trannies with the TH475. Are there some folks here that have run a TH400 or TH475 for a long period of time or under extreme conditions behind a Cummins 4BT that can suggest whether they're tough enough?

How about a record of repairs needed, or upgrade parts suggested also?

Please, I know about brand loyalty and all that, but we're talking about people installing one and then potentially being faced with thousands of dollars worth of problems and years of grief, so how about 'Just the facts'?

Thanks!
 

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What are you putting it in? The load on a tranny is as much dictated by what you're hauling around with it, as what motor is in front of it. I don't think many of us are looking at anywhere near that weight, so the load on the transmission is pretty minimal compared. Also, when you look at something like a dove bid auction, you have to remember that these things have been on the road for many years, driven hard, and retired when it is decided that it is cheaper to replace than rebuild. in many cases the trannys were in the vehicle long before the engine since they originally had GM diesels in front of them.


The short answer, is that a TH400 is more than up to the task, but the one that comes in that dove bid auction will have already been used up and will very likely require a lot of repair.

Doug
 

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Discussion Starter · #3 ·
Doggone it, Doug, that never occurred to me! They re-popped the ENGINE, not the trans! So these TH475's may have had 200,000+ on them before the Cummins install and already due for a rebuild, or rebuilt previously. Plus the fact that no tranny cooler was used, and a puny torque converter. Add it all up and that explains it. Thanks for the new perspective. Seems like a good trans with a cooler and a good converter should be just fine, and as you say, especially in lighter vehicles used for different purposes. Good thought!
JimmieD
 

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a th400 will hold a 454 425 hp and 500lbs of torque so dont you think its strong enough to handle a 4 bt ? yes it will handle it.....
 

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As far as brute strength is concerned, I believe the TH400/475 is more than up to the task. It is not, in my opinion, ideally suited to a diesel application, however. The main reason being the lack of a lockup torque converter and, secondly, it's requirement for vacuum to operate the modulator valve in order for it to function as designed. The retrofitters got around the modulater problem (sort of) by replacing the vacuum actuator with a device that fixes the modulater valve in one position. The problem I see with this arrangement is that it defeats the whole function of the modulater, which is to raise or lower line pressure, and, consequently shift points and firmness according to load. Just conjecturing here, but this may be a contributing factor to the number of bad TH400's behind 4BT's out there. The TH400 backed 6.2L diesels provided this vacuum input to the trans by means of a vacuum pump and a valve on the side of the injection pump that varied the vacuum signal as the throttle linkage opened and closed. With a little ingenuity, I'm sure such an arrangement could be made on a 4BT and would, in my opinion, contribute greatly to driveabilty and longevity.
 

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Discussion Starter · #6 ·
Thanks for some excellent technical advice, Flyin71h, I'm sure it will help a lot of guys!

JimmieD
 

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Discussion Starter · #7 ·
Somehow I missed your post before, Bob. Anyway, yeah, it seems the TH475 'Problem' is pretty simple: High mileage trannies that are plumb wore out! That's not the time to bolt a Cummins diesel up to them, without a tranny cooler.

In similar genius of thought the latest decision by one of the biggies, either UPS or Frito Lay, can't remember which, is to switch their new fleet of trucks back over to gasser engines :deadhorse: :deadhorse:

Maybe they'll add little weenie tires on 19" rims and some trick jumping hydraulics to finish the plan....
 

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In similar genius of thought the latest decision by one of the biggies, either UPS or Frito Lay, can't remember which, is to switch their new fleet of trucks back over to gasser engines :deadhorse: :deadhorse:
I was told by a mechanic friend at Frito-lay they are going to the gassers for economic reasons. The fleet rate for overhauling a van was a tad more than $8000 a couple of years ago, with the biggest portion of that expense being a remanufactured Cummins engine. They figure they can buy a lot more brand new Ford gasoline replacement engines for a lot less money. The most stupid idea they have is changing the Cummins oil change intervals to 15,000 miles and not doing oil sampling. It doesn't matter to them because the vans are on their way out and headed for DoveBid.
 

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Discussion Starter · #9 ·
It doesn't matter all that much to them 'cause they can write so much off as losses on taxes, if they pay any at all. Their cost per mile is irrelevant because they pass that on to the consumers. Makes you wonder, well, no, I think we're beyond wondering now....
 

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Vacuum vs cable vs Fixed

THM 475--I pulled my fixed modulator and replaced it with a cable modulator (same as HMMWV), but after one day working with it, I went back to the set modulator. I set the fixed shifts for 1400 or so, and it shifts like glass. With the cable modulator, no matter what I did my shifts were too FIRM. Everything you said is true. However, at set shift RPMs, it is smooth as a Cadillac. I can pull the shifter back and max out RPM on shift. My tranny auto-override shifts at 2350, or just below governor.
If it didn't shift so good, and drive so smooth, I'd change to something else. Still have a new cable shifter. Bought new for a diesel civilian HMMWV.

Wayne
PS: The van my engine came out of was junked because the setup tore the differential bearings up. You set the fixed shifts too high and it ripps driveline up or the transmission. Like I said, everything you stated was right.

As far as brute strength is concerned, I believe the TH400/475 is more than up to the task. It is not, in my opinion, ideally suited to a diesel application, however. The main reason being the lack of a lockup torque converter and, secondly, it's requirement for vacuum to operate the modulator valve in order for it to function as designed. The retrofitters got around the modulater problem (sort of) by replacing the vacuum actuator with a device that fixes the modulater valve in one position. The problem I see with this arrangement is that it defeats the whole function of the modulater, which is to raise or lower line pressure, and, consequently shift points and firmness according to load. Just conjecturing here, but this may be a contributing factor to the number of bad TH400's behind 4BT's out there. The TH400 backed 6.2L diesels provided this vacuum input to the trans by means of a vacuum pump and a valve on the side of the injection pump that varied the vacuum signal as the throttle linkage opened and closed. With a little ingenuity, I'm sure such an arrangement could be made on a 4BT and would, in my opinion, contribute greatly to driveabilty and longevity.
 

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Ok, this is what I have been trying to find out for the past couple of days. I wasnt sure what the extra shaft coming out of the pas side of the tranny was. Now unlike you guys who are driving your trucks, and want a nice clean shift that dosent rattle your teeth, I am wanting the quickest, hardest shift possible for the quarter mile. So far everything I have seen as far as gaskets, clutches, steels, ets has been the same as the TH400 except for the planetaries being a heaver strait cut version.. So I guess what info im looking for is.... Would a full manual valve body/ shift kit from a TH400 bolt up, or do I actually need to start out with a TH400 and build it to the max? Thanks guys, and let me know if you have any other info that might help me out. Later guys...:beer:
 

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Discussion Starter · #12 ·
I did a lightweight web search and came up with very few hits on the TH475. From what I could find out it's a beefed TH400 used in heavier duty light trucks, with some saying it has 'straight cut gears' with no explanation. Planetaries are usually straight cut anyway, aren't they? Whatever, it probably has heavier clutches and bands but that's just a guess, and no idea if it has heavier input, main or output shafts. Novak kinda goes overboard on just how stinkin' wonderful a TH400 is, if'n ya ask me and nobody did, but here's a link to read it for yourself...

http://www.novak-adapt.com/knowledge/th400.htm

One problem with autos and diesels is the famous off-idle torque, where all kinds of power is being put to the torque converter before it has spun up. That seems to mean you'd want a lower stall TC so it gets a fluid lock at lower rpm, and maybe heavier vanes to take the torque twist.
 

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Im hoping that I can use the 475 untill the beginning of next year, and then I would like a overdrive tranny from suncoast like the one in our other drag truck. I have tons of friends who are running 400s behind small and big blocks with nitrous, chargers, and blowers with no problem. Its just the amount of scratch you want to throw in it, and how fast you want to go... Well back to welding....;)
 

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As far as brute strength is concerned, I believe the TH400/475 is more than up to the task. It is not, in my opinion, ideally suited to a diesel application, however. The main reason being the lack of a lockup torque converter and, secondly, it's requirement for vacuum to operate the modulator valve in order for it to function as designed.

The logic of your case is absolutely based on rock solid fact, and I have been trying to figure a way around it. I may have an answer that will be of use, and I really want to look deeper into it.


Point of fact: Diesel engines generate no vacum

We do, however work in a boosted environment.

Th400's have been used for decades behind supercharged gasoline engines, so someone has already figured it out for us.



If anyone has any information on how the modulators are set up in supercharged applications, that information is directly applicable to our uses, So please post up any information you might have.


Thanks

Doug
 

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The logic of your case is absolutely based on rock solid fact, and I have been trying to figure a way around it. I may have an answer that will be of use, and I really want to look deeper into it.


Point of fact: Diesel engines generate no vacum

We do, however work in a boosted environment.

Th400's have been used for decades behind supercharged gasoline engines, so someone has already figured it out for us.



If anyone has any information on how the modulators are set up in supercharged applications, that information is directly applicable to our uses, So please post up any information you might have.


Thanks

Doug
BDS makes a little valve that splits the vacuum line from the modulator into two lines, one goes to the usually-pressurized intake manifold the other to the appropriate carburetor port.

CD
 

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12 volt electric vacuum pump, OEM GM unit, AC-Delco # 215-119 was discontinued but replacement manufacturer Standard Motor Products had them last I checked. Standard p/n VCP-103 was $140.68
 
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