turbo 400 kickdown solenoid

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Dr1rrrr

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I have a 1998 c1500 with a 4bt and a turbo 400, I haven't had any auto cars in my life until this one (and I've had 5) so I'm learning as I work on it. It is very clear that the person that did the swap didn't know much about autos as well, so I have a single prong connector on the drivers side of the transmission which from what I've read is for kick down and should receive 12v at WOT which is simple enough what I was wondering is didn't some of the older 2500s of these trucks come with a turbo 400s and if so did they use a switch on the pedal or on the linkage at the engine to send the 12v to the solenoid the reason for asking is I'm trying to keep as many parts factory gm as possible its been my experience this makes later trouble shooting much easier. Also my transmission has a vacuum modulator delete as shown below what I'm wondering is how can I use a stock modulator since my engine is boosted and being a diesel it is almost always in boost unless at idle. I've read where some have used a check valve to bleed off boost but this was in gas boosted engines where boost only really hits after like 2000 rpm. what I'm worried about is that if I use the check valve it might just be the same as having no modulator at all cause I always have boost going to it. Also ignore the 2.865 million oil leaks they are being addressed as well.:Big Laugh:
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packer0440

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1988-90 trucks could have a turbo 400; it was an option in some 1/2 tons but mostly HD trucks. I’m not sure about what it used to shift it was either a modulator or possibly something to do with the TPS.
 

1998_K1500_Sub

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what I was wondering is didn't some of the older 2500s of these trucks come with a turbo 400s

The THM400 and 700R4 are both mentioned in the 1988 factory service manual which you may find here:

GMT400 CK Service Manual Project

From the 1988 manual, note these figures and the wiring:

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and if so did they use a switch on the pedal or on the linkage at the engine to send the 12v to the solenoid

This image (below) and related THM400 info come from one page in the attached .pdf, taken from the 1988 manual, which tells the tale.

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Also my transmission has a vacuum modulator delete as shown below

What is the "vacuum modulator delete" and how does it work? How is the line pressure controlled w/o it? Shifts and other things rely on such things. Does that trans have a manual valve body and/or is used for competition?

I guess I could Google "vacuum modulator delete", but...

Something has to modulate the line pressure in the THM400, and AFAIK that's the vacuum modulator's function. I think of it as being akin to the TV cable in the 700R4, which modulates the 700R4's line pressure according to the extension of the cable. In both cases, their function is to modulate the line pressure as function of the engine's delivered torque to, among other things, effect proper clutch timing / firmness.

Too, AFAIK in both transmissions the modulated line pressure works in concert the with the governor's pressure to effect shifts at the proper RPM/speed.

what I'm wondering is how can I use a stock modulator since my engine is boosted and being a diesel it is almost always in boost unless at idle. :Big Laugh:

I see the 1988 manual mentions the diesel and the THM400. I don't know if that diesel was blown or not, but if so the factory manual might provide you with some insight.
 

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Dr1rrrr

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thank you for your reply that was extremely helpful but as for how it shifts, magic maybe I from what I've read you are correct but it isn't full manual valve body I can put it in drive and despite all the stuff i stated it actually shifts decently if runs the gears out just a little more than I like and it doesn't like to down shift but i would think it would be a lot worse all things considered.
 

Dr1rrrr

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so here's what I found from looking at service manuals the 88 to 92 vehicles used and 6.2l none turbo diesel and before they went to the 6.5 turbo diesel the swapped from the turbo 400 to the 4l80e which eliminates the vacuum modulator and fixes the problem I'm bring up with it so I will have to look to the after market to solve this issue I know Chevy put turbo 400s behind 4bts in bread trucks so their has to be a solution I just don't know where to get service manual for those trucks
 

1998_K1500_Sub

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I will have to look to the after market to solve this issue I know Chevy put turbo 400s behind 4bts in bread trucks so their has to be a solution I just don't know where to get service manual for those trucks

I wish you the best of luck.

There are plenty of smart people on GMT400, hopefully someone can help you further :waytogo:
 

Erik the Awful

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Something has to modulate the line pressure in the THM400, and AFAIK that's the vacuum modulator's function. I think of it as being akin to the TV cable in the 700R4, which modulates the 700R4's line pressure according to the extension of the cable. In both cases, their function is to modulate the line pressure as function of the engine's delivered torque to, among other things, effect proper clutch timing / firmness.
In older transmissions, line pressure is a constant and it's balanced by throttle pressure. The throttle pressure works in opposition to the line pressure, and it's managed by the vacuum modulator. You can buy an adjustable vacuum modulator pretty cheap to adjust your shift points. You simply stick a hex key in the vacuum port and turn the screw inside to move all both of your shift points higher or lower. I have one in my Jag's TH400, but I haven't had to adjust it yet.


Edit: This was tickling my brain and I had to do some more research to clarify my memory.

Doh! Your asking how to get the vacuum modulator working with boost. I don't have an answer for that, but a quick Google search gives some options.

To explain the vacuum modulator a little better - throttle pressure is developed from line pressure by the vacuum modulator. Line pressure is more or less constant. The more vacuum present on the modulator, the lower the throttle pressure. If you romp on it, vacuum drops and the throttle pressure increases. At the same time line pressure is used to develop governor pressure. The faster your governor is spinning, the more pressure it produces to induce a shift. When the governor pressure overpowers the throttle pressure, the transmission shifts. Without a vacuum signal to the vacuum modulator your shifts are late and hard. It's a natural process of not having vacuum present, and it's also a good default if the vacuum modulator goes out. With a boosted operation, you'll always be defaulted to max throttle pressure. Some guys just run it, but I'd correct it before your transmission slams itself to death.

You don't necessarily need a kickdown solenoid, especially in a high-torque application like a diesel. My Jag has a big-block Caddy motor, and produces serious torque from idle. Dropping down a gear is detrimental to acceleration. Supposedly the kickdown solenoid does boost line pressure, and if you want that you just have to put a switch on your throttle pedal or linkage that provides 12v to the solenoid at full throttle.
 
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Schurkey

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A diesel CANNOT use a vacuum modulator since there is no manifold vacuum. Boost would be even worse.

The exception is a diesel with a vacuum pump--and even then, only if the vacuum is regulated based on engine load, to provide a pseudo-manifold-vacuum signal like a gasoline engine would have.

There used to be--and likely still are--kits to convert the vacuum system to a throttle-valve system, where the modulator valve isn't operated by a vacuum modulator any more, it's operated by a cable connected to the throttle lever.

Edit:

www.tsr-racing.com/th400-mechanical-modulator.html
 
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Dr1rrrr

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so it sounds like 1 I should defiantly get the modulator working and 2 the easiest and best way seems to be getting an throttle valve I've heard adjusting those can be tough but I'm hoping the companies making the conversion would have good instructions to do so
 

1998_K1500_Sub

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so it sounds like I should defiantly get the modulator working

I think so. How you accomplish that IDK, but I would pursue the "bread truck" solution since that's tried and true.

the easiest and best way seems to be getting an throttle valve I've heard adjusting those can be tough but I'm hoping the companies making the conversion would have good instructions to do so

IMHO that TV cable solution @Schurkey showed is best used on a carbureted gasoline engine. I think he mentioned it just for reference and not to suggest that it be used here. (@Schurkey, sorry if I put words in your mouth.)

The "bread truck solution" is likely your better bet.

For the TV cable option, the next question would be "What should be the attachment point for the throttle valve cable on the 4bt engine?"

Assuming the 4bt's injector pump is a mechanical one and has a "throttle lever" on it (I haven't looked at it myself yet), then... yeah, maybe attach it there, but...

- you'll probably find it best to attached the cable to the throttle lever so that you get the proper throw on the cable, e.g., presumably "fully in" at "idle" position on the lever and "fully extended" at "WOT". However, what happens in-between those settings will depend on the orientation of the cable to the throttle lever, and it''s not obvious (to me anyway) what the best orientation should be. The best orientation will depend somehow on the engine's delivered torque, based on the injector pump throttle lever's position.

- there's another assumption that the position of the injector pump's throttle lever is more or less directly related to the engine's delivered torque. IDK if this assumption is true (if the injector pump's got a governor it won't be directly related, but IMHO a truck injector wouldn't have a governor, whereas a tractor injector would).

- there are other details I can think of but will save for another time
 
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