Shifting a diesel th400

[obijuank5]

I have a truck I need to fix that has been swapped to a 4bt.
It's an 87 k30 with ac and originally a th400 truck. The 4bt is not long enough to install a vac pump from a 6bt so I'm looking at electric options to run a vac system. Most likely a pump from a Ford 6.0l diesel.

How the hell does the th400 shift? if it has a constant vacuum will it shift ok? I have always thought the modulator does the shift control but they were gas trucks where vacuum varies.

Then the ac system I assume works off of constant vacuum but it would be nice to have the trans shift before the ac.

Am I at least on the right track applying full vacuum all the time?

 

[Justin V]

@Greg Ducato

 

[black dawg]

The diesels used a VRV on the injection pump, full vac to vrv, and it provided vac to the trans similar to what a gasser would, to create shift points. Full vac to trans will make super early shift points.

vrv acted like a tps, except creating a vac signal instead of voltage.
somebody does make a cable actuated vac modulator for a th400

 

[sweetk30]

why not vac pump from a 7.3 ford diesel that runs off the engine serp belt . whip up a little bracket .

and i think they make a kit to convert from vac modulator to a cable .

 

[dyeager535]

Modulator is what determines line pressure based on (theoretical) engine load. High vacuum (light throttle), light shifts. Low vacuum (heavy throttle), harder shifting.

Governor is the main mechanism that determines shift points. It will shift without vacuum, at maximum line pressure IIRC. But if the governor weights stick, it won't shift.

Others have expressed different results, however my experience with a TH400 that the vacuum line never stayed on was that I could not tell in regular driving that the line was on or off. Under full throttle it might have given me 5MPH more before it shifted. I had installed a shift kit, and the shifts with or without the vacuum line felt the same.

 

[Greg Ducato]

The vacuum controls line pressure according to throttle demand and acts against the shift valve and governor pressure to determine shift points. If you run constant vacuum you will have very low line pressure and sloppy, early shifts. If you have a low rpm engine like the 4BT you can experiment by adding weight to the small inner weights of the governor until you reach a satisfactory shift point without any vacuum. This was common practice in diesel converted Bread and Delivery trucks in years gone by. We still do it for some custom diesel mining trucks we build transmissions for. There are also mechanical modulators on the market that can be used with a tv cable to vary pressure if you are so inclined. We use a spring behind the modulator valve in the case and an adjustable modulator to keep line pressure around 150psi constant on these applications. You can accomplish the same thing by grinding on the modulator valve end where it fits up into the vacuum modulator to make it a bit shorter, just a little at a time.

 

[fourxpete]

on my 88 with a 6.2 it had something like a tps sensor that would modulate the vacuum to the tranny ill get a pic of it tomorrow for you

 

[diesel4me]

Greg Ducato fixed me up with a modified govenor for my TH400 /6.2 ,and the spring he mentioned that goes behind the modulator valve--and that made it shift much better--the vacuum pump on it was dying when I first got it,and another one a member here sent me worked for a few months ,then the vacuum started dropping and was not constant,I think the diaphram has a hole in it--it was fluttering between 5 and 10" of vacuum and the modulator .

When I first got the truck and it refused to shift into 3rd until you hit about 45-50 mph,and if you let off the throttle before it had upshifted,it would free wheel in second,until you apply the throttle a little bit..(still does that ,but to a much lesser degree).
I tried by-passing the vacuum regulator valve that varies the amount of vacuum to the modulator and adjusted the screw inside the hose nipple on it to firm up the shifts..
This got me by until the vacuum pump failed completely..then I got the 2nd pump, and Greg sent me the fore mentioned parts and that made it upshift into 3rd around 28-32 mph,which was much better than 45-50 mph ..
I also hooked the vacuum regulator valve back up...now the pump isn't putting out full vacuum.but it still shifts at the same speeds,the govenor weights and that stronger modulator valve spring seems to make the modulator not so important..it shifts very firm too,no slippage..

There are vacuum pumps for 6.2's that were belt driven on the later ones,I have seen a few in salvage yards,one was on a G series van..the VRV valve on the injector pump is getting hard to find new,and costs over $100,so if you see any used ones in junk yards I'd grab them,if they'll let you "break up" a complete injector pump..

Or you can use the cable operated modulator,which last time I looked were selling for about $125..I think some military CUCV's used those too,instead of vacuum pumps..

 

[Fordum]

Don't really have a dog in this hunt.
The only two diesels in my life are my 100KW genset, no shifting or brakes involved there. Its not going anywhere.
And my Mahindra tractor. Manual shift and mechanical brakes. Plus if you have a disc harrow or plow hooked up, dropping the lift stops you really quickly.

But just out of curiosity, I did a search.
Only looked at the first hundred, but saw plenty of both electric and belt driven pumps for under $100. It would require some mounting work, adding a pulley and belt for the engine driven one but once installed should fix the problem.
https://www.summitracing.com/search/part-type/vacuum-pumps-street?PageSize=100

 

[fourxpete]

got these off pm if you want them came off 6.2 w 400 tranny

 

[obijuank5]

Thanks but this truck has a 4bt. Understanding of the system makes these parts make a little more sense. The mechanical modulator with a 700r4 tv cable seems to be the ticket on this truck.
Thanks for the guidance but I have yet to actually click buy it now.