Can someone share the proper vacuums one should see if I tested with a vacuum gauge?
First off...I am going to claim ignorance and humbleness and say I don't know...and I have not seen any factory data on what the vacuum should be....
But I will claim some experience with diesel calibration C-6's firsthand from the 80's and 90's...
An original diesel C-6 has a slightly different valvebody than a gas engine one...probably because of the expected operating parameters with the lower RPM's and more dwell time in each gear...and the tendency to hold the throttle closer to WFO longer...
Also...with the VRV controlling the vacuum to the Vacuum Modulator I expect that the engineers probably took into consideration the constant vacuum signal from the vacuum pump instead of the normal rise and fall busyness inherent with the gas engines...the vacuum modulator type and pin length was tailored for diesel, as well...
But...
Back in the 80's and 90's it was common for busier transmission shops to just toss in an exchange unit and try to 'adjust' it to work with the application...those that insisted on this probably had a lot of time spent on road tests and such...when they should have just custom built the original unit and had done with it.
So...what I am saying here is that the unit stands a good chance of not having the right valvebody and governor assy for the truck and engine. If it is all original then you are good for starters...
This is the long way to explain that you should adjust your expectations of what you really can achieve here...but if you are fortunate...you just might be able to get it satisfactory...
When it comes to vacuum signal to a transmission with a vacuum modulator...the size of the diaphragm and the spring tension in the can is critical to its response...newer cans have a preload adjustment...but it is a fine adjustment ...not a way to make the wrong modulator work where it is not supposed to be.
A good general rule on vacuum signal is that if your pump can muster 19-20 inches of mercury before it is regulated at the VRV then you have a good start...you might be able to get by with less...but remember that less vacuum is translated to more percieved 'load' and will delay the upshift relative to vehicle speed and throttle position.
Once the vacuum gets to the VRV it gets reduced...not increased...you have what you have going in...and it gets worked with at the VRV...it is just a matter of how much reduction it produces relative to throttle position...it varies up and down and you have some provision for adjustment there...by turning the VRV forwards and backwards.
Now to testing vacuum....first...put vise grips on the part of the vacuum line that goes to the brake booster...this eliminates the possibility of a leaking booster making you chase your tail...
If it were me...I would put two 'T's in the vacuum lines and cap 'em...one for the source vacuum and the other for the signal vacuum...check the source with a guage first and find out how the pump is doing at all RPM ranges...steady, strong vacuum...? you are good. Signal low and varies with RPM...eeennnhhhh....you are just setting yourself for a lot of test driving and frustration...
See to it that you have the correct diesel application modulator...do not rely on the kid at the parts store that can't focus on anything but his device...seek out a good transmission pro on that...
When you install the can...look to see that the pin that connects it to the valve in the case is not too long...if you have a LOT of preload...not real good...someone tried to compensate for another problem...
When you got that straight...cap the source vacuum and uncap the signal vacuum and look at that while raising and lowering the engine speed....Idle...high vacuum....WFO...low vacuum...I would consider somewhere around 17" HG to be good at idle and somewhere around 2-3-4-" Hg to be adequate at WFO...and the sweep of the guage needle should be smooth and steady relative to throttle position...
You will find this method in the shift kit instuctions provided by Transco in their diesel C-6 valvebody recalibration package...by far the best shift kit company I used to deal with..if your valvebody is not an original diesel one...their kit will enable you to convert a gas VB to a diesel one....should you find you need it...and...it makes the transmission response more forgiving...
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Good luck...Skylabtech...