85 300SD Too Much Vacuum to Transmission
 

Hi everyone, I'm currently working on making my transmission shift properly and did some testing at various points in the vacuum system. The output from the pump is within spec (22-24 inHg), however I believe I'm getting too much vacuum going to the VCV and too much at the output of the Vacuum Transducer (Blue UFO). I've attached a diagram with the locations I tested marked. I got about 22 inHg at point 1 at idle and while it did decrease with throttle I believe that is too high. At point 2 I also got about 22 inHg which I suspect to be high as well. If anyone knows the specifications for vacuum readings at those points on an 85 model it would be much appreciated. I'm also curious if the vacuum signal to the transmission is the same for the 722.3 and 722.4 because my engine and trans are from an 81-84 SD so I believe it has the 722.3.

Diagram with Test Points:
https://i.imgur.com/VmNiSs0.jpg

peterschmidtransmission.com

722.3 and 722.416 series transmission adjustments

Are all of the restricted orifices in diagram present and in the proper location. Those are 63, 63a and 64b in the diagram.

If you can describe what you believe to be im-proper shifting, it may be helpful.

It’s fairly erratic, sometimes it shifts alright but at full throttle it won’t upshift until I let off and it occasionally flares when upshifting. I read a PDF I found here about transmission tuning and there’s only a short page at the end about the ‘85 system and I’m not sure if they’re supposed to have 10” of vacuum at the trans at idle like the older system. I have a red restrictor at 63 and a black one at 63a. I also see that the Transducer has an adjustment screw, could that be part of my issue?

Shift points are controlled by the tension on the bowden cable. If it's adjusted too tight, your shifts will be late and can be excessively harsh.

Vacuum at idle on the vacuum amplifier setup is usually somewhere around 12-15" of vacuum. Higher isn't necessarily bad at idle. You should see the vacuum rapidly drop to zero on the line to the transmission modulator when the accelerator is actuated. By 1/4 throttle or so, you should have no vacuum at the modulator if everything is adjusted properly.

It's important to know what the vacuum levels are at the VCV line on the blue UFO to properly diagnose its operation. You haven't given us that info yet. Without boost, the blue UFO output to the transmission modulator should largely correspond to the vacuum level at the VCV line. The blue UFO is there to refine the shifting based on engine load (boost pressure) rather than solely from throttle position like the earlier cars.

Don't adjust the blue UFO. They can wear out internally. If you always have full vacuum going to the transmission modulator regardless of what the throttle position is and the vacuum level from the VCV line is, you could have a failed blue UFO. Rule out all other possibilities first.

I’ll take some more measurements and post them here tomorrow. When I tested the vacuum at the line from the UFO to the trans it did decrease with throttle but only to about 10-15” at nearly full throttle. I’m not sure if I tested the line to the VCV on the injection pump with varying throttle. Is it supposed to pull that entire light red section labeled Vacuum (2) on the diagram to atmospheric pressure?

You need to measure vacuum at the VCV nipple on the blue UFO. The VCV drops the vacuum in the red line connected to it to atmospheric pressure. The switchover valve shown in the circuit is to bypass the VCV and apply full vacuum to the blue UFO during cold starts to help with the rough 1-2 shifting. It is SUPPOSED to drop out when the block heats up and allow the VCV to operate normally. If it is remaining energized, or is stuck, you can have full vacuum applied to the VCV nipple at all times.

Just did some more testing, the vacuum at the VCV nipple was 16" at idle and only dropped to about 8-10 at almost full throttle. I got the same measurements at the line to the VCV. (location 2 in the diagram). I let the car warm up to about 90 degrees celsius or so and the switchover never activated, there was still 12V at both terminals.