1986 M1008, inconsistant shifting

[smokeeater495]

I have a 1986 M1008 with a little over 50K miles. Since I have had the truck it shifts pretty hard, chirping the tires sometimes at the 1-2 shift. 1-2 and 2-3 almost always hit pretty hard. This morning I took my time getting off work and let it run and warm up for about 30 minutes. On the drive home the shifts were buttery smooth. Any thoughts as to what could be going on? I've driven the truck around town for a few months and it definitely got up to temp before.

 

[Keith_J]

Have you a vacuum gauge, vacuum line tees and extra vacuum hose? Use it to measure vacuum at the pump, then at the vacuum regulator. If that checks out in accordance with the TM, then it could be transmission vacuum modulator.

 

[smokeeater495]

Vacuum pump was brand new when i bought the truck a few months ago. i'll look at vacuum modulator. If it was sticking could that cause it to shift to hard? Then getting it good and warm may have broken it loose.

 

[Keith_J]

The vacuum regulator attached to the injection pump can be wonky. They get dirty and refuse to work. I'm batting 500 at repairing them, the only success was washing it in denatured alcohol.

Remember, to regulate vacuum, it must admit air into the system. That air comes from the dirty under hood area, without good filtration. Yes, it could be thermally sensitive.

I've made a replica of the gauge block mentioned in the TM to hold the throttle shaft in a certain position. Then vacuum is applied to the bottom port and a vacuum gauge to the top port. Adjust the valve for 8 inches of mercury.

 

[lostintexas]

Mine was shifting hard, much like you described. Found some broken vacuum lines and replaced. Now you can't even feel the shifts she so smooth.