Automatic Transmission weird shifting

Automatic Transmission weird shifting

1990 Ford Ranger 4X2 Super Cab V 4.0L
A4LD Transmission

My vacuum modulator failed, automatic transmission fluid was being sucked
into the intake. The part has been bad for a good while about a year. I
discovered it as my fluid level has been slowly dropping over the last year.
The truck ran fine.

I purchase the part, Niehoff # TM115 fits 1987 - 1990 Ranger, when I ordered
the part, there were two to choose from; Niehoff also had a TM332 which was
for 1983- 1996 Ranger. Not sure what the differences are, altitude?

When I went to replace it things were tight with the cat so I took it to a
transmission shop gave them the part and they installed it. The also changed
the few inches of vacuum hose at each end of the hard-line as they were full
of transmission fluid.

As I drove away from the shop I noticed that at 15 mph it would shift into
2nd (which is normal), then at about 22mph the rpms would go up a good bit,
was it shifting back to 1st? Then after 1-2 seconds it would shift back into
2nd. Then as I would pick up speed it would go into third. This was Sat and
the shop closed as I left, so I couldn't go back.

It repeats this pattern every time, I drove around all day and it would do
it every time. If I took off and kept my foot into the throttle a bit it
would not happen. It happens if just drive easy.

The truck never did this before. At first I thought that I was getting use
to normal shifting since the vacuum modulator was replaced, but the more I
drove it I know that this rpm increase, slipping sensation was not normal.

So what could be wrong? Did I put in the wrong part? Is the new part bad? Is
there an adjustment involved?

And I did check to see that my transmission fluid is at the correct level.