Transmisson slams into reverse

[Campbell]

I have a 98 ranger 3.0 automatic,when i got this truck there was no engine or tranny,i installed another engine and tranny, and it runs like a million bucks,on occaision when putting into reverse it slams hard like the mount is broken,also the speedo sits at 20-25 when the engine is off and when battery is disconected it goes back to 0,hook battery back up and it goes to 20-25 mph,also the oil pressure guage after it has been running for awhile the guage goes crazy back and forth,replaced pressure sending unit and guage in dash,oil pressure is fine 20 psi hot,what is going on? im at my witts endHELP!

 

[superj]

add a ground to the cluster to see if it fixes the gauge issues.

 

[RonD]

Did you just replace the oil gauge or the whole cluster

Whole cluster might be needed, 1996-2003 are plug and play

The speedometer does have 12volts full time, fuse 25 in cab fuse box, but its electric motor should be OFF or it will run down the battery

Oil gauge is either not grounded 0psi, or grounded "Normal" range on gauge
The sender on the engine is an On/Off setup, below 6psi no ground, above 6psi grounded full time
So the jumping around "could be" the ground, but because of the speedometer issue it could be a problem in the 12v distribution in the cluster as well