Hi all,
my 89 5.3 gearbox makes a humming noise on the overrun. As the car has had a lot of refurbishment work done I'm keen to eliminate any basic things before I start major surgery. The cable was adjusted and I wonder if this could be something to do with it. Will double check the fluid level as it's been changed not long ago. Is there anything else it could be? Drives fine. Many thanks in advance

 

What cable? The shift points on the TH400 are controlled by vacuum. If you mean the shifter cable, that will have no effect.

 

Ok thanks that rules the cable out. I feel its something simple but not sure what to check. Only happens on deceleration.

 

Probably normal, as you are probably hearing the one way clutches (or sometimes called a sprag) inside. I don't have a car with a TH400 anymore to go have a listen.

 

Just dipped the trans fluid and it looks a bit high , would this have any effect do you think?

 

A little high is probably fine. Was the fluid hot ( at least 15 miles of driving), engine running and in park when you checked it?

 

No. I checked it cold.

 

I think there is a cold and hot mark on the stick isn't there? Hot is considered more accurate.

 

I will check it again hot , probably at the weekend. Do you think this ayup be the cause of the problem?

 

If it drives ok, you may not have a problem! Are you 100% sure it's the transmission? The rear differential can have a humming sound under coast if the backlash isn't quite right. The sound will go away under power and come back when coasting. Might be a U joint too.