1991 pontiac bonneville transmision problem

the 91 bonneville stopped having forward gears but was drove back in reverse about 4 miles,reverse appears to work fine does this thing need new transmission or is there an easier fix

Years ago I had a '78 Caprice that did the same thing. I found the trans filter was almost completely clogged with debris, since it had never been changed.

So, I suggest drop the transmission pan and have a look If its very dirty, change the filter, clean out the pan and refill with new fluid. This might fix the problem, but no guaretees.

If you are very handy, try removing the valve body and blowing it out with compressed air, since, you may have a valve hung open due to debris.

Removing the valve body on this car is not going to happen unless the sub frame is leaned on the drivers side enough to get the side pan off... And even then you might just as well remove the trans..

This is and was common of the THM440-T4( AKA 4T60 ) Transmission... Reverse is a Manual apply,

Try Manual 1st gear and see if the car moves... If not you might just consider the trans done..

The manual pressure valve issues with these transmissions is one of the reasons GM went to an Electronic Version... The 4T60E is simply an upgraded a little and has electronic controls, These transmissions share a lot of parts...

if nit does have manual 1st gear then what am i looking at

if it does have manual1st gear then what am i looking at

Did you ever get a diagnosis of this problem? I have a '90 Celebrity with the same transmission (4T60), with the same symptom (reverse, but no forward gears). Although I wasn't driving it when it happened, I believe it rather sudden instead of slipping for a while first.

The pressure levels are fine, so it's not the pump. I'm assuming the servo seal must OK (???) if the pressure is good, so my guess right now is either no hydraulic flow to the servo, or the band is worn.

No matter what this is a major internal failure, so it must come out and apart.

Yes, it most likely needs to come out. I just hate going in on something so time consuming without a good idea of the problem, and especially if it's something subtle (like a check valve ball) that might not be found after taking it out.

Thanks for the info.