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|All Messages in Thread||Author||Date|
|Allison VS2 Transmission ?||Chuck||1-9-10|
|I am running a vintage (70s?) Detroit Diesel non turbo 671 motor and a Allison VS2 Transmission in my 1942 GMC conversion Motorhome. The current transmission shift points are creating a serious power loss and therefore a performance issue that I am tryng to remedy. |
The Allison is a 2 speed with a "fluid drive" 1st gear and then a direct drive with a overdrive for freeway speeds. The rear end gear is long legged and it runs 65mph at 1600 RPM. The issue lies with a shift out of "fluid drive" 1st gear to direct drive - occuring at 1600 RPM (the motor is just coming into it's power curve when it shifts). If this shift takes place on a incline the vehicle immediately loses power and starts to descelerate. The vehicle will shift back to fluid drive at about 600 RPM (20mph) and the cycle continues (600-1600-600-1600) until the incline is crested.
The other gear shift points are at 2200 RPMs which is perfect for the 671s torque curve. Is there some way I can adjust manually that fluid drive 1st shift so it occurs inside the engines power range?
I have asked every transmisson shop and diesel mechanic locally here and no one has been able to shed any light on this. - Thanks for your time, Chuck Sutfin email@example.com
|Re: Allison VS2 Transmission ?||BUBBA THE BUS TECH||1-11-10|
|Two things 1st it is tuned this way to reduce the huge jerk these transmissions create when shifting at higher RPM from first to second. 2nd this retune is quite dificult and requires a transmission expert that used to fix problem units back in the day. The last one I knew retired a few years ago. When we drove these in service the technique was to not get into this situation or keep it in first by use of throttle feathering untill you could get into second and power band. Has it over heated doing this yet? Sorry and Good Luck|
|Re: Allison VS2 Transmission ?||keith shepard||4-28-17|
|I have a 1969 gm transit bus conversion vs2-8 transmission|
looks to be over full of tranny fluid at idle in neutral
do I check the level while engine is off or running?
bus sometimes wont shift to reverse thought maybe to much fluid could cause this
any help would be a big help
|Re: Allison VS2 Transmission ?||Chuck||1-12-10|
Thanks for the quick reply. Answering your first question - This was the configuration it was in when I purchased it. The previous owner (builder) had on a couple of occasions (crossing some big grade mountain passes) some overheating issues with it, which is probably due to this poor shifting range condition.
I have the Allison Service Manual for the VS2 and the recommended up shift point from 1st to 2nd is 1300 RPM and my VS2 right now is already shifting out of hydraulic (1st) at 1600 so I am pushing it already as you said. I noticed today that once bogged down after the shift to 2nd it drops all the way down to 700 RPM (a painful 15mph) before shifting back to hydraulic drive (1st) which could be an adjustment that would probably help alot because the Service Manual calls for a 1150 RPM downshift not 700 RPM. Still not much of a first gear 1150 to 1600.....ugh
Maybe a new gear for the rear end is in order, one not so long legged I suppose? Then the issue becomes where to find one?
Thanks for your input - Chuck