Can someone educate me on proper operation of an under/overdrive trans?

[89square]

I picked up my 1989 R3500 crew cab dually on Christmas eve with 34000 original miles from the original owner. I've never owned a 1 ton truck so this is a new world for me. They left me a binder with some info about the truck, one thing stuck out is that it has a Gear Vendor GV-400 2 speed under/overdrive trans installed, not sure if this was a factory option or if they had it put in.

I've done some reading and I think I understand the concept of the overdrive trans, but I'm not sure I fully understand how to operate this thing and was hoping you guys could set me straight. It has a foot activated switch just to the left of the brake pedal and a control module installed on the dash, the control module has a standby light and a toggle switch that is labeled 20 on the left and 40 on the right. The notes that the owner left me stated that the overdrive should be engaged at all times when not pulling a load. Is this sound advice? Additionally, when I push the switch in on the floor board, it will turn the standby light off and on, on the control module. If the standby light is on, is the overdrive engaged? Whats the difference between the 20 and the 40 on the toggle switch?

Also, they noted that the overdrive trans was due to be serviced, anything special I need to be aware of when I take it in? The only other info I have related to it was that in 2014 they had the transmission overhauled and had a HD Allison torque converter installed.

Thanks in advance!

 

[Matt69olds]

I have a gear vender in my Olds, it works well. If you leave it in the auto mode, it engages around 45ish mph, and automatically disengages at 20. If you put it in the manual mode, you can engage and disengage at will any speed above 20 mph. You really shouldn’t run it without the controller, it prevents damage from low oil pressure. The pump in the gear vender is operated by the driveshaft, if vehicle speed is too low, it won’t spin the pump fast enough to develop the needed pressure. No pressure, things slip, and it’s all downhill from there.

As for the benefits, let’s say your pulling a heavy load. There is a slight hill coming. You could either downshift the engine to pull the load up the hill, which isn’t really needed for the small increase in power needed for the hill. Or, keep the gear vender in overdrive and pull the trans down one gear. Basically, you “split” the gear ratio between 2nd gear and direct drive. In other words, you now have a 6 speed trans. Imagine pulling away from a light with a heavy load. Pull the trans into low gear, when the engine has past the sweet spot in the torque curve, engage the overdrive. Once again, back into the sweet spot of the torque curve. More speed, kick off the overdrive, put the trans in 2nd, more speed, engage overdrive, and so on until you have the trans in the highest gear.

Hope that helps. I’m sure there are YouTube videos that can show the benefits far better than I can explain them. I have had mine in my car for probably 9 years, it’s the only thing I have ever successfully bid on eBay.

 

[Blue Ox]

Pretty much what Matt said. You can leave it to it's own devices, it will engage and disengage itself around 20 MPH or 40 depending on your switch setting. Personally I find it tends to be harsh engaging and disengaging depending on throttle position so I operate mine manually with the foot switch. I also rarely use it for splitting except the occasional downshift from third to second OD. Also GV does not recommend coasting in OD when towing or heavily loaded.

Maintenance; it requires a specific synthetic you can get from either GM or Jezus Chrysler. But it's basically change fluid, clean filters. You also need a spanner to pull the filter out. Check GV's site or the manual for the fluid spec.