Electrical, Overdrive, Tranny

Gearvendor Controller Broke? How I Replaced my Controller with a Simple Circuit

Gearvender Overdrives are a great addition to the rock-solid C6 transmission on these 1980’s Fords.  It helps bring the RPM’s down, helps elongate the life of the engine and transmission, improves MPG, and all that jazz. I’ve written a lot about my GVOD, including advice on how to shop for a used one, how to install a Maximum Overdrive-brand Deep Sump, how to access the internal filter, and its impact on improving fuel economy.

The only downside to adding a used GVOD is access to replacement parts.  Buying parts from Gearvendor HQ is an option, but they ain’t cheap.  And for the average 80’s truck/van owner, they are not driving these things for the nostalgia.  So I’m always looking for alternatives to the old parts, whenever possible.

The Tailshaft Housing is the blue part in this image. It is required for installation of a Gearvender Overdrive Unit.

The Tailshaft Housing is the blue part in this image. It is required for installation of a Gearvender Overdrive Unit.

The GVOD is more than just the overdrive unit.  It is comprised of the OD, tailshaft housing, GM yoke, tailshaft seal, speedocable adapter and extension, electronic controller, manual/automatic override switch, and a few other minor accessories.  By far the most difficult items to obtain are the tailshaft housing and the electronic controller.  The tailshaft housings are usually ignored by junkyards, because they resemble the C6 housing so closely.  So when the GVOD unit is put for sale, they usually are divorced from their respective tailshaft housing, which incidentally is crucial in the installation of the unit.

The other item that is difficult to find is the Gearvendor electronic controller.  Well, I should amend my statement; the controller is easy to obtain, but will it work?  Usually not.  The electronics on these things are thirty years old; it doesn’t take much for a capacitor or resistor to fail, rendering the unit dysfunctional.  My controller arrived from the junkyard working ok, but after a few months, it started randomly shutting down and starting up.  It was a pain, because on the highway, the last thing I wanted was a surging overdrive.

I took my controller to a local electronics shop, and they quoted me a few hundred dollars to investigate the device and solder in whatever component failed.  And even then, if they replace that one electrical whatchamabobber, there was no guarantee that the other whatchamabobbers would fail shortly after.  I even asked them to reconstruct the existing controller with all new components, but the quote was so high, I contemplated learning how to build electrical circuit boards myself.

I’ve mentioned my simple-circuit replacement on several IDI forums, and have been asked about it on occasion.  Recently, I received a question from FTE user Jake_TheGreat regarding how I wired up my Gearvendor Overdrive (GVOD).  I thought this would be a great opportunity to publish our conversation, so others could benefit this information.

From Jake_TheGreat:

  • Hey Genscripter,
    I’m currently trying to figure out how to wire up the solenoid manually for the Gear Vendor. I know that the solenoid has 2 posts, I purchased a simple On/Off switch. Do i need to run a line from the battery to a fuse, then to the switch, then the switch to the solenoid? Then ground out the solenoid?Sorry, I’ve wired up lights before but never a solenoid.

I responded:

  • Your plan is exactly what I did, after my controller broke. The solenoid runs on 12v, so a simple switch circuit will work for you. It’s pretty simple. If you have set up lights, then you’ll be able to wire this up easy.


  • Thanks a lot. I managed to get it wired up and it works! Which is great news.


  • Glad I was able to help. I remember the day my GV controller busted. It was randomly turning off and on, so I pulled off in Mesquite NV in the desert. I had read on the forum from years ago that some guy had a similar problem, and he ran a simple wire switch circuit which he claimed to work fine. I figured I was stuck in the desert so I would give his strategy a shot. If it didn’t work then I’d be driving without the OD regardless. Luckily, it worked fine.I should have mentioned that you should have wired it up to “key-on” power. I’m sure you did, but if not, you might want to consider rewiring it to a relay on something like your radio power supply wire. If you run it straight from the battery, you might forget to turn off the OD when you come to a stop. If you leave the OD on while in park or with the vehicle off, the vehicle won’t be “in park.” With the OD powered on, it will let the vehicle coast as if it’s in neutral. I had that happen to me once and it was super scary. Luckily, I was still at the helm, so I just put on the brake and then turned off the OD. From that point on, I had my OD powered on a “key-on” circuit with a relay. You can leave your current wiring in place, just put a relay spliced in on something like the radio power supply.

    If you got that done, then another luxury item would be something I did to prevent driving my OD in reverse. I put an NC relay on my reverse light circuit, and then spliced that into my GVOD solenoid circuit. Basically, if the reverse lights come on, that would mean the driver put it in reverse, that would mean the NC relay would “open” the GVOD circuit. Just in case you don’t know, “NC” means “normally-closed” which means until the relay is powered up, the circuit is always connected. Once the relay is powered up, then it breaks the circuit, thus disabling the GVOD. this is a pretty decent upgrade if you expect to have some guest drivers who are not as acclimated as you with your own vehicle and its unique overdrive switch. You don’t want a guest driver to slow down on the highway without killing the OD power, and accidentally back up into a parking spot and destroy your GVOD.

    I’d also recommend upgrading to the far superior Maximum Overdrive Deep Sump. I have a post about that here: https://www.nickpisca.com/diesel/engi…mum-overdrive/ With our big trucks and vans, the GVOD was designed for smaller vehicles, and up until the P-type, they were used in things like little Volvos and whatnot. Even the P-type is a little undersized for a full size Ford Truck, especially under load. Ever since I installed M.O.D.S., I never exceeded 160 deg F overdrive temp, which had extended the life of my unit and increased the oil capacity by 1.5 quarts. Also the cooling fins help too.


  • So, the only thing that will truly destroy the GVOD is reversing with it engaged? Taking off with it engaged will work, but it will be incredibly sluggish? Also I did not know what an NC relay is, so thank you for explaining that.


  • Well… more things will grenade a GVOD. Going in reverse was the biggest no-no according to GV Headquarters, whom I’ve called several times when repairing and maintaining my GV. But other things will destroy it as well, like not cleaning the filter regularly. Something like every 15,000 miles, IIRC. Also, running the unit low on oil. The tailshaft seal (it’s a GM seal, i forget the part number) will degrade over time and let some of the GVOD oil leak out. If you don’t regularly top it off, it can get dangerously low, thus causing long-term abrasion damage. That’s also why I love the Maximum Overdrive deep sump, because it increases the capacity to a level that would make it very hard to forget. As for starting in overdrive, I didn’t read anything in particular saying it’s a bad thing, but I just wouldn’t do it out of personal preference. I know guys run these over- and underdrives on all sorts of performance vehicles so they get the advantage of splitting gears, so in any forward-drive, it should be a robust unit. As for performance on the starting line, with the OD engaged, it’ll likely act like the vehicle is under a moderate load. That’s nothing for these IDI engines, so you’d likely not experience much for sluggishness, but there would be a little reduction in power. 

I’d like to thank Jake for his great questions and letting me publish our interchange here for others to peruse.  I’ll summarize my major points in a list below.

The 2 spade connectors on the Gearvendor Solenoid are in the upper left of the image.

The 2 spade connectors on the Gearvendor Solenoid are in the upper left of the image.

I forgot to mention that the Gearvender solenoid has two spade connectors, and it didn’t matter which way the circuit was wired.  I am not 100% sure, but I think I wired it up positive to the top and negative to the bottom spade connectors, and both ways activated the solenoid fine.

To briefly recap, these are crucial factors when using and modifying the Gearvendor system, and any deviation from this will lead to severe damage to the overdrive, engine, transmission, vehicle, and possibly the occupants and nearby bystanders.  Have your vehicle maintained by a professional if this seems too difficult for you.  You don’t want to accidentally run over a person by leaving the GVOD powered on when you walk away from your parked truck.

  • Wire the solenoid on a “Key-On” power supply ONLY.
  • Use relays to prevent overloading the stock “Key-On” circuit.
  • Use an NC-relay on the taillight or reverse power circuits to disengage the GVOD if someone accidentally puts the vehicle in reverse while the unit is powered on.
  • Never drive the vehicle in reverse with the GVOD powered on.
  • Never let the oil level go low and make sure the filter is cleaned  regularly.
  • Give the vehicle some accelerator when turning the GVOD on or off.  It’s like shifting a bike: pedal through the gears.  Failure to accelerate when turning the GVOD on or off will result a clunk that, over time, will damage the unit.
  • NEVER EVER leave the GVOD powered ON in Park or with the vehicle OFF.  IT WILL NOT BE IN PARK AND WILL ROLL THE VEHICLE.

My Long Term Plan:

One of my long term goals is to run a small tablet with a raspberry pi or PC Controls receiver allowing me to retrieve sensor information from all over my van, and render it on a custom display.  This would allow me to eliminate a lot of the disparate gauges scattered all over my dash, and showcase my vitals on the screen.  It could do things like display my speed, engine temp, tranny temp, intake temp, overdrive temp, boost levels, EGT’s, IP inlet temp, biofuel temp, biofuel capacity, fuel pressure, and all sorts of values.

I could also use this tablet to automate some of the little things on this non-computerized IDI van.  Like for example, I could have it automatically switch on a relay to turn on the GVOD at a predetermined speed, like 45mph.  And when I dip below 40mph, it can turn off the OD.  Could be a fun project.  I would have started working on this already if I hadn’t burnt up all my spare time this winter and spring rebuilding my 7.3 and starting up the VRV prototyping projects.  Until then, I’ll be switching my GVOD with the manual for now.


No warranty. You are responsible for your vehicle. For novelty use only. Not responsible for anything or anyone. Not responsible for damage to your vehicle, you, or anyone or anything.

Copyright 2000-2018 Nick Pisca 0001D LLC



Steven Longmire

Hey, great site. I’m trying to milk my E350 for all the efficiency I can get. I’ve got a massive steel bumper with a winch on the front, but it’s got so much room, I could easily fabricate some way to attach an air dam under it that I could remove when off-road (since it’ll be going to baja and the mountains). Lots of awesome inspirational information. Thanks for the link from OilBurners


Thanks for the kind words.

I have more eco-mods on this article, that might help you.

These IDI vans can go everywhere. I just got back from a 12,000-mile trip from Los Angeles to Tuktoyaktuk in the Canadian arctic. I’d love to invest some time to put in a front wheel drive and adding rear posi. That would help a bit when the conditions get muddy.


“”but I think I wired it up positive to the top and positive to the bottom spade connectors””, So you put + 12v on both connections on the solenoid at the same time?


My bad. I mistyped. That should read that one side is positive and the other connector is negative. I’ve wired it both ways now, and it is irrelevant how the electrical flow is for that solenoid.


What size fuse did you use


I think it was a 5amp. I usually don’t use a big fuse for a small solenoid

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