Gmt400 cruise control

[Bonanza7895]

I have read on various forums that you can use a gmt400 cruise module in a squarebody and I can't find too much info on it. Wondering if anyone has done it and if anyone has any wiring or other information on it. This would be on my 78 with no cruise. I live in Canada and the other cruise options are 1000+ dollars .

 

[legopnuematic]

I recall @Bextreme04 has talked about adapting the GMT400 cruise setup before.

 

[Bextreme04]

I have read on various forums that you can use a gmt400 cruise module in a squarebody and I can't find too much info on it. Wondering if anyone has done it and if anyone has any wiring or other information on it. This would be on my 78 with no cruise. I live in Canada and the other cruise options are 1000+ dollars .

You would only need a rostra or similar cruise stalk, some relays, and a GMT-400/600/800 module that can attach to your carb. The only hard part will be getting a VSS signal. What transmission do you currently have?

 

[Bonanza7895]

My truck is 4speed 2wd. Could it be done with a factory 78 cruise switch?

 

[VTXRider]

I have read on various forums that you can use a gmt400 cruise module in a squarebody and I can't find too much info on it. Wondering if anyone has done it and if anyone has any wiring or other information on it. This would be on my 78 with no cruise. I live in Canada and the other cruise options are 1000+ dollars .

I've done this on my 1980 K2500 4x4, and have a ton of info on the cruise systems. I have several spare cruise control modules as well. It is a reasonably easy install once you have all the components and wiring diagrams.
You will need:
-the cruise control module - auto wrecker, or I can sell you one of my spares.
-correct length cable from the module to the throttle - auto wrecker
-cruise control cable bracket mount at the carb. (i have Edelbrock ProFlo on mine and made up my own)
-a stalk switch - auto wreckers have various models that you can refit or modify to fit your column
-a speed signal to match the cruise control module. I have a 2 wire signal generator for my overdrive that provides a
signal to a Dakota Digital signal generator to get the correct signal. Likely the most expensive part you'll need.
-the wiring diagram.
i did a lot of research on this and have multiple cruise control manuals, diagnostic charts, and I think 7 modules that
have all bench tested as good, with the 10pin harness connector.
Mine works well. I can shoot you some pictures if you are interested. I'm in Canada as well, BC just out of Vancouver.

 

[Bextreme04]

My truck is 4speed 2wd. Could it be done with a factory 78 cruise switch?

The early cruise switch was just a single button. It won't provide the correct signals for a modern cruise control. You could use the factory one if you are ok with only having the "set" feature. The rostra or dakota digital one looks like a factory GM switch and attaches at the factory location. This is the one I have in my '80.

https://www.summitracing.com/parts/...h9uUpTmID3MLzGJCiDy1me-q0Nk-QqmOg3IViwbJj360k

Also, if you don't "@" someone(type the "@" symbol and then their screen name to tag them in a post like this @Bonanza7895 ) or reply to their post in your post, they won't know that you are talking to them and won't get a notification that will take them to the thread being discussed.

 

[Bonanza7895]

Mine works well. I can shoot you some pictures if you are interested. I'm in Canada as well, BC just out of Vancouver.

@VTXRider You could message me that would be greatly appreciated. I have a few questions you may be able to answer.

 

[Bonanza7895]

The early cruise switch was just a single button. It won't provide the correct signals for a modern cruise control. You could use the factory one if you are ok with only having the "set" feature. The rostra or dakota digital one looks like a factory GM switch and attaches at the factory location. This is the one I have in my '80.

https://www.summitracing.com/parts/...h9uUpTmID3MLzGJCiDy1me-q0Nk-QqmOg3IViwbJj360k

Also, if you don't "@" someone(type the "@" symbol and then their screen name to tag them in a post like this @Bonanza7895 ) or reply to their post in your post, they won't know that you are talking to them and won't get a notification that will take them to the thread being discussed.

I also want a tilt wheel would it be possible to upgrade to a whole 88-94 steering column that would give me a tilt wheel, the correct cruise switch, a better steering wheel and they are easy to find.

 

[Bextreme04]

I also want a tilt wheel would it be possible to upgrade to a whole 88-94 steering column that would give me a tilt wheel, the correct cruise switch, a better steering wheel and they are easy to find.

I don't know if there are any other considerations when switching between major model years like that. My 80 has a tilt column... they were an option in your year range. The late GM cruise switch would be a direct wire in to the GMT-400 style cruise module, no relays required. The rostra stalk requires two relays to be wired up. I have the harness schematics for the rostra stalk and GMT-400 cruise wiring somewhere.

The other consideration for the later column is that the wiper controls and high beam switch are now in the column for those years instead of the dash and floor respectively.

 

[Bonanza7895]

I don't know if there are any other considerations when switching between major model years like that. My 80 has a tilt column... they were an option in your year range. The late GM cruise switch would be a direct wire in to the GMT-400 style cruise module, no relays required. The rostra stalk requires two relays to be wired up. I have the harness schematics for the rostra stalk and GMT-400 cruise wiring somewhere.

The other consideration for the later column is that the wiper controls and high beam switch are now in the column for those years instead of the dash and floor respectively.

It won't bother me having things on the stalk that don't work. Seems to be the easiest amd cheapest way to get what I want. That's if it will swap over. I would also appreciate the wiring diagram for the gmt400 cruise

 

[VTXRider]

It won't bother me having things on the stalk that don't work. Seems to be the easiest amd cheapest way to get what I want. That's if it will swap over. I would also appreciate the wiring diagram for the gmt400 cruise

I just took a couple of pictures of my stalk switch and the cc modules I have. You'll need a clutch switch as well. I can send you diagrams, diagnostic info, etc.

 

[Bonanza7895]

I just took a couple of pictures of my stalk switch and the cc modules I have. You'll need a clutch switch as well. I can send you diagrams, diagnostic info, etc.

Wiring diagrams would awesome @VTXRider

 

[VTXRider]

Wiring diagrams would awesome @VTXRider

I'll attach the manual, a diagram I used, and an explanation of how I got mine working.
The GM manual is great info and I would recommend reading it all before you start.
You can bench test the module if you set up the circuits. I did that, and as I said, I have multiple modules that all bench tested good.
The diagram attached is with a Dakota Digital SGI-5 module, which is an older discontinued module.
I purchased and used the Dakota Digital SGI-100. It's simple to connect, adjustable and you can monitor the signal.
You need a 4000 pulse per mile square wave signal for the cruise to work, and the SGI-100 provides that. I'll have to get you
a picture of the module and pinouts I used. You will need a signal generator to get a signal to the Dakota Digital module.
There are several aftermarket units out there. I used the one that is inline on my speedo cable that provides a signal for my
Gear Vendors overdrive. You could get one from them. It's an 8000ppm sender, so the DGI-100 adjusts that down to 4000ppm. I just spliced into the signal wiring and plugged that into the SDI-100. The Dakota manual gives you all the info for that, and I'll get you a picture of how I wired mine in.
As you said your truck is a manual, you'll need a clutch switch as well as brake switch inputs. If your brake switch has 2 connectors, you can use those and then just install & wire in a clutch switch. You do need both connectors on the brake switch, as the module needs to "see" the brake light circuit is valid before it will allow cruise control to engage.
One brake circuit provides 12 volts to the brake lights, and cuts the cruise off when you step of the brakes, and the other circuit has 12 volts and goes open when when you step on the brakes.
I'm using LED brake lights and they won't work, so I just wired in a clearance light bulb in the circuit under the hood to test it and get it working. I may swap that out to a resistor down the road, but it's an easy way to check that circuit if needed.
My cc module and stalk switch came off a 1999 Suburban. I took the stalk switch apart, removed the wiper components, rewired it, cut and threaded my old turn signal lever, and threaded the stalk switch on. I didn't take pictures of the process at the time, so sorry about that. Depends on how mechanical and patient you are for that part of the process. I wanted just the cruise switches, and already have a tilt column, so it worked out well.
I also wired in a cruise engaged indicator LED. I had the LED already and it's easy to wire in the circuit.
You only need light gauge wiring for the stalk switch, indicator light, and speed signals, as it's all just signal circuits.
Power and grounds can be 16 gauge as well. I used the old choke circuit for power to the module as it was already in the engine bay, not being used, and ignition controlled.
Once you get your switches in and wiring run to where you are going to mount your module, you can use a DVOM, or a test light if necessary, to check function of all the circuits. DVOM is better so you can see if you have a good ground, and if there are any volt drops on your circuits. The CC module has a 10 pin connector, 9 get used, 1 is left empty and capped with a plug. if you don't wire in the cruise engaged light, then 2 pins are left empty. I have a list of the pin ID's and what to test each circuit for so I'll dig that up as well.
Ok, this is long enough for now. I'll go get some more pictures of my install for you. Hope this all helps out.

 

[VTXRider]

I'll attach the manual, a diagram I used, and an explanation of how I got mine working.
The GM manual is great info and I would recommend reading it all before you start.
You can bench test the module if you set up the circuits. I did that, and as I said, I have multiple modules that all bench tested good.
The diagram attached is with a Dakota Digital SGI-5 module, which is an older discontinued module.
I purchased and used the Dakota Digital SGI-100. It's simple to connect, adjustable and you can monitor the signal.
You need a 4000 pulse per mile square wave signal for the cruise to work, and the SGI-100 provides that. I'll have to get you
a picture of the module and pinouts I used. You will need a signal generator to get a signal to the Dakota Digital module.
There are several aftermarket units out there. I used the one that is inline on my speedo cable that provides a signal for my
Gear Vendors overdrive. You could get one from them. It's an 8000ppm sender, so the DGI-100 adjusts that down to 4000ppm. I just spliced into the signal wiring and plugged that into the SDI-100. The Dakota manual gives you all the info for that, and I'll get you a picture of how I wired mine in.
As you said your truck is a manual, you'll need a clutch switch as well as brake switch inputs. If your brake switch has 2 connectors, you can use those and then just install & wire in a clutch switch. You do need both connectors on the brake switch, as the module needs to "see" the brake light circuit is valid before it will allow cruise control to engage.
One brake circuit provides 12 volts to the brake lights, and cuts the cruise off when you step of the brakes, and the other circuit has 12 volts and goes open when when you step on the brakes.
I'm using LED brake lights and they won't work, so I just wired in a clearance light bulb in the circuit under the hood to test it and get it working. I may swap that out to a resistor down the road, but it's an easy way to check that circuit if needed.
My cc module and stalk switch came off a 1999 Suburban. I took the stalk switch apart, removed the wiper components, rewired it, cut and threaded my old turn signal lever, and threaded the stalk switch on. I didn't take pictures of the process at the time, so sorry about that. Depends on how mechanical and patient you are for that part of the process. I wanted just the cruise switches, and already have a tilt column, so it worked out well.
I also wired in a cruise engaged indicator LED. I had the LED already and it's easy to wire in the circuit.
You only need light gauge wiring for the stalk switch, indicator light, and speed signals, as it's all just signal circuits.
Power and grounds can be 16 gauge as well. I used the old choke circuit for power to the module as it was already in the engine bay, not being used, and ignition controlled.
Once you get your switches in and wiring run to where you are going to mount your module, you can use a DVOM, or a test light if necessary, to check function of all the circuits. DVOM is better so you can see if you have a good ground, and if there are any volt drops on your circuits. The CC module has a 10 pin connector, 9 get used, 1 is left empty and capped with a plug. if you don't wire in the cruise engaged light, then 2 pins are left empty. I have a list of the pin ID's and what to test each circuit for so I'll dig that up as well.
Ok, this is long enough for now. I'll go get some more pictures of my install for you. Hope this all helps out.

Ok, here's more info.
Cruise control module connector pinouts. Don't worry about colors unless you want to match up for OEM wiring.
Pin A: gray - cruise control on switch signal: 12VDC with cruise switch on
Pin B: dark blue - cruise control set/coast switch signal: 12VDC with switch pressed
Pin C: gray/black - cruise control resume/accel switch signal: 12VDC with switch pressed
Pin D: brown - cruise control release signal: 12VDC with brake pedal not pressed: 0V when pressed
Pin E: black/white - ground
Pin F: brown - ignition voltage: 12VDC with ignition on
Pin G: white - stop lamp switch signal: 0V with brake pedal not pressed: 12VDC when pressed
Pin H: dark green - cruise control inhibit signal: 0V if PCM detects an inhibit condition - Leave this pin empty/not used.
Pin J: white - cruise control engaged signal: I connected 12v through an LED to this. It goes to ground when CC is engaged and the LED comes on.
Pin K: dark green - vehicle speed signal: 1 to 5 volts depending on speed. - i wired to the Dakota Digital SGI-100 OUT4. see picture and the manual I posted earlier.
CC module to throttle body cables come in different lengths, so make sure you get the length you need. Lots at the auto wreckers. You might have to make up a mount at the throttle, and add a pin on the linkage. Holley makes pins.
Brake switch - yours might already have the cc contacts that you can wire in. If not, aftermarket (Napa) switches are available.
I did play around with the output signal on the Dakota Digital a bit, just to make the cruise a little smoother for my truck.
Hope all this helps. If you have anymore questions, feel free to ask.