Speedo Mystery Module

[Triumph1]

Anyone know exactly what the blue module is on the back of this speedo? It's from a 1986 GMC Sierra. I'm stumped.

[hatzie]

Cruise III optical speed pickup.

[mrolds88]

Should be cruise control.

[Triumph1]

Figured it had to do with the cruise control. So here's another question. I'm putting all new Autometer gauges in (electronic). Anyone have experience with doing this and connecting the cruise as well?

[Ski-me]

Originally Posted by Triumph1

Figured it had to do with the cruise control. So here's another question. I'm putting all new Autometer gauges in (electronic). Anyone have experience with doing this and connecting the cruise as well?

It probably is possible but will take some work to get it all in sync. The 90 and 91 both have electronic speedos and there is an extra box that makes this electronic conversion. Also, the two gear rings in your transfer case (and speedo cable) will be replaced with an electronic sensor and tone ring. It's possible to switch everything at least to the "factory" electronic and make everything work (cruise control included).

I made the swap from cable driven speedo to electronic speedo by adding in a DRAC box. I forget the pulse rate off hand but it was something like 2000 or 4000?!? Anyways, definitely possible to go to factory electronic so I would assume one step further to make the Aftermarket ones work too.

[hatzie]

GM calls that box a Vehicle Speed Signal Buffer (VSSB). It amplifies the optical sensor signal to a level that the cruise control module can interpret.

Does the cruise control work now?

The Cruise III parts are going or gone obsolete and getting harder to find.

You may need to investigate the VSSB signal with electronic test equipment (Oscilloscope, frequency counter etc) to see what it is and if it can be mimicked. Or just install an aftermarket cruise control unit...

Look in the 1986 GM Light Truck Service Manual... Cruise III Service Section 9B starts on PDF pg 1378.
The GM STG (Service Training Group) 1984 Cruise III training manual has more details but it's mixed with the automotive versions.

PDF copies of both manuals can be found in my service manual post.

[hatzie]

Does Dakota Digital require you replace the output gears?
I thought they used a VSS box that replaces the cable and hangs on the speedo gear output.

In the late 80's cars with Cruise III the VSSB provides three signals. 4K PPM (pulses per mile) to the speedometer and 2K PPM to both the Cruise III controller and ECM. You probably have some leeway like 1,900 PPM to 2,500 PPM will make it work... The Cruise doesn't know about MPH it just knows you set it to hold steady at X pulses. The system cuts out at 30-35mph so lower PPM is not the right direction. I don't know what pulse rate will overdrive the Cruise III controller input circuits but guaranteed there is a limit.

I don't know the signal levels but it wouldn't be hard to find out. Power the VSSB with nice clean 14vdc and then drive the speedo at a steady MPH. Hookup a scope probe for signal level and then frequency counter probe to the Cruise III signal wire. Do a little high school math. Viola PPM...

My nice old Analog Tek 2236A has a built in frequency counter but I don't have a working Cruise III VSSB in my 1989 Suburban cluster. Someone let the magic smoke out of it before I pulled it.

Is the Dakota Digital VSS signal 2K or 4K PPM? If you can get 2K PPM then you may be good to go. If the Cruise III module robs too much signal or the Dakota Digital VSS is 4K then add a second 2K VSS unit.

This is all purely theoretical. It assumes your Cruise III is in working order and it won't get pissed off with your speed signal.
Check voltage levels with a meter before you hookup the expensive stuff. Magic smoke is hard to put back in.

[gchemist]

If you want to ditch the stock cruise, look at electronic models from www.rostra.com. I have an older vacuum unit from them. Works great!

[Triumph1]

Thanks for the replies. I thought about ditching the stock cruise as well but it still works fine so I might just wait until it takes it's final journey! I've done a little research on the pulses too. Looks like the Autometer 5291 sending unit might do the trick.

[Eyegore]

It's big brother tracking all your moves

[bigd65]

Drones !!!!!!!!

[FritzA]

Some of the Autometer electronic speedos already have an output terminal on them to send to the ECM or to cruise control. You have to make sure the output is compatible with what is needed.

Or

Dakota Digital makes a little box that takes input from the electronic speed sensor and has multiple outputs for electronic speedo, ECM and cruise. The outputs are also adjustable so you should be able to get what you need.