Cruise control rose to the top of the two year list...success! I checked forum, photo site, and general web and found very little advice other than "troubleshoot and replace transducer", at $100+ for rebuilt (only option available or used) I figured nothing to lose and took it apart.

Album of seven pics:
http://www.gmcmhphotos.com/photos/g6858-cruise-transducer.html

Excellent GM service guide (we need the first section of the following linked book for GMCMH)
http://www.73-87chevytrucks.com/techinfo/7387CKMans/Training/1968-1982_Cruisemaster_Cruise_Control.pdf
which was a great compliment to our full GMC motorhome repair manual, which had a multipage diagnostic but a little different walk through troubleshooter and some different pictures. Descriptions and internals were better described in the linked guide above.

I used the GMC manual to troubleshoot all the electricals. Short version: power comes through brake pedal switch if adjusted correctly. OEM cruise shares power with reverse lights, if they work you should have power at brake peddle switch. When you look at top of brake pedal, two physical switches - the electric actually is a two-in-one for lights and separate/opposite action for cruise (peddle up power to cruise, peddle down power to brake lights), and there is a vacuum switch/dump that has a piece of hose connected to it. Unhook the six pin connector with five wire placements connected to three miniwire thing(this goes up stalk to switch inside turn signal) and test according to manual. Black wire from steering column to actual transducer needs conductivity, brown wire needs conductivity with 40 ohm resistance.

The indicator light in the dash gets its power from the dash, and actually is grounded at the transducer when the solenoid gets activated. Test dash light by unhooking wire lead from transducer, and jumper wire to ground on engine block.

I'll add here that I had added a ground wire last year from transducer to engine valve cover. OEM is transducer to body and several have suggested that path is intermittent.

Got my brake switch feeding power, and my light bulb replaced and bright in the dash; now working on transducer. My ground wire is good. Supposed to not do anything until over 30 mph. Mine would now turn on anytime/speed including idle, but not hold at any speed for more than twenty seconds or so. I tested all the vacuum stuff in parts and together (if you take the "T" vacuum hose off the high side of the transducer and plug it goes unregulated vacuum - effectively full throttle - zoom! that part could pull).

I could type for as many hours as I worked on all this trying to describe the guts of this thing, but let me say to go through the two manuals well first, and I'll add my tips that made a difference here.

1. If you take the "T" electric connector for hold and engage at the transducer and try to make it so they are backwards, you can only put the middle wire on the outside connector pin (this is buried in the linked manual explained better). this effectively bypassed the low speed cutout switch and when you push the switch up in the turn signal, the solenoid will thunk thunk if working (do this with engine off, key in run position).

2. DO NOT unscrew the adjustable vacuum regulator thing all the way out unless you are ready to take the unit apart! It is very much like a carburetor main jet EXCEPT the piece it slides in to work properly is on a spring. You can not put back on unless it is open.

3. One easy electric test not in the walkthrough of either book will tell you if your low speed cutout switch is closed (not correct): continuity from the "hold" pin to the "engage" pin on the transducer when all external wires are disconnected.

OK. I took my transducer apart at this point. The long and the short of all this is that the vacuum mechanism is controlled by a spinning magnet that looks like a top, working like a speedometer needle (old school of course). The disc should be free, a little brass plate pressed in the transducer body sets the depth - and mine from wear, age, construction, etc, was not free to rotate (by spring power) to its intended position. This meant at idle the low speed cutout switch did not work, and the drum was not responsive enough when it tried to hold speed to change the vacuum correctly. From the outside of the transducer body, I just barely tapped the little brass flat plate (to reseat it) that is pressed in just above the two external vacuum ports. This moved the shaft in and allowed it to "float" in the magnetic plane. Careful reassembly (tip: the internal vacuum tube orifices should be half covered by the black plastic block at rest. the spring that goes around the drum straddles the solenoid at the bottom which allows reassembly. both pieces of info buried in the linked manual).


I'm sure much of what I have written isn't clear, some notes with the pics on gmcmhphotos may clarify a bit as well. I have assumed you are using both official GM manuals, and this is just what I did to get mine working. Anything you do is at your own risk. I suggest you make sure your vacuum and electrical cutoffs are working BEFORE you try and get the rest working if you have multiple issues to troubleshoot like I started with (brake switch, dash light, and transducer low speed cutout and vacuum control drum were all inop). I still may purchase a rebuilt unit, but I will be doing so more to assure that the bearings and gears that spin the speedo don't bind and take out the little plastic gear at the transmission end (a scenario that leaves you with one gear until you put your spare plastic gear in, which I do have). Open road, good visibility, and low traffic volume are great for cruise control - I now have that option!