'78 K10 Cruise Transducer

[RyanAK]

Brought the truck out of town for work the last two weeks for a little shake down... and because I'm bored to tears in the hotel after work and thought I'd do some small projects in the parking lot.

Other than the vibrations I'm chasing at 45-55 and 65-70 mph, the truck is pretty darn comfortable and seems reliable.

Rig is a '78 K10 Suburban Silverado with a GM Crate 350, TH350/NP203, 3.73 gears.

This week my little project was to replace the one-piece speedo cable (which was working perfectly...) to the two-part so I could run through the cruise transducer. 160 miles of interstate each way would be so much nicer with cruise, and everything seems as if it's in place and hooked up. Wires, vacuum lines, etc. I assumed the guy that dropped in the crate motor just bypassed the transducer because it takes a little effort to figure out which cables are needed. I found the cables and install should be easy-peasy. 15 mins and I had the new cables from the transfer case to the transducer and from the transducer in to the speedo. After 5 feet of test drive... BOING! Cable snapped at the transfer case. Apparently the transducer is seized or almost seized up. The speedo DID move for the 3 seconds before I pretzled the lower cable. Anyone know if these Speed Master transducers can be freed up? I'm finding it tough to source a reman'd unit.

This afternoon I'll put the old cable back for the drive home. Thought I'd be cruising in style with GM OE cruise. Nope.

Good grief. Old trucks.

R

[Matt Man]

On my 81 I did the same thing I bought a 2 piece cable and tried to engage it and nothing happened. Then last November I was driving down the road and the transducer started making a horrible racket and did not want to break anything so I went back to a single cable.

I did do some basic trouble shooting and could never track it down and once the transducer went bad I just removed most of it (I still have it in a box). I thought I read somewhere that the wires going from the turn signal stalk often break and that is why they stop working.

[RyanAK]

I’m about to start looking at the electrical. You’re right, the wires on the turn signal stalk are a weak point. But everything seems to be in place. Even all of the vac lines are where they need to be. But the transducer physically won’t spin... hence the pretzeled cable at the transfer case. I wonder if it needs to have power to it to allow it to move? I have all the service manuals related to this cruise system, read all the threads, but it’s unclear if the transducer needs 12V to allow the speedo cables to spin.

Anyway. Most of the literature has “replace transducer” as the fix when something isn’t right with that part of the system. It’s has to be fixed by a “certified transducer technician”. Since I can’t seem to find a new/reman’d unit, I might need to dig into it and see what I can see. “Certified” or not.

[hatzie]

There used to be a write-up from one of the C3 Corvette gurus on rebuilding the Cruisemaster transducer. He posted it on a UK .edu site and it's gone into internet oblivion.
Basically you carefully strip it down on a towel or large rag, douche it out with carburetor cleaner, lube it, and re-assemble.
If it's seized up you probably can't make it worse. I would take pictures or video the process so you can look at the footage when you get stuck on re-assembly.

The 1979 GM light truck service manual in the manual post has a fairly decent Cruisemaster troubleshooting section on it and the GM Cruisemaster service training textbook is digitized in the manual post as well.

[RyanAK]

First, thanks hatzie for compiling those manuals. I’ve saved them and refer to them often. You’re a good egg, brother!

I’ve found one Corvette Cruise Master write-up here: http://www.splashedpaint.com/cameron...iseControl.pdf

Not sure if that’s what you’re referring to. I will most likely give tearing down the transducer a try. Like you said, can’t make it worse. I wish I could find more photos/info specific to the transducer. Your manuals and the training textbook say “replace” rather than service. Since I can’t replace, I’ll give servicing a try.

My system doesn’t have power at the moment. The attached pic shows a few wires that are disconnected near the fuse block. The blue connector has a blue plug, but looks like it comes out of a plug that’s in the fuse block. No idea what the orange wire is. Everything else on the truck seems to work...

[RyanAK]

Power and vac all seem to be good to go. Pulled the transducer to see what I could do by opening it up. Man this thing is neat inside! But... the big spinning magnet is shattered, so not repairable my mortal man. And without a reman transducer unit to be found in the known universe, I’m stuck without usable cruise. Hopefully I can find time to call around or visit a decent pull-it yard. The long, mountainous interstate commutes would be more enjoyable with the factory cruise working.

First-world problems, eh?

R

[hatzie]

I found a post on the GMC MotorHome forums with some pictures of the innards of the non resume Cruisemaster Transducer.
Since there's such a minuscule amount of information about what's inside these I'll post a link to that thread.
http://gmc.mybirdfeeder.net/GMCforum...t=msg&th=35113
I'm going to archive the photos for the eventual catastrophe that causes them to evaporate. Hopefully it's not gauche to post a link to that forum thread here.

GM used these units from 1968-1982 on cars and light trucks. As long as the unit has the same functions as the one you're replacing... Resume or the more common without resume... you can pillage them from any GM car or light truck made in that timeframe.

There's an evilbay auction advertising rebuilding 1968-1970 Cruisemaster Transducers. I'd ask the guy if he can do anything with yours.
https://www.ebay.com/i/352460776127

The other option is to retrofit the Electromotor Cruise box from an early 88-93 GMT400, 90's F-Body, or late 80's early 90's B-Body. They used em through at least 2005 on the venture vans and other cable throttle cars and light trucks. When they moved to Fly By Wire they stopped using them so the end date in some models is choppy.

They are quite compact, simple, and still fairly easy to find. As long as you can hook up the cable to your throttle and get a VSS signal without disturbing the speedometer cable too much the rest of the hookup is fairly easy.
A passthrough cable speedometer VSS was available from Jags That Run and Dakota Digital among others. The 82 and later optical VSSB will work too if you're willing to change to an 85mph speedometer cluster with the optical pickup hole.
The on-off, set, resume turnsignal stalk for the 73-83 columns was available from Dakota Digital.

[hatzie]

Info from Novak on the GM VSS options...
https://www.novak-adapt.com/catalog/...onics/sensors/

[RyanAK]

I'm hoping to not go down the retrofit route... but may need to to get this running. I've seen the guy on eBay... Might be worth a shot. Or cruise a u-pull-it. If it was a something like a bad electrical connection or gummed up gears, any handy guy would have a shot at fixing this. But with the magnet in pieces, no-go.

The write-up on the motorhome site is really good. He steps through it well.

I'll take some photos of the inside of the transducer if you like. It might be helpful to archive with the Cruismaster Product Service Training manual.

Appreciate the follow up!

[hatzie]

Definitely take some pictures and post them in this thread.
I'll add a PDF of the photos to the 73-91 & 67-72 manuals page.

[RyanAK]

There be magic inside...

You can see the shattered magnet in the last photo.