Does anyone have any bright ideas for testing the cruise tranducer on an '80? Mine's never worked. I spent a while checking for bad connections the other day, all the vacuum hoses, switches and wires seem to be good, but it's hard to tell. If i could bench test the transducer and find out if its broken, then at least I'd know to order another one, rather than waiting to get one over to the uk, fitting it, and then finding out that nothing works still!

If only i had a friendly local parts dealer! I think i need a trip to the states with a big suitcase!

Hey Theo:

Well I can't say I have any bright ideas regarding the testing of the transducers but I've monkeyed with mine enough to know I still don't have it down. I replaced it because the speedo needle had the shakes. Once replaced...the speedo needle smoothed out but can't seem to get the cruise dialed in.

I'm now at a point where I can set at 60 but it will increase and decrease up to 65 then decline to 60...gradually. I've place with the orifice setting but can't seem to peg it. Perhaps, someone can help us both.

I have the same issue that it loses speed after it's been set.

I also have a VERY bouncy speedo needle, but I figured that means I need a new speedo cable.

My needle reacted as yours did until I replaced the transducer...do the transducer before cable. I did cable and it had no effect...just cost me $!

Hey guys,
Glad to see it's not just me who has this problem! Since the original post, I've had the transducer in bits, partly to see how one works and partly to see if it was obvious why mine didn't work...
There's a magnetic disc in there, which spins directly from the transmission. When the disc is spinning fast enough, a little pair of points will close and arm the system (hence you're not able to set the cruise below 30 mph). In principle it's the same as a speedo, where the spinning magnetic disc induces movement in the needle, except that here it's a pair of points and not a speedo needle. When you're above 30 and set the cruise, a solenoid snaps across and closes the valve between the vaccum "inlet" and the "outlet" to the throttle servo, so the engine vacuum is now pulling the throttle open. This on it's own would be no good, since the speed would be all over the place as the engine tried to reach some balance - obviously throttle position determines vacuum, but also vice- versa with that solenoid valve shut, kind of a feedback situation. So theres's a third vacuum pipe in there - with a speed controlled bleed valve. The position of this valve is also determined by the speed of the magnetic disc, just like the points. This keeps things steady: if you run away down a slight incline, the disc naturally speeds up, the bleed valve opens a little more, "letting the atmosphere in", the vacuum in the piping will reduce and the throttle servo relaxes. If you've set your speed and hit a hill, the engine begins to labour, and the disc slows down. The bleed valve will then close a little bit, so as to "make the most" of the vacuum available and pull back on the servo, opening the throttle.

In the end, it looks like some of the light weight copper mechanism that closes the points has broken off inside my transducer and fallen out the bottom - the whole thing was full of ground up dust that probably used to be something really important!

I guess in theory, the system is supposed to keep itself in balance. I suppose that speed fluctuation you describe, hunt4cleanair, could be a dodgy bleed valve (this is total speculation, since I have had zero experience with these crazy little devices) but you say your transducer's new, so perhaps not. There's suppsoed to be some adjustment on the orifice/bleed valve, as you said - but i think thats to do with the final speed you obtain, not the "floating around" you're seeing.... I may be wrong on that one - mine's an 80 and yours is a
I wonder if these things are meant to be matched with a certain carb/inlet setup - i don't know enough about engine vacuum to know if that matters, but perhaps you've got a different set up to stock?
As for bouncy speedo needles, I've heard that a tight bend in the cable can cause this, but with our system, the cruise unit would seem to be the most likely candidate - if the gear wheels in the unit were worn, this would cause the speedo cable spin speed to be "loose" and uneven, especially if triggered off by a shock - i bet your bouncy neeedle got momentarily worse going over bumps? Mine does.
I think a new transducer is in order - Ecklers do one for $85, does anyone know if there are cheaper ones about?

Hey guys..

Just in case you're still having problems, check out this well written explaination of how the cruise transducer works - its at
http://iotech.no/corvette/index.htm, in the technical articles. I reckon this may explain how you're getting problems pegging the speed constant... although it seems more like a basic problem with the design, so perhaps that'll give you less to worry about!
Having seen what goes on inside a new one, i'm not surprised mine is dead, as half of the internals have simply vanished into thin air....
No choice but to get a new one

Good luck with it all