Well, I have owned the '79 DeVille for over a year now.... one of the little quirks that got to me was the cruise control... I am an avid CC user, so I like when it works....

but you turn it on, set it, and it will keep accelerating the car...

so I finally got around to checking it out, to make a long story short, I got the diaphram part and the electrical connection on the side of it from a '79 Coupe DeVille in the junkyard.

Well, my diaphram worked, as the car accelerated before, the little extra part that went along side it, one of the little nipples for the vaccum line was broke off, I didn't realize it...so that was useless...

So that left me with an electrical connector that went into the diaphram thing, I noticed the contacts were rusty, so I used emery cloth and cleaned them up.... I put it all together and went on the road.... to my surprise, it worked! CC worked just as it should.... I wonder if my module thing worked, but the contacts were corroided up... but it is ok, $25 and I got the cruise control to work, I am very happy.

I still don't know what the difference between "cruise" and "auto" is, in 1980 it was ust off and auto... but I turn it on, get up to speed, hit set, it holds the speed, accel/resume works, as well as coast, it is nice.

Couple things i notice, it is not as good about keeping the speed as my '89 Oldsmobile and '93 DeVille are... I think mostly in part because it is MPFI and a more current system, also, on the newer cars if I tap set/coast 5 times, the car will loose 5mph, on the '79 if I tap it, it'll gain speed, I gotta hold coast until I get to the speed i want, then let go... it is fine though.

I had the cruise set at 70 on the highway, so going up hills the speed would drop down to 67 or so, I think 66 at one time, then slowly catch back up, that is the only "bad" thing, and honestly, I coudln't care less, with equipment that is 26 years old, probably some vaccum leaks (we replaced some of the lines) and electronics/connectors that are showing their age, I would say the system does a hell of a job, definitly makes me happy.

But that is another thing, tonight I went on a road trip to nowhere :) I drove a total of 70 miles, all on the Thruway, all with the cruise set at 70mph, why? it is a nice speed, and at 70, the 425 is turning over at exactly 2,000RPM, where the torque peak is, and I remember reading an engine is most efficent at its torque peak...

So I thought I would get an honest 20mpg, the engine is in very good tune, and steady 70mph with the cruise set, as little change in speed as possible.... but after the 70 miles, I filled up (again... 3rd time filling up today) and was surprised to see that I was only getting 16mpg.

But driving around town, and I drive it hard (accelerate fast etc..) most the time, I get about 12mpg.... I thought I should be getting 18-20 on the highway... not 16.... is this normal for the 425 or may there be something else?

16 isn't all that bad. The 75 barely gets 2-3 more on the highway.

part of the mileage may be the 70mph cruising speed. The engine may be most effecient at the torque peak, but drag increases with the square of speed, so small increases in speed especially on the highway will make big increases in drag.
I also don't see much difference in mileage with my '70 either. A bit over 15 on the highway, and 13 on my last tank with mixed city/highway.

As for the decel function - I think that is how the older Caddy system is supposed to work.

How low will your controller let you set the speed? I think my '70 will set at any non-zero speed. I have had it set and hold at as low as 10mph. It is not very smooth at low speeds like that though.

Beats my old 91 Bonneville, it had one of those aftermarket cruise controls, it would drop 3-4 mph going up a hill and then keep on it till you were doing 5mph faster than set speed after the crest of the hill before it would backoff. Annoying and ticket bait if you ask me! I always had to override it on hills. Smallish hills were oh, eh? But larger ones it would fly over the otherside. And it didn't have tap up/tap down, which I missed a lot, my 85 Cutlass had it. I love that feature.

There is some fine tuning in that old cruise, I can see if I can copy it for you from my 1976 Olds book. Unless you have a FSM, there is some wrench adjustments for the speeds. But this is only for the "Cruise o Matic" cruise unit, which if you don't see a name, it isn't. My Turbo T/A cruise (non cruiseomatic) would do that acceleration thing when it died, and that isn't a good thing going to boost when you hit cruise!

Interesting....Maybe you can help with mine. Mine works fine until I hit an incline. The car loses about 10-15 MPH going uphill and the cruise doesn't try to accelerate to compensate. Then after the ground levels out it doesn't kick back up to speed, it stays where it fell. It's strange but it's discouraged me from using the CC. Also there's some sort of hissing from under the dashboard that gets quieter as I apply my foot to the gas and gets louder when I let off. My guess is that's the auto parking brake release that's leaking since that doesn't work and runs off a vacuum line but then again so does the CC. I just replaced a badly cracked and dryrotted vac line going directly into the carb and haven't tried the CC yet but if you Wolf or anyone else has any thoughts, you know what to do.

Beats my old 91 Bonneville, it had one of those aftermarket cruise controls, it would drop 3-4 mph going up a hill and then keep on it till you were doing 5mph faster than set speed after the crest of the hill before it would backoff. Annoying and ticket bait if you ask me! I always had to override it on hills. Smallish hills were oh, eh? But larger ones it would fly over the otherside. And it didn't have tap up/tap down, which I missed a lot, my 85 Cutlass had it. I love that feature.

There is some fine tuning in that old cruise, I can see if I can copy it for you from my 1976 Olds book. Unless you have a FSM, there is some wrench adjustments for the speeds. But this is only for the "Cruise o Matic" cruise unit, which if you don't see a name, it isn't. My Turbo T/A cruise (non cruiseomatic) would do that acceleration thing when it died, and that isn't a good thing going to boost when you hit cruise!


I have the FSM

I am not worried one bit about the MPH, it'll only loose 3mph or so, and isn't even an issue for me, it gains the speed back pretty smooth too, it is a 26 year old system that isn't restored or anything, I am sure it has its share of little leaks and stuff..... but I am just very happy, as I love cruise control, and now I can just set the speed and be done with it....

Interesting....Maybe you can help with mine. Mine works fine until I hit an incline. The car loses about 10-15 MPH going uphill and the cruise doesn't try to accelerate to compensate. Then after the ground levels out it doesn't kick back up to speed, it stays where it fell. It's strange but it's discouraged me from using the CC. Also there's some sort of hissing from under the dashboard that gets quieter as I apply my foot to the gas and gets louder when I let off. My guess is that's the auto parking brake release that's leaking since that doesn't work and runs off a vacuum line but then again so does the CC. I just replaced a badly cracked and dryrotted vac line going directly into the carb and haven't tried the CC yet but if you Wolf or anyone else has any thoughts, you know what to do.

hmmm, sounds familer...

it isn't accelerating? I bet it is a leaking vaccum line, or the little electric module is bad.

I would first start by checking all the vaccum lines, make sure there are no holes or cuts, then remove the little electronic pieces (2 or 3 of them) and use a wire brush to clean all the contacts... that should solve the problem

FWIW, I have seen cars with out the speedo cables going through the cruise unit be converted to the new electronic cruise (the one without vacuum).

But if you have the speedo cables going through (like a 79), then you would need to use one from that era. I think what happens is seals in side dry up and start to leak, causing it not to work. The key is opening it without destroying it. Then, if you find something wrong, being able to fix it.

Does GM still have replacement ones? Or are these going by the way of Unobtainium?

hmmm, sounds familer...

it isn't accelerating? I bet it is a leaking vaccum line, or the little electric module is bad.

I would first start by checking all the vaccum lines, make sure there are no holes or cuts, then remove the little electronic pieces (2 or 3 of them) and use a wire brush to clean all the contacts... that should solve the problem
Ok thanks, can you tell me where that is located?