So my cruise control doesn't work. Not a huge issue, but I wonder if it's anything to do with the fact that the actuator rod (5) runs freely when the spindle is turned, rather than pushing or pulling the bellows. Is it just a matter of ensuring that the rod (5) is fastened in the bush (4)?
Or might this have been deliberately disengaged as something else is gibbled, and putting it back will do something nasty like hold the throttle wide open?
Basically, I imagine it's a very simple issue to just put it back, so I am wondering if there are any gotchas I need to be aware of, such as why such a simple operation was not done before....

 

The way it works is vacuum is applied to the bellows and that pulls rod 5 to open the throttle. The rod needs to be free to move when you drive the car using the throttle pedal, that's why it should be free to move through bush 4.

You can test the system just by applying vacuum to the bellows and see if it moves the throttle spindle. You can do that with with the engine off, using a vacuum tester. Or suck really hard if you have good lungs!

 

Just as well I didn't try and insert it, then.... I have tried pushing the bellows flatter, which it does fairly readily, but it doesn't engage the rod even then. Perhaps it needs the length adjusting with the screw. It's quite possible there's a vacuum leak, however. I'm not sure how to test that, though. I've found this, and a few other sites, so it might pay to get one anyway - still have the vacuum air-con flaps issue, after all....
https://www.2carpros.com/articles/ho...e-vacuum-gauge

 

If you flatten the bellows by hand it should move the throttle. It can be adjusted if it doesn't. Just make sure that the rod isn't so short that it holds the throttle open.

At that age it's entirely possibly a rubber fitting has deteriorated and you have a vacuum leak. That would account for low fuel economy too, as the ECU uses a vacuum signal to determine the load on the engine and how fuel to give it. are all the fittings on the backs of the intake manifold sound? there are a number of vacuum lines there and the rubber can split. Sometimes if you listen closely on a running engine you can hear the hiss of a vacuum leak, but it can be hard to hear over fan noise.

Vacuum at idle depends on engine condition, it's state of tune and elevation, but at sea level I would expect to see something in the range of 16-20" Hg with the engine warm and idling.

Leaks can be hard to find, it's usually a matter of removing fittings one at a time, plugging where they came from and see if that makes a difference. Unfortunately, there are a lot of vacuum lines and fittings on a V12, and many are difficult to get to. Could be the dump valve too, those leak over time.

 

I'll try that tip re the rod length. If that doesn't work, then a vacuum leak is likely. I'll check the fittings when I next can, though previous random explorations of the engine bay haven't turned up any obviously bad hoses or connections. I'm not too concerned about the cruise control, but would like to see if mpg can't be improved. On a good run she'll do much better (even north of 20 mpg cruising steadily about about 1500-2000 rpm on a flat road) but there's a lot of stop-start in town....

 

I just remembered, you might have a vacuum pump rather than using manifold vacuum to power the bellows. Worth checking that works. Not sure if you have the separate brake pedal switch for cruise either. I had that on a 92 DD6 and the switch had shorted out internally so the cruise thought the brakes were on so disabled cruise.

I'd have to look at the parts book to see what version you have.

 

You're probably right - under the bit on Speed Control System (p. 476) my ROM says there's a vacuum pump and regulator valve. As well as an ECU, dump valve, and the various operating switches, which include a brake switch.
Thanks for all your help and advice - much appreciated.

 

What's the condition of the white plastic washer, part 4. This tends to soften and lead to problems. Worth replacing at that age anyway as can cause the rod to drop and the cruise to stick on.

 

It's yellow now, but appears fine. The adjusting rod is very very tight, however - I need to apply CRC or something to try and loosen the screw, as it will NOT turn.....

 

Well, after some more reading the ROM, and more fiddling with the adjuster bolt, I managed to tighten the actuator rod almost all the way to the grommet. Then when I was poking around under the passenger knee bolster area looking at a/c stuff, I noticed the fuse box cover was a bit dusty on the inside, so wiped it clean, and then compared the fuses with the key. And interestingly enough, the cruise control 3A fuse was missing. Luckily there was a spare 3A fuse in the horizontal spares bit just below (at least I assume that's what that is!) so I inserted it, and took Lady Mary for a drive. Found a nice bit of reasonably fast, reasonably open road, and....

...the cruise control worked! She hummed along at 70 nicely, though it wasn't as smooth as the cruise control I've used on late-model Cadillacs (a more mechanical action), and the brake switch worked to slow down, and the resume button worked to speed up and resume. Sweet.

Thanks to Jagboi64 and Malc4d for their advice! This is the first thing I've repaired myself on my car that was more advanced than screwing in a screw.

And to make today even better, I managed to pry off the fascia wood quite easily, using an ANA airline knife (very flat) and knowing where to put pressure thanks to other threads in this forum. The PO had put a cupholder in the vent, and I hated it, and while I had managed to remove most of it, one prong was stuck in there, and in trying to get it out I dislodged two vertical vanes.

With the fascia wood off, the vents pulled out easily, and I could force the bloody prong off, which was hard enough even then. Then I could rescue both vanes from the a/c duct inside, reinsert them, and bang the wood back on. Now I am finally free of that damnable cupholder prong!

 

Glad it worked for you! Good detective work on the fuse.