At 03:29 AM 8/24/03 -0400, Allen Fox wrote:
>"Do you know of any aftermarket cruise controls suitable for an MGA?"

Yes, I have installed and used a few of them. Some are not as bad as others. And I'm not sure any cruise control is suitable for an MGA. This is just a matter of my personal philosophy. The cruise control unit costs money, adds weight and complexity to the car, could add to future maintenance problems, and is not required to make the car go down the road. I will not post any pictures here, as each unit is a little different, and the installation instructions will come with the kit when purchased. But they do all have a lot in common, so I can relate some general observations.

My first CC installation on my MGA was in 1989 just prior to a 13,000 mile trip around the U.S. I had successfully installed CC's on other cars before that. My last CC installation in my MGA was in early 1997 just before a 19,000 mile Alaska trip (and that one failed before the end of the trip). In between there were several other noble tryouts. I have gone through several cruise control units over the years. There were a couple of failures of control modules, a couple which failed to hold a constant speed, a couple of physical failures (broken parts), a few different mounting configurations,and a couple different basic types of control unit. All CC's which were ever installed on my MGA have eventually ended up in the trash. Even though most of them were not terribly expensive, and not difficult to install, those that didn't fail eventually gave more maintenance problems than it was worth time fiddling with. I guess the bottom line is that they either didn't perform well or were not reliable. And they must have all been bad in one way or another, because I have long since given up and have not had cruise control on my MGA for the past several years. If I have to fiddle with something, there are other things I would rather be fiddling with.

Aftermarket cruise controls are quite generic in nature. They are not very expensive, not hard to install, and work on almost anything that moves. Most of them have a pull cable similar to a throttle cable which you hook up to tug on the throttle arm of the carburetor. About half the work of installation is the fairly simple job of rigging up a bracket to hold the cable outer jacket near the carburetor. The actuator on the other end of the cable goes anywhere you find space on a convenient mounting surface, and mounts with a couple of screws. A small control panel or button assembly mounts somewhere on or under the dash, on the steering column (or turn signal stalk in newer cars). It only has a power switch and two or three buttons. A small wiring harness comes with the unit, already connected where possible. Excess wire length can be coiled and tied up out of the way. It needs power and ground, speed signal, and shut-off signal.

Most inexpensive CC units are vacuum powered, so you need to install a small hose barb somewhere in the intake manifold for a vacuum tap, then run a small hose from there to the actuator. The actuator module will also commonly contain the logic module and a couple of solenoid valves to vent air in and out of a vacuum diaphragm, which in turn pulls the cable.

A speed input signal is commonly picked up from the primary side of the ignition coil, the wire to the ignition contact points, which works great for a manual gearbox car. With an automatic gearbox, where the engine speed can float some in relation to the propshaft speed, it is common to wire tie 2 small magnets onto the gearbox output shaft. For front drive or mid engine cars you can tie 4 magnets to a drive shaft for one wheel. A thumb size hall effect sensor picks up the signal from the rotating magnets, in which case you also need to fashion the bracket for the hall effect sensor.

A cut-off signal comes with a pair of wires connected to the brake light switch. You could also wire this in parallel with a switch on the clutch pedal and/or a neutral switch on the manual shift lever. Many of the aftermarket units have a speed variation trigger in the control module such that a 20% increase or decrease in engine speed shuts off the control. Then if you're going up a steep hill where the engine does not have enough torque to pull it, the unit will shut off when the car loses too much speed. Or if you depress the clutch or shift into neutral, when the ground speed drops the engine speed may try to increase, and the module will shut down immediately. With these units you don't need a clutch switch or neutral selector switch (but you do still need a wire or two to the brake light switch).

The vacuum operated units will not work on a diesel engine which runs with an open intake (no throttle plate), because those engines have very little vacuum in the intake manifold. For that application you need a mechanical vacuum pump and vacuum reservoir, or a non-vacuum type of CC unit.

There is a similar related problem with small 4-cylinder engines, or any engine which spends any extended period of time operating near full throttle condition. If your under-powered car is lugging up a long steep hill (or at high speed into a head wind), the CC unit will try to pull and hold full throttle. With wide open throttle there may not be sufficient vacuum in the intake manifold to operate the vacuum actuator module (aggravated by high altitude). If this condition is temporary, like less than a minute cruising over a short hill, then it can be accommodated by installing a small vacuum reservoir. This is commonly a plastic container about the size of a softball with two hose connectors and one check valve. The check valve connector gets hooked to the intake manifold, so when there is sufficient vacuum it will evacuate the reservoir to a moderately high vacuum level sufficient to operate the CC unit. The CC unit then operates from the vacuum in the reservoir. When the engine is required to go near full throttle for hill climbing the CC unit can still operate from the vacuum reservoir (for a while). This will ultimately fail to work on a long mountain hill climb that goes on for several minutes.

I did install one more or less common type vacuum operated CC unit on a MGC with 3-liter 6-cylinder engine. That one worked very well, because most of the time the engine was loafing along at partial throttle with plenty of vacuum at the intake manifold.

There is another type of CC unit that does not require a vacuum supply for operation. These use an electric servo motor to mechanically actuate the pull cable. Otherwise the installation, control and operation is similar to the vacuum powered units. I believe the non-vacuum CC units should be much more reliable for a small engine car, but I have never actually installed one (yet). The servo motor actuated CC units will be somewhat more expensive than the vacuum actuated units.

When original appearance is a concern, or if you don't want to clutter the engine bay with this additional hardware, you can mount the mechanical actuator almost anywhere you find a place where it can pull on some part of the throttle linkage. At one time I had a CC unit mounted under the dash in my MGA, and had the pull cable acting on the pedal arm just behind the firewall. The only parts visible in that installation were the small vacuum hose running from the manifold through the firewall, a pair of wires running to the brake light switch, one wire running to the ignition coil, and a button module attached to the bottom edge of the dash panel. Everything else was hidden behind the dash. That worked okay. The final demise of that unit came with the fracture of a small plastic vacuum connector nipple on the actuator module. This was in such a location as to be non-repairable.

One problem with aftermarket accessories of any type is the difficulty of availability of repair parts. When some small part fails, sometimes the most expedient form of repair is to chuck the entire unit in the trash and buy another one. If you keep good records of the installation and the manufacturer of the unit you may have a better chance of procuring repair parts. Otherwise the first time it gives you a problem it may be instant junk. I did have one unit replaced twice under warranted within the first year. The first time it failed outside of the warranty period it went directly to the trash bin. If you don't use the car much, the CC unit might never fail. But most people want a CC unit because they do use the car a lot, and often on extended road trips. When you drive a lot of miles you have a much higher chance of ultimately having a problem with the aftermarket part. At the very least, keep good documentation of any modification to your car.

I just took a quick peek in the J.C.Whitney catalog, and they are currently selling CC units from $99 to $229. You can probably find a small assortment of CC kits at any moderate size discount auto parts store. This is just the sort of impulse accessory that Pep Boys or Autozone would keep in plain site these days. Installation instructions and some generic sort of mounting brackets come with the kits. A little head scratching may be required to adapt one or two brackets to your particular car. Expect the installation to take a couple of hours. But you should also be prepared for the possibility of needing to fiddle with testing and set-up adjustments for an hour or two more to get the smoothest operation. Being generic kits, like one size fits all, these things are not always totally compliant with your particular car on initial installation. For newer cars which did have a factory CC unit available as a factory option, you may be able to special order a CC kit with installation bits specific to your model, which could make installation and setup a little easier.

For a typical cruise control installation instruction sheet, see:
cruise_inst.pdf - (1.1-MB pdf file).