"Zoomamatic" Repair Information for Your Imperial

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Question from Dale (1970's):

Got a question on the 70's style cruise units used in Imperials and other Chrysler products. I had to install a replacement unit as mine failed. Installed a used cruise I obtained at a salvage yard. Everything worked fine except when I engage the unit, it keeps on accelerating! I tell you that old car will fly done the highway! SO what is the deal? Not sure if this is an electrical or a vacuum problem. Did not have this problem with the old unit so I am doubting a problem with the wiring harness or the switch. Any ideas?

Reply from Steve:

I looked in my '75 Chassis Service Manual...and unfortunately it doesn't say anything about the symptoms that you have in the Troubleshooting charts. When you wrote that it keeps accelerating, I would think that when you engage Resume....that it keeps on speeding up, passed the pre-selected speed even though you do not have the Resume button still pressed??? There is a lot of emphasis in the manual to the lock-in screw adjustment you mentioned near where the 2-prong elect-connector plugs into the servo unit. Possibly something is binding??

Question from Larry (1982):

Has any body worked on the speed control system of a 82 Imperial? The FSM gives me some clues of were to start But if somebody has an idea or has trouble shot this system before I could use your help. It does not work. The FSM said to jack up the back wheels block the front wheels run the car to over 35 MPH. This seams a little scary to me I do have jack stands for the back wheels. However I could just see me trying to explain to my wife how the car ended up in our back yard, and the whole in the garage wall.

My Imperial seems to be missing a cable from the speed control servo. The FSM shows two cables, a cable from the transmission, and a cable to the speedometer. The one that goes to the speedometer is missing!! Could this provide data input to the brain for the speed control?? I haven't found the cable in the FSM.

Does any body know what it does? The speedometer seams to be working-without this cable!

Reply from Dick:

The speedometer is operated by an electrical pulse. There is no speedometer cable that goes to the dash. There is an interface device that appears to be a parasitic growth on the speedometer cable that goes to the cruise control - you'll find two wires originating in that lump on the cable, those are what drives the speedometer and odometer. If the speedometer is working, you know for sure that the cable is OK.

Most problems with the speed control are related to the 4 wire switch that is operated by the doo-dads on the end of the turn signal lever. You need to study the wiring diagram for that switch, then find the connector from it to the speed control master unit. Then take your trusty voltmeter and see if the electrical signals are all working. Chances are your switch has failed from lack of use, and if this is the case, you may need to just exercise the bejabbers out of it to get it working again, similar to the dash illumination controls on lesser cars.

With your demonstrated abilities with things electrical as witnessed by your success with the garage door opener, I'm confident you can dope this one out also.

The control unit for the speed control is somewhat tricky to take apart, but I have done it and successfully gotten it back together and operating again, but as I say, usually, the problem is in either the control switch or perhaps in the brake light switch adjustment.

Question from Luke (1981 - 1983):

My 1981 (a "backyard" carb.-conversion) is my daily driver and is generally a VERY reliable car. Lately, however, the cruise control has become quite erratic, whereas it used to work perfectly. When I am cruising on the freeway (euphemism for "tollroad" in Florida) and set the cruise control, it drops off of the set speed considerably, sometimes by nearly 10 mph. Even when I get it set where I want it, as soon as I traverse a hill (there are a FEW of them here, mainly on overpasses!) the speed drops way off, then as the cruise catches, it accelerates 3-5 mph above the set speed, then drops back below it again, slowly hunting until the set speed is attained, at which point I have to step on the brake to slow for a toll plaza or because some idiot in a mini-van or Lincoln Town Car has cut me off, exiting from the left lane.

I have replaced all of the associated vacuum lines under the hood and some other unassociated ones too. The speed control line appears to be attached securely to the carb. throttle linkage, with little play. I have also adjusted the set screw on the servo control (one full turn counter-clockwise, per the Service Manual,) which did not help at all. Do I need to replace the entire servo control?

Reply from Brett:

Well, it depends on how brave you are. The mechanism inside these old time speed controls dates from the earliest days of the breed, with whirling doodads and flying weights etc. They were never intended to be disassembled for service but you can do this. If you've ever repaired a spinning reel (A spinning reel is very similar in concept to a Virginia Reel. The main distinction is in the parts that are spinning. In the latter case, the parts are much more interesting than in the former. Another distinction is in the proximity of the former to the smell of old fish guts! Fortunately, this is not usually a problem with cruise control servos.), you'll feel pretty comfortable inside one of these doodads. You just have to study the parts and think about what each item does, and when. Probably you are going to find some worn parts and stick sliding thingys in there, and a general tightening up and lubrication of all moving parts will restore something close to the original performance.

I enjoy playing with things like this, but I readily admit that the modern electronic cruise control devices work 1000 times better, with rock solid speed constancy, quick engagement, 1 MPH increments when you tap the coast (-) or set (+) button and so forth. It's a curious fact, but the 4 wire control that is built into our cars is a direct match to the controls of a modern system, so if you don't mind hiding an electronic box somewhere out of sight, and gluing a little permanent magnet to your driveshaft, you can convert the car to modern cruise and no one will every notice. I have the modern type on all my really old cars (the ones that predate cruise control) because of a physical problem that means I have to use cruise control if I'm driving more than a short distance. I did rehab the original units on the cars that are new enough to have factory cruise, but that's much more work for a much less satisfactory result.

Question from Leo (1983):

I am having problems with my cruise control in my '83. It won't engage about 1/2 the time. The other day, while cruising down to Interstate, I used the lane change feature and it started to accelerate, and wouldn't quite until I turned the cruise off!. Would this be the control under the hood, or a switch problem? Also, the not engaging problem; under hood control or switch?

Reply from Dick:

Both problems are almost certainly in the switch itself, which is right inside the turn signal knob on the end of the wand, as you probably know. The acceleration was caused by the switch thinking you were holding the handle in the "accel" position, and the refusal to work sometimes is caused by a worn out contact in the "set" position. There is a small possibility that you have some frayed wires where the cable goes into the steering column - if this is the case you may be able to repair the bare wires and make the connections reliable.

These controls are a 3 dimensional SOB to get apart without damage (been there, tried that, gave up!) - your best bet is to find another control wand and change out the whole shebang. Bob Baker can probably help you with one.

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