Adding Cruise Control to a Mk I

By: Marcus Corbin

Yes, adding cruise control to an MR2 that didn't come with it can be done relatively easily! - at least to an '88NA like mine. The parts all bolted on, but the car was not, repeat, not pre-wired for cruise control. There are reports that some earlier cars did have the wiring already in the looms. Toyota must be clever if they can save money by customizing wiring harnesses with or without a few wires for different electrical options.

Sacrilegious, some of you will say to install cruise to a sports car like an MR2! Maybe, but I like to keep my spirited driving to the twisties, and allow for a little more relaxation on the monotonous drives the U.S. National Defense Highway system provides. Take your pick.

The description below may look long, but the job had none of the usual physical challenges or complexities. It would have been extremely easy if the wiring was in place, but even without it, there were no particular difficulties. The good news was I took all the parts off a junkyard '87 for just $40, and that was without even bargaining! Another yard quoted $100.

In my early driving test I found the cruise control had a range of about 2-3 mph up or down on moderate grades. Disadvantage of adding cruise control for weight freaks is some extra pounds, at a very rough guess perhaps 4-5 lb?

I wrote up some notes based on my experience. An incorrectly-operating cruise control is obviously a very dangerous thing, so anything you do to yours, do at your own risk.

Special tools needed were:

wire stripper
steering wheel puller
Optional:
side-handled ratcheting screwdriver or socket wrench screwdriver attachments for steering wheel centerpiece removal (a convenience)
small alligator-clip connecting wires with rubber boots (this and next for testing purposes)
vacuum gauge/pump

Parts to get at junkyard or elsewhere:

Control switch stalk on steering column - Tip: The wiring guide shows only the wire colors coming out of the farther (outboard-from-the-switch) wiring connector, but the first (inboard) connector wires are different colors, so make sure you write down the order of the wires as they enter the inboard connector, or else you won't know which wire is which based on the book alone. I cut my wires before the first connector, just downstream from the switch. Or you can just cut the second connector too and take it with you from the junkyard to compare the relative wire positions at home (bearing in mind that, when installed, the connector top-catches of one connector is up, the other down). If you can, take the second connector and the junction strip that both it and the first connector plug into (that part may not come off easily), then do so, because having the connectors is useful for later circuit testing.
Steering column trim, upper piece - the normal one doesn't have a hole where the stalk comes out. One could presumable cut a hole in the old one.
Brake light switch, located above brake pedal arm - it's two switches in one, the normal switch won't do, since it only has one internal switch.
Vacuum switch (this and next two are located at left front of engine bay, behind the battery).
Vacuum pump with long vacuum hose.
Actuator with accelerator cable.
Accelerator pulley bracket on the cylinder head cover - the normal one is nearly identical, but it does not appear that you can add the second cruise control pulley, the old one is heavily riveted on, so get the special one.
Cruise control computer - located behind the panels at the left front of the rear luggage compartment. It says what it is on it, mine was light green.
Get all the wire/hose clips, bolts, and screws, I recommend label-bagging them.
Quite a bit of new wire, multicolored to ease differentiation, and some wire ties.
The wiring diagram book and the BGB.

Cut the wires at least several inches away from their parts so you have room to make new wire connections.

Installing the parts:

This is straightforward as all the threaded bolt holes were ready and waiting on my car. Memory refreshers are at BE-6 and -7 in the BGB (although I believe the vacuum hoses going into the vacuum pump may be backwards for my year). I took off the lower instrument panel and the air vent duct too for easy access (careful, the lower panel is only a couple inches wide at one point and can easily snap). The old brake light switch above the brake pedal needs to be replaced with a cruise control version with a second circuit in it. When installing the new one, how far it is threaded in determines when your brake lights come on, so make sure it's at the right distance, it's quite sensitive to position.

Tip: When taking off the steering wheel to install the switch and stalk, find a reference point so that you can put the wheel back on aligned exactly the same. Otherwise your steering wheel will be off-center.

Attach the vacuum hose to the throttle body, and tie it to things along the way as it goes around the engine bay, to keep it secured.

Running the wires:

Besides the wiring diagram book, there's an additional but less informative wiring diagram in the BGB at BE-39.

Quite a few wires run from the steering column switch all the way back to the computer. I ran them along the lower left footwell area, under the plastic trim on the sill in the other wires' channel, up behind the carpet on the firewall behind the drivers seat, through a large wiring harness rubber grommet in the firewall, along the left side of the engine bay behind the fuse box, through another large grommet into the trunk, and the computer is right there. Since going through the middle of the grommets would require a large amount of destructive untaping of the wiring harness, I cheated and just put the wires through a small hole in the outer part of the grommet ring. There's a potential for small water intrusion then, but the new wires could be taped up to the grommet similar to what Toyota did. Tip: to get the wires through a grommet that you can't reach easily from one side, slide a spare wire through from the other side, tape it on to the real wire you're threading, and then pull the real wire back through with the spare wire.

Wires to and from the actuator/vacuum pump are right along this same path.

It takes several steps to get the carpet and insulation behind the seat up so one can access the forward grommet, including taking off the three trim panels in the area, the t-top locking pin cover, and the top seatbelt bolt.

Tip: remember to thread the wires between the seatbelt and the side of the frame, and under whatever wires/hoses you want in the engine bay, otherwise you'll have to unbolt things to get the wires in your desired final locations.

Connecting the wires:

I ran about 17 wires in the end. I didn't do the parking brake wire, nor the ECT/EFI wires. I didn't do the clutch cutoff wire at first, but plan to - if you don't do this wire, and you change gears while operating under cruise control, the engine can overrev very quickly because the cruise control computer doesn't know there's no load when you push in the clutch pedal.

The cut-wire to cut-wire hookups are straightforward, but the splicing-in locations have options: For power to the cruise control switch, I spliced into terminal 3, junction box 3H connector (referring to the wiring diagram for the '88). The junction box is beyond the dead pedal. Its wires are very short, so it's definitely easier to unscrew it to get a little more twist access to the panel.

For the ground out of the switch, I pushed the wire into a spare crimping channel on one of the wires going into the indicated ground at "cowl left."

The speed sensor (in the instrument cluster) looked tricky until I realized the wiring for it goes close to the cruise control computer in the trunk on its way to the main EFI/TCCS computer located further right in the trunk. So it was just a matter of splicing in to the trunk wire rather than messing around behind the instrument panel.

The original brake light circuit I spliced into after terminal 6, junction box 3G connector, and also into the harness going into the left brake light assembly in the trunk.

Testing:

The BGB has some tests that can be done before a road test. The stalk lights serve as a fault code blinker similar to the check codes from the instrument panel warning light. There's also a procedure to check the accelerator cable freeplay.

Good luck!