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Discussion Starter #1 (Edited)
Mods, feel free to move this thread if this is not the appropriate place.

I converted my non-cruise 91 to OEM Toyota cruise control about 2 years ago. It has been functioning without a hitch with extensive daily use ever since. I did the conversion because my MR2 is my daily and I have a 100 mile round trip daily commute. Having cruise control really makes that drive much more pleasant. It took a lot of research and some engineering work to do the install. The intention of this post is to document all the work I did, so others can use it to either install cruise control on their own car, or troubleshoot their cruise control. This guide is applicable to both Turbo and NA models. Automatics are a little different - but the majority of automatics were equipped with cruise control anyway. 93+ Cars will be a little different because they have electronic sending units for the speed. 95% of the information here will be the same for 93+, however most 93+ cars came with cruise control.

Total out of pocket cost was ~$400
Total install time (excluding doing the R&D) was about 8 hours. (The R&D took weeks, but I've already done that for you.)

The vast majority of the information you need can be found in the linked (below) excel sheet.

MR2 Cruise Control Conversion Walk-through

I'm in the process of creating a similar document detailing my 93+ (REV2) OEM ABS system install on my originally non-ABS MR2. I'll create a separate thread detailing that when complete.

Section 1: Required Parts

See the "General Information" Page on the Excel Sheet. The "Helpful Pictures" Page has pictures of most of these items.

You will require need to acquire the following parts:

-Cruise Control ECU, P/N 8824017140
-Speed Sensor, P/N 8826120070
-Cruise Control Switch, P/N 8463017030
-Cruise Control Stop Light Switch, P/N 8434030070 (Not 100% sure it is the right P/N; make sure it has a 4-pin connector)
-Cruise Control Actuator, P/N 8820017070
-Actuator to TB Cable, P/N 8373117010 (Turbo P/N, N/A might be different)
-Floor Harness, P/N 8216117300
-Speed Sensor Coupling Key, P/N 8379820020
-Cruise Control ECU Bracket Lower, P/N Unknown (should come with the Cruise Control ECU)
-One Cluster light bulb and socket (smallest ones)
-C17 Pigtail Connector (Cruise Control ECU connector) - can be taken from a junk yard, common Toyota connector.
-S9 Pigtail Connector (4-Pin stop light switch connector) - can be taken from a junk yard, common Toyota connector.

Optional, but recommended parts to acquire:
-Cruise Control ECU Upper Bracket, P/N 8829217030 (Provides a 2nd bolt in point - hard to find part)
-Cruise Control Actuator Bracket, P/N 8829117030 (Provides a 3rd bolt in point - hard to find part)
I have neither of these installed on my car because I could not find them anywhere. If you can find them great! If not, don't worry about it, they won't affect the functionality or reliability of the system.

Optional, but not-recommended parts to acquire:
-Cruise Control Cluster (you can make the non-cruise one work easily)
-Cruise Control Throttle Cable (you can make the non-cruise one work with careful routing)
-Cruise Control Clutch Switch P/N, 8828020010 (non-cruise one works fine, more on that below)
-Dash Harness with Cruise Control

Some notes on the above not-recommended parts.
1) Cruise Control Cluster: The non-cruise cluster has the cruise control indicator in it and the required wiring; it doesn't have the bulb or socket installed. Additionally, the contact pads for the light are not exposed. The plastic film of the ribbon cable is still covering them. Use some fine sand paper or emery cloth and sand the plastic film off of the contacts until the copper is exposed. Be gentle. Then cut the center section of the plastic film out so you can insert the bulb. While you're at it, do the same procedure for the Power Steering and ABS light sockets. That way if you ever decide to install these systems you won't have to take the cluster out again.

2) Cruise Control throttle Cable: Very hard to find part and difficult to install. If you have one and don't mind swapping it out, I recommend doing this as it will be the cleanest solution. Otherwise, the existing non-cruise throttle cable will work fine. You will need to loop it around a few times to take up the extra slack. Make sure there are no tight bends. I used the non-cruise cable for 2 years before I found the right throttle cable.

3) Cruise Control Clutch Switch: The cruise control system uses a second clutch switch. It activates when you first press on the pedal. The clutch start switch activates when the clutch pedal is at its maximum throw. The cruise Control Clutch Switch is a pain to install. I didn't do it. You can either leave it unwired, or splice it into the Clutch Start Switch. If it is left unwired, the cruise ECU will rev the engine to redline if you press on the clutch pedal without pressing on the brake. The rev's increase slowly enough that you can shift without issue. If you wire it to the Clutch Start Switch, the cruise ECU will stop the revs, but only when the clutch pedal is at maximum throw. You shouldn't be shifting or using the clutch with cruise control engaged anyway so I didn't see installing this switch as worth the effort. In 2 years of daily use, I've only stepped on the clutch (without stepping on the brake) with cruise engaged twice.

4) Dash Harness with Cruise Control: Non cruise dash harnesses do not contain the required cruise wiring paths (the body harness has the cruise paths). You can either swap in a Cruise Dash Harness, or add the missing paths by making and installing your own harness. I made my own. I think this was probably much easier than finding and swapping our the dash harness.

Some notes on the Speed sensor:
The 91-92 Cruise Control Speed Sensor assembly consists of the 6 parts shown below. All parts are extremely hard to find. If you find them all, then great! If you can't, then you really only need F and E. and you can reuse your non-cruise speed sendor in place of A,B,C, and D. If you do this, you will need to modify part E. Both ends of part E are keyed. You will need to grind one of the keyed ends flat to interface with the non-cruise speed sender. This may be slightly different for turbo and non turbo cars, so double-check fitment before grinding.

Section 2: Installing the parts:

Install the parts as follows referencing the below picture:

-Cruise Control ECU: Bolts under the dash to the right of the glovebox. You need to remove the govebox.

-Cruise Control Actuator: The Motor Assembly bolts in with 3 bolts (2 if you don't have the lower bracket) to the rear or the RR strut tower. The pulley assembly bolts in with 2 bolts to the front of the RR strut tower.

-Cruise Control TB cable: Hook the Cruise Control TB cable up to the Throttle, and the other end to the pulley assembly on the cruise control actuator.

-Throttle Cable: Hook the throttle cable up to the cruise control pulley assembly. If using the existing throttle cable find a new way to route it with long sweeping bends to take up the slack.

-Brake Switch: Swap out the Brake switch with the 4-pin cruise control version.

-Install the Speed sensor in between the speed sender unit on the transmission and the speedo cable. Plug in the pigtail to the body harness (the body harness has this connection even on non-cruise cars). You may need to hunt for it a bit. CLEAN THE CONNECTOR AND PINS BEFORE PLUGGING IT IN!

-Install the Cruise Control Switch in the steering wheel. You will need to remove the airbag. I recommend disconnecting the battery until you have this job done as you'll probably end up blowing the horn a lot. Also if you leave the battery connected, you might get an Air Bag light when you remove the airbag. The clockspring/spiral cable has the required wiring paths for the cruise control switch even in non-cruise models. Your cruise control switch should come with one of the air bag brackets spot welded to a ground wire coming from the switch. Use this new bracket. If the bracket it missing, you will need to ground that wire to one of the airbag brackets in the steering wheel.

-Install the Cruise Control light in the Cluster (if you haven't done so already). Leave the cluster uninstalled since you will need to get back in there to install your custom wiring harness.

-Install the floor harness: The Floor Harness AKA the cruise control harness runs from the passenger footwell to the cruise control actuator. It also carries the ABS sensor wiring for the RR sensor. If your car has ABS, then you already have this harness. If you do not have ABS, then you will need to install this harness. It connects to IK1 (Circled below) in the passenger footwell, and then runs under the carped along the door sill towards the back of the car (red pointer). It then goes through the firewall and snakes it's way back up over the RR strut tower to the cruise control actuator. You will need to remove the door sill plastic covering, the Passenger Rear seatbelt/speaker cover, and pull back the rear firewall carpeting

C5 is the Cruise Control Actuator connector, and A2 is the ABS sensor connector.
Floor Harness 2.png

Section 3: Create the Custom Cruise Control Wiring Harness

The final step is to create and install your custom cruise control wiring harness. Sheet "Existing Wiring and Conn C17" in the excel sheet lists all of the wires coming from the Cruise Control ECU connector (C17) that need to be connected. Red highlights are paths that don't exist on non-cruise cars, green highlights are paths that do exist on non-cruise cars, and yellow highlights are paths that I didn't verify their existence, but ultimately don't matter.

Note: There are two more wires that are not listed on this sheet because they does not go through Connector C17. One connects the cruise control actuator to the stop light switch. The other is the grounding wiring for the cruise control switch (C14) that connects to the airbag bracket in the steering wheel. Wiring from C17 Pins 9, 21, and 22 are for Automatic Transmissions only. I didn't include these paths in my new custom harness. Ignore if you do not have an automatic transmission.

Sheet "New Harness Wire List & Diagram" has the wire list and wiring diagram for the new harness I designed that needs to be installed. The wiring Diagram is shown below as well.

The rest of the sheets show extremely detailed info for all of the wiring paths and the connectors they pass through.

A couple of notes that will make your life A LOT easier:
1) Obtain a Cruise Control ECU pigtail Connector (C17) and a 4-Pin Stoplight Switch Connector (S9). These can easily be found on ebay.
2) Cut off the 2-Pin Stop Light Switch connector and Splice Pins 1 and 3 of your S9 pigtail in its place. The pin order doesn't matter.
3) On the diagram below, the unterminated lines (far left and far right) connect to existing wiring now on the car.
-For example, on the left of the diagram, all of the lines labeled C17-XX connect to the C17 (Cruise ECU) pigtail.
-On the right, S9-2, and S9-4 connect to the S9 pigtail wires 2 and 4 you just installed.
-The ebrake switch wire (P1-1) is spliced into the wire coming off of Pin 1 on the ebrake switch.
-All other wires such as IK1-XX, IE3-XX, etc connect to existing connectors on the dash harness. The catch is these connectors are missing the required pins. The best way to make these connections is obtain some Toyota connectors from a junkyard, salvage their pins, and create pig tail stubs for each of these wires that you then insert into the existing connectors.
4) The Connectors starting with MJ and MP are Molex Connectors I added to make the new harness modular. You don't have to include these, but I recommend it. It makes installation and troubleshooting easy. You can build the harness outside the car, then just install it in one go like a normal wiring harness. The majority of the branches should be about 6' long, except for the IK1 branch which only needs to be a few inches.
5) The ground lugs bolt to any good chassis ground.

Section 4: Final Hints & Lessons Learned

1) There is a lot more info in the excel sheet. Almost everything you would want to know is in there compiled for quick access.

2) Test the Cruise Control Actuator and Cruise Control Switch, and 4-Pin Brake switch in accordance with the BGB prior to installing. These parts do go bad (some of mine were DOA).

2a) Test the Floor Harness for continuity prior to installing.

3) If you get a Cruise control error light when shortly after starting the car, randomly when driving, or soon after disengaging cruise control with the brake pedal, the stop light switch is bad. Another symptom can be cruise control erroring out in colder weather or wet weather and then the error goes away as the day warms up or dries out. I estimate 75% of cruise control problems are stop light switch related. It's also frustrating the that codes thrown by a bad stop light switch rarely ever point to the switch. If you are having cruise control error codes, replace that switch before doing anything else even if the switch tests good.

4) Extremely sudden and hard braking while cruise is engaged can on occasion throw a cruise control code. Nothing is wrong, just cycle the ignition to clear the code and continue on.

5) Don't test the cruise control system with the car on jackstands. The system will freak out and you will think something is broken or not installed correctly.
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