Cruise control actuator - how to repair

[pingu]

I describe below how to repair the cruise control actuator. The problem was due to a bad connection between a motor inside the actuator and a printed circuit board (PCB) inside the actuator.

I don't know what the symptoms were as the problem occured when my Indy was replacing the actuator cable to the throttle (the old cable was was badly frayed). Because the cruise control wasn't working (they said something about the car racing ahead), they disconnected the actuator - I never actually experienced the symptoms myself. Although they offered to attempt to fix the actuator, it was easier for me to do it myself.

An excellent start for repairing the actuator can be found at http://www.bmw-540i.com/bmw-technica...oads/index.php

The actuator is held to the car by three nuts. You'll also need to disconnect the wiring loom from the actuator, and disconnect the actuator cable from the throttle (it's fiddly but quite easy to disconnect the actuator cable). Then start taking the actuator apart - you'll need to remove the cogs, the spring (it's pretensioned by one turn) and the rest of the gubbins.

Once you've dissassembled most of the actuator, you need to remove the PCB. To do this you first need to remove the PCB securing screws. Then you'll need an "L" shaped tool that you can get underneath the PCB. At several locations around the PCB, gently work the PCB out of the actuator (you need to be quite firm as the PCB is tightly attached). As you work the PCB out, you'll be disconnecting the PCB from the motor and also disconnecting the PCB from pins that protrude from the housing into the interior of the actuator.

One of the attached pictures shows the L shaped tool (cyan arrow), pin sockets (green arrows) that connect the PCB to the pins that protrude into the interior of the actuator housing, motor sockets (red arrows) that connect the PCB to the motor, and a plastic aligner (dark blue arrow) that holds the pin sockets together.

Another of the pictures shows the plastic aligner (dark blue arrow) mounted over the pin sockets (green arrows).

The final picture shows the housing pins (green arrow) that protrude from the housing into the interior of the actuator, and also shows (red arrows) the two pins of the motor.

The problem was that over time, the motor sockets (red arrows) had lost their springyness and no longer made a decent connection to the motor pins. The document I linked to above gives the motor resistance as 10 ohms; I was getting 18 ohms (due to the bad connection). After repair, I got about 9 ohms for the motor resistance.

I repaired the actuator by pressing in the contact splines of the motor sockets. Each of the two motor sockets has two contact splines that are meant to grip a motor pin. I pressed the two contact splines inwardly so that they touched (when there was no motor pin inside the motor socket), and then did the same for the two contact splines of the other motor socket. By making the contact splines touch, you ensure that the contact splines will firmly grip (and thus make a good electrical connection with) the motor pins.

If I was to do it again, I'd probably get rid of the motor sockets and solder a small bit of wite to each of the motor pins, and then solder the wires to the PCB. It's your call.

When it's time to put the PCB back, you first need to put the plastic aligner (dark blue arrow) over the pin sockets (green arrows). Then, if you haven't removed the motor sockets, align the motor pins (red arrows) with the motor sockets (red arrows) and also align the pin sockets (green arrows) with the housing pins (green arrows). Firmly push the PCB back into place - the plastic aligner (dark blue arrow) acts as a depth stop.

Reassemble the actuator, re-attach it to your car and enjoy cruising!

[t.wak]

Thanks for the great writeup! I think this would be the problem with mine, also...

Only one issue for me, it's a new car, and I don't know where the Actuator is; fill me in? Haha, thanks!

[t.wak]

Nevermind, I found it near the end of that PDF file. Thanks again.

[pingu]

Good luck with the repair - it saved me over £300.