Cruise Control

About a year ago (mid-2008), after reading Lin Rose's paper on cruise control, I decided that I wanted this addition to my Austin Healey Works Rally replica which is intended to have grand touring functionality.  I don't plan to revisit what Lin has already contributed in his well presented material.  I will touch on the differences in my installation and application.   

To begin, I will provide a link to Lin's excellent write-up on this website (link to Lin's write-up).  Lin has recently stated (June 2009) that this was the best modification he made to his car for his 8,000 mile coast-to-coast-plus tour (Going Mobile).  Most of the differences in my installation were driven by the electronic fuel injection computer (ECU) that I have installed in my car.  That led me to use an electric cruise control rather than the vacuum style that Lin uses.  I seriously doubt that there is any appreciable difference in the functioning of the units; it is just a different approach.

I have a Rostra Precision Controls Cruise Control Part #250-1223 with a #250-3592 switch with an engagement light (link to Rostra).  Rostra was the provider for Audiovox, who supplied Lin's Cruise Control (CC) unit (albeit through J.C. Whitney).  I purchased my Rostra CC through Brandon Distributing in Elk River, MN at 763-241-4172.  Brandon Distributing told me that Rostra no longer supplies cruise controls to Audiovox.  So if Audiovox is still selling the Rostra cruise controls apparently it is their existing stock.  I talked to Joe who was extremely knowledgeable and helpful.  As stated earlier, my CC unit is an electric cruise control, and it must have a VSS (Vehicle Speed Signal) wire or an available signal generator input to function.  This signal tells the cruise control module how fast your car is going.  My ECU provides a VSS connection for the cruise so that makes this portion of the installation relatively easy as there is no need to add a sensor to my driveshaft.  If you don't have a computer with a VSS connection point, and want an electric cruise control, you can add a Signal Generator or Magnet & Coil Pick-Up Kit #250-4165.  Lin details the installation of a magnet & coil pick-up in his article. 

I'm sure that for Austin Healeys there are positives to both vacuum and an electric cruise controls.  For one, to state the obvious, the vacuum unit isn't electric, so there is one less electric component to fail.  Both require that the actuating units be placed somewhere in the engine compartment.  For me the electric unit was the easy choice since I didn't want to tap into the vacuum supply for the cruise control since vacuum fluctuation is sensed by the manifold absolute pressure (MAP) sensor and the ECU adjusts the system based on that input.  There may be a way to isolate the two components, but I didn't really research it since there was a viable option for me with the electric cruise.  

I followed Lin's lead and put the cruise control switch in the ash tray.  This is very slick as it is a perfect fit. (Pic 1,2,3).  In Pic 1 you can see the metal spacer I made to put the switch at the proper height.  Be careful not to put the switch to high or it will interfere with the pivoting of the ash tray cover.  Pic 2 & 3 show the switch installed in the tunnel on my BJ7.  The switch wiring harness comes out the side of the tunnel, and I made it pluggable for when I need to remove the tunnel to service the clutch or transmission. 

The electric cruise contol module and the throttle actuating mechanism are packaged as a singe unit.  I located mine behind the front left (driver's side in the USA) wheel well housing.  While this was certainly easier with the fender off, it is doable with the fender on.  It would be a bit tight working past the exhaust pipes, but I do believe there is least I sure hope so because if maintenance is required this is how I plan to get to the control module.  (Pic 4) 

Over the years, Austin Healeys have had a few throttle linkage configurations.  In Lin's article he addresses a different version than what you see here.  You may have to get creative.  On my tri-carb linkage, the section that I wanted to add the cruise connection to was a solid shaft of linkage and fittings.  So for the cruise throttle attachment, I cut the throttle shaft, inserted a brass throttle lever I had from some previous AH linkage acquistion, and welded the shaft back together.  I found it easiest to cut the shaft in the center to give myself the most material to work with for the reweld.  I made a little jig to keep the shaft straight during this process.  The results appear very factory (works in UK speak). (Pic 5)  To attach the cable I used some aluminum cable tie downs I got from a hot rod supplier.  (Pic 6) 


When I tried to wire up my #250-3592 Switch with Engagement Light, I couldn't resolve the wiring instructions in the kit with the electric cruise control wiring instructions.  So, I called the Technical Support number listed in the Rostra Installation Manual.  The technican (Tom) immediately understood the issue and told me how to wire in a 5 terminal Bosch relay to resolved the problem.  I have enclosed that wiring diagram..... see below.

The only issue I had when I took it out in the neighborhood to test it was that the unit didn't engage properly (read the thing just didn't work!).  After going through the excellent "Troubleshooting" guide provided in the installation manual, I was led to the brake pedal disconnect switch.  This function turns off your cruise control when you depress the brake.  It seems that the AH brake pressure switch just doesn't do the job for the cruise control module.  Once again, I called the Technical Support number, and the extremely knowledgeable technican (Tom) quickly resolved the issue by telling me how to wire a relay into the brake disconnect circuit...this did the trick and everything worked as advertised. (See relay wiring diagram below.)

Warning, the diagnostic test light is in the control module (Pic 4) so before you make the final fit up with the module in a difficult to reach location test it out (it would not be a good idea to put the fender back on or think you are done working around your exhaust system.... UNTIL YOU TEST IT OUT).  Also, don't be afraid to use the Technical Support number!!....Tom is outstanding!! 

Now let me see, what other electronic wizbang can I add to this machine!!  :-)  How about 400 Watts of hidden stereo with 6 speakers!!....oh, yeah, I've already done that!


Steve Thomton

This project paper was originally posted in June 2009