Inside Econocruise Tester

On another thread and in “The Book” Kirby gives a simple way to test cruise control. That made me curious what is inside the factory “Installation Test” AE Econocruise Tester. Honestly it’s not much…


If you have it on hand, you could even produce the schematic…!

I have had one of these for years, and an 83 series 3 just came in that wants the cruise fixed. Of course I can do a pin-out test, but I would love to try this out. Problem is, I can t find any instructions. Any ideas?

Glenn of Glenn’s MG in Florida? Welcome!

My understanding is that you unplug the big cruise control plug in the module under the dash, and plug it in the tool. Then you plug the tool into the module. Turn the big cruise switch on console and you’ll get 9 to light up, step on brake and 6 should light, etc. that’s all done idling. On the right the big red bottoms are actually lights also, and with the black buttons on side you should be able to dump the vacuum etc. mine had no instructions but this is how it was explained to me.

In reality, there are three things possibly wrong… no power, a cruise bellows leak, or rubber foot fell off the dump valve itself… “ Yet another common failure is that the tiny rubber seats on the solenoid plungers fall off, and the solenoids no longer seal the ports. To repair this failure, merely glue the rubber seat back on the end of the plunger. After removing the bellows, the solenoids can be removed by prying them out of the housing to get at the plunger. If you’ve lost or damaged the rubber seat, use a hole punch on a bicycle inner tube to make a new one. Make sure the sealing surface is flat and smooth before reassembly.”


We had already freed up the stuck valves in the bellows assembly and had tested that. I was able to get the tool hooked up, although there were three “extra” plugs that I did not have a purpose for. I merely hooked them together since they were obviously supposed to tie into some sensor or switch.
All the components checked out except the engage switch on the steering column switch, which we had already ordered a new one for since the clamp was broken. The system is working now that we replaced that switch. Thanks for the feedback. I am going to keep my eye open for the actual instructions for this tester. They were not in the Dealer Factory repair manual which I do have a copy of. It is a wicked cool tool, showing car the speed signal, as well as every signal that is required to make the system work.

Instructions for use are in the “XJ-S HE Electrical Fault Diagnosis Handbook”, AKM 9051, edition 2, page 30.

Thank you. I will check to see if I have that here in my collection. I thought that it might be in some other Model manual like that.

If you can’t find it, let me know. I may be able to find a pdf of it.

I found it in an XJS factory manual. I have scanned it, and also added a photo of the inside of the module showing the two adjustment pots for set speed, and low speed lock out speed.

I tried to upload it here but got an error message about being a “new user” and that I could not upload files.

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Upload it to a public access portion of Google drive, and provide a link here.

The instructions are also in the XJ-S Repair Operation Manual (ROM), Supplement A, page 86-14.