I am looking for information about how the speed sensor wire used by the cruise-control control unit (and power steering control unit on ps equipped cars) works on my 84 Series 3 (I do not have ps).

Is there a varying voltage down this wire based on the vehicle speed as read by the vehicles’ speedometer?

The reason I ask is because I am thinking about installing an electric fan on the air-conditioning condenser to boost the cooling of the condenser when idling in traffic. The cooling currently relies on the engine fan which is only running at idle speed and can be inadequate on a tropical summer’s day.

It is strange that there was no such electric fan, factory installed on the RX7, when other Mazda cars in the 84 model year like the 626 and 929 had them.

Advice from the local Auto Electrician was to wire the electric fan in to run when the air conditioner compressor is compressing (electro-magnetic clutch is engaged).

This would have the electric fan operating at speeds when it would not be required (such as cruising on the highway) so I thought that it would be better to (if possible) use the speed sensor as input as well so that the electric fan would only operate when the vehicle was moving below a certain speed AND the air conditioner compressor is compressing.

All advice welcome.

 

your auto electrican is correct. An aftermarket electric fan t-stat is equipped with extra wire that connects to the compressor's + terminal. it will immediately turn-on the fan when the a/c is running.

with the speed sensor, I did some reserarch on that wire too as I noticed a problem with my P/S over flowing. Actually, its power assist not power steering. A power steering will make the steering soft at any speed. With the RX-7, its only soft on low speed. My search for an answer didnt last as I eliminated the p/s pump completely.

 

You asked for 'all input' so here you go;

The Cruise Control (CC) system uses engine vacuum to determine vehicle speed to set, which accommodates going up and down hills to adjust the throttle position, and thusly the vehicle speed. The CC, being vacuum operated, does not have any leads which may help you with the fan activation other than the CC 'on' lead going to the ECU. This lead tells the ECU that CC is engaged and overrides previous ECU maps so that the EFI system goes into closed-loop fuel injection mapping. The 5th gear on the transmission also supplies a lead for this purpose to the ECU, known as the 5th gear switch. Similar function.

I agree with your assessment that an aux cooling fan specifically for the A/C condensor would be useful on hot days, but probably simpler to just rig it to the A/C compressor electro-mag clutch. This will spin freely when underway and draw very little electrical load when the front air dam is feeding the condensor.

You're increasing complexity to use the CC leads when the net result could be a lot simpler and more reliable. HTH,