How to make 1 gauge do twice the work

How many of you have wondered just what the OE temperature gauge was telling you? While it was nice of Mercury to provide us with a needle instead of an idiot light, it leaves a lot to be desired. How many of you could tell the outside temperature if the thermometer had no numbers? I can't. If you can then you won't be interested in this little tip. If you can't, read on.

We all know that space is a commodity in our Cougars. If you have an XR7 then the dash is full. If you have a console then under the dash is full. If you have a '69 or '70 with console you can install a Shelby gauge pod and add 2 more instruments. But which two? I chose to install a vacuum gauge and an electric transmission temperature gauge. The Trans Temp gauge is what we are going to talk about.

After installing the gauges, I could tell you exactly what my Trans Temp was, but my engine temp was still Cool, Normal, or hot. I had no idea what the Ford guys considered "normal" and if my sensor was not the correct one then the gauge would not read right anyway. (I discovered this after installing a new sending unit and seeing my gauge read about 50% higher. Turns out '69 XR7 sending units are unique and VERY hard to find.) So I got to thinking that the gauge does not know if it is sensing oil or water. It simply reports what the sensor tells it. And the sensor is stupid too. Water, Oil, Air, it doesn't know what fluid it's measuring and it doesn't care. Now we get to the numbers part. By running two sensors and switching between them, I can get real number readings from my engine, and not have the redundancy of multiple Engine Temp gauges. So I installed a SPDT (single pole, double throw) switch and a second sensor to create my 2 in 1 temp gauge. (BTW - this will work with your factory dash temp gauge if you want to keep originality but just want to keep an eye on the A/T.)
Here's how I did it:

    Parts List

  1. A second sending unit for your temp gauge. Since I used AutoMeter gauges, I purchased a # 2258 sending unit.

  2. Number 2258 sending unit

  3. An SPDT switch. I used a Micromini Toggle Switch from RadioShack. It's a bargain at $3.49 and comes with 4 colored sleeves - Red, Green, White, Yellow so you can pick which one you like, or leave it "chrome".
  4. SPDT

  5. Enough wire to go from each sending unit to the switch, and one from the switch to the gauge. I used 16ga just because that's what I had handy. I'm sure that 18ga would be fine since there is no "load".

    Hooking it up

  1. Pick the location for your SPDT switch. I mounted mine between the gauges in the gauge pod:

  2. Switch in pod

  3. Install the new sending unit (sensor) in the engine. Make sure that you install it into the water jacket, not vacuum.
  4. Sending Unit

  5. Run a wire (yellow in drawing below) from the engine sensor to your SPDT switch.
  6. Remove the AT sensor wire (yellow in drawing below) fom the back of the gauge and re-route it to the SPDT switch. Replace this wire if necessary.
  7. Attach a wire (red in drawing below) from the center post of the SPDT switch and route it to the gauge. Hook this wire to the post that you removed the AT wire from.
  8. Attach the wire from the engine temp sensor to one of the outside posts on the SPDT.
  9. Attach the remaining wire (from the AT sensor) to the remaining post on the SPDT switch.

  10. Wiring Diagram

  11. Mount the SPDT switch with the supplied nut.
  12. Secure your wiring so it is safely out of the way and won't get burned or chaffed.
That's it. Turn on the ignition and move the switch lever from side to side to read Engine or Transmission temperature.
(You may have to start the engine to get some heat in it, but you are all done.)
You now have 1 gauge reporting the status of 2 different components.

  • I installed my AT temp sensor in the pan. This worked out great because I also installed a B&M AT drain plug and the AutoMeter sensor replaced the drain plug.

  • You can do this with your factory in dash gauge by installing a second sensor in the AT. I don't know what the limit for a stock gauge is, but an AT can go over 200F easily, so I don't recommend it.

  • Instead of a toggle switch you can use a rotary selector type switch.

  • This project cost me less than $20.00. The most expensive part was the sensor, which cost $13.99 at my local speed shop.

  • The gauge pod is available from most classic Mustang vendors for about $40.00 and the AutoMeter gauges were about $40.00 each.