Installing a Transmission Temperature Gauge
Paradise Garage

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© 1999 Brian F. Schreurs
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This is an ex-parrot!!

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The problem with automatic transmissions is that there's no good way to tell how they're doing. They run fine until one day it shifts a little slower or makes the dread "funny noise." Even those few who can find the trans dipstick often don't know the proper test procedure, and even under the best conditions diagnosis usually ends with "whoops, better start saving up."

We did not want this fate to befall the visiting 1995 Mustang GT. Even though we're dying for an excuse to drop in a five-speed, total transmission failure is not the route we have in mind. We've already installed a transmission cooler but we thought it'd be nice to keep an eye on things. To this end, we're installing an AutoMeter electrical transmission temperature gauge (#2640) which we're mounting in a Mustangs Unlimited dual- gauge A-pillar pod (#77L409).

Remove the transmission pan. It's held on by a zillion 10mm bolts. The best way to avoid geting soaked is to remove the side bolts first; then the rear bolts; then the outer front bolts (now you'll start seeing fluid); and finally, slowly undo the last two front bolts. Be sure to have a catch pan and an oil eater mat.

Blecch -- transmission fluid everywhere.
Once the fluid is done pouring all over the place, carefully remove the pan. Remember -- there's still fluid in there! Pour the excess into the catch pan but leave the catch pan under the trans to catch the dripping from the valve body. Clean the pan and the magnet -- yecch. If you still have a dust thingy, keep it! It'll amaze the next guy to service it.

Now you need to put a hole in the transmission pan. Our kit came with two different plugs, one substantially larger than the other. We decided to install the larger plug because it gave the sensor much more contact area with the trans fluid. By some happy coincidence, the larger plug is the exact right size to use a 3/8" hole cutter on a power drill.

The hole needs to be placed somewhere on the pan where it will not interfere with any trans parts. Clearances are tighter than they look so be careful making your selection. We chose the flattened corner on the left-rear of the pan.

Once the hole cutter has done its work, the hole will likely need to be widened slightly with a rat- tail file. We do mean slightly -- just enough to get the plug to screw in. By the way, when you screw the plug in, it goes in with the hex-side out.

Here's the sender plug in position...
...and now rather permanently brazed in place.
Not that it'll do you much good at this point, but test-fit the pan with the plug in place to see whether you picked a good spot. If you screwed up, throw out your old pan and get another. Lucky for us, we chose wisely and were able to continue.

If you're like us, you don't have fancypants brazing equipment lying around. Find yourself a welding shop to do this part because it's important. Get that plug brazed in place so that it won't move or leak. It should only cost a few bucks -- $15 in our case, done by Erin's Welding in Woodbridge, Va. And no, thread sealer or thread tape are not acceptable substitutes for getting it done right.

Insert the temperature sender, using thread sealer to keep the trans fluid where it belongs.

Put the pan back on. (That missing bolt is in the drain, no need to thank us.)

Wire up the sender. You'll also need a ground wire so you might as well take care of that right now by grounding to one of the transmission pan bolts. Be sure to use 18-gauge wire, and remember it has to reach all the way to the A-pillar so use a lot of it.

The sender is in place, wired, and grounded.
The sender wires head past the master cylinder and through the grommet.
Both the sender wire and the gauge ground have to make it into the dashboard (it's important to ground the gauge near the sender; if the ground resistance differs greatly from the sender resistance, the gauge could read funny). To get there, run the wires along the bottom of the transmission (and through a convenient unused bolt hole), then across the front subframe, up to the master cylinder, and through a firewall grommet located on the passenger-side of the master cylinder.

Had we been thinking, we would have poked a hole in the center of the grommet and run the wires through that. We weren't thinking, however, so we just took the grommet out and ran the wires through the big gaping hole. When we went to replace the grommet we had to cut a notch in it for the wires. Oh well.

If you're attaching more than one gauge, stop here and wire the other sender as necessary. We also hooked up an oil pressure gauge.

The rest of the work is done inside the car. Note that if you're installing more than one gauge, all these instructions apply to all gauge wiring.

If you haven't already disconnected the battery, do it now! You'll be working around the air bag and they don't like being disturbed.

Remove the plastic trim covering the A-pillar on the driver-side. It's easy; just pull hard.

Take the dashboard apart. To get the front fascia off, you must first remove the headlight knob. Pull the knob to its full-open position and look for a slit near the base. Supposedly there's some sort of clip in there that you just loosen with a pick or punch, but we dunno. We just fiddled with it for about a half hour till it fell off. It'd probably be easier to just buy a new one and break the old one off.

Whack dem posts off with a hacksaw.
With the headlight knob out of the way, it's possible to pull the fascia off. It's held in place by two Torx bolts, size T-15. They are located along the upper edge. With those removed, gently pry the fascia off.

This will give you access to run wires from the grommet to the A-pillar. Instead of trying to shove the wires up through the dash (you can't push a rope), use a spare length of wire as a guide line. Start at the A-pillar and route it down through the dash, to where the sender wires are waiting. Tape the sender wires to the guide wire and pull them back up. It may take more than one trip since you'll likely have a fairly large wad of wiring to cram through some fairly small openings.

Here's a quick inventory of the wires you'll be running:

  • sender wire [two if running two gauges]
  • ground wire [two if running two gauges]
  • gauge power [two gauges can be spliced together in the A-pillar, for just one wire]
  • illumination power [two gauges can be spliced together in the A-pillar]
  • illumination ground [two gauges can be spliced together in the A-pillar]

The wiring is only confusing if you didn't carefully label them.
So if you're just installing one gauge, there will be five wires. If you're installing two gauges, there will be seven wires (assuming you splice together wires where you can).

You're already routing the sender and ground wires up. You'll also have to route the power and illumination wires back down. Before you connect the wires, you should position the gauge in the A-pillar pod.

Basically, the gauge has two mounting studs that get in the way of everything. Use a hacksaw to cut them off. Now the gauge should just press into the pod without any trouble.

With the gauge in the pod, continue the wiring job.

The sender wire and ground wire should already be attached on the chassis end. On the gauge end, they attach to the gauge posts as labeled. If you lose the label, it's also stamped into the housing. In fact, all the wire posts are carefully marked on the gauge end. Just follow the diagram on the back of the gauge, and we'll help with the chassis side.

The gauge power wire needs to get ACC-on power (i.e., it's off when the car is off but it's on with the key in the ACC position). We already have ACC-on power because of our Foglight Switch but you'll find plenty of wires to choose from under the dash. Just use a multimeter to find a wire that meets the criteria.

Now, the gauge needs to light up when the parking lights are on. The headlight switch is right there, so it's easy enough to tap into that and run the gauge in series with the parking lights.

Wrong wrong wrong wrong wrong!!!

Ford has some bizarre setup in the headlight switch, wherein it receives power when it's off and no power when it's on. If you tap into this, your gauge lights will work exactly opposite what you want. Not that we did it or anything. Not us. Nope. We heard about it somewhere. Yeah.

The illumination wires need to tap into power and ground for your parking lights. You can look around for the harness under the dash if you want. It's gotta be there somewhere. We cut out the middleman and went straight to where we knew the circuit would be the way we wanted: we spliced into the wiring harness at the front-driver parking light.

Patch into the parking lights to get the gauge lights to work properly.
If you want to do it our way, lift the plastic flap covering the driver-side headlight. From there you'll be able to access the bolt (sorry, we forgot to write down the size) that holds the side light in place. Then just disconnect the wiring harness from the housing. As usual with these darn plastic clips, they're brittle and likely to break. So be careful.

Use a blade to cut away some of the plastic insulation, then use a multimeter to determine which power wire is the parking lights (you don't want your gauges to blink with the turn signal do you?). Run the illumination wires out to the harness (we just followed the route we already used for the Fan Switch) and splice the power wire into the harness. Then put the insulation back on and tape everything up.

The ground wire can ground on one of the green screws just next to the battery, on the radiator support. Easy. Then put the marker light back together.

Trying to jam all those wires under the pod can be tricky, but they'll fit.
The gauge should be all wired up now. All that's left is to mount the pod to the A-pillar.

Put the factory A-pillar trim panel back in place. We're using self-tapping screws; the tape is worthless. Wedge the pod as far down on the A-pillar as it will go. It should sit there on its own, although it might pull away a little. No matter. Check for fit and to make sure all the wires are tucked away.

The trans temp gauge is on the bottom.
Home Depot sells plastic caps to hide the screws. These are cool, but hard to open once closed. To fix this, use a knife to cut a small slit in the side of the cap. That way you can jam a pick in there to pop it open when it comes time to service the gauge.

Slip the screws into these caps, then get in the car and hold the gauge pod in position. Line them up, one on each side of the pod's curve, and just push. They'll screw into the aftermarket and stock trim panels with a little effort.

Close the caps to hide the screws.

Put your dashboard back together! It should all go okay, without surprises, except that the headlight knob will be as hard to get on as it was to get off.

Hook up the battery and go! You're done!

Man oh man that was a lot of work. But the new gauge looks really, really cool. With this much engineering involved, AutoMeter is a little misleading about the ease of installation. That's okay though, most people probably pay a pro to do something like this. For us it's another notch in the tool chest.