Aftermarket Water Temp Gauge Installation On Spec E30

Race cars have numerous systems that must be monitored. One of the most basic is the water temperature. While my Spec E30 has its stock temp gauge intact, I wanted something a little more accurate and reliable than the blue, black, red sequence of my stock gauge – what most call a dummy gauge or idiot lights. I know this isn’t rocket science, but I can tell you there are a lot of people out there still running idiot lights, so maybe this will motivate them to add this little piece of security.

For this application, I chose the Autometer 2-1/16 inch, 100-250 degree, Air-Core, Ultra-Lite (pn 4337) Water Temp Gauge for its reputation and affordability. It will give me a reliable understanding of how hot my motor is running. What it does not do, is to tell you when you’ve sprung a leak and run the car out of water. For that, I’ll be installing a pressure switch and warning light, which I’ll detail in another article.

The package comes with a gauge, mounting bracket, temp sender, various threaded brass adapters, optional light with different color options, and instructions.

Now to devise my plan. The gauge needs three things to work properly: First, is a fused 12V power source. Second, it needs a signal from the sender, and finally a good ground. Before wiring the gauge, I need to find a place to mount the sender. Having researched a number of alternative installation points, I decide to mount it in one of two covered ports in the thermostat housing.

The fitting on the left was the perfect spot to drill and tap for the sending unit.

Neither of the adapters that came in the package fit into the threaded ports in the thermostat housing. Rather than buying an adapter, I decide to tap the original nut. In this case, the threads on the sender are 1/8 NPT (National Pipe Thread), and I found a tap and drill bit that would work nicely for it.

Make sure to secure the nut tightly in the vise and drill slowly making sure you have it centered and don’t overheat the brass.

Once the hole is drilled, the tapping part is pretty easy. Just take your time, make sure you have it lined up squarely, and that you run the threads all the way through.

When screwing the sender into the tapped cap,  make sure to use Teflon tape to keep everything leak proof.

Now that the tedious part is done, I prepared the gauge and sender for wiring.

The power and ground wires will be hooked up under the dashboard. I know generally where I will mount the gauge and where I will source power and ground from, so I cut the right length of wire, about 18 inches each. For the gauge end of the wire, I found the correct size connector, stripped the end of the wire, ran a piece of heat-shrink tubing over it, and crimp the connector on. A little heat from a lighter shrinks the tubing snug and now my connector was ready. I installed circular connectors for the other ends in the same way, with heat-shrink tubing to insulate both ends of the wire.  I used circular connectors on these ends as I know they will be screwed down.

You can get a little set of these connectors and heat-shrink tubing at any auto parts or hardware store.

Make sure to push the wire into the connector and get a good firm crimp on it. Maybe give it a somewhat firm pull to double check it’s really on the wire.

After you’ve checked your crimping skills, pull the heat-shrink tube over the end of the connector and use a lighter to shrink it.  Make sure the flame doesn’t touch anything. We don’t want scorched wires and connectors!

With connectors ready, I moved to finish my work under the hood.

First, I ran a long wire through the firewall and back under the dashboard. This wire will go from the gauge  to the sender. I make my life a little easier by running a longer wire than I’ll need, which I can cut to fit once everything is mocked up.

I used this access port in the firewall on the engine side for ease of access, and ran the wiring under the dashboard as you see here.

It’s a tight fit under the dash, so make sure you feed enough wire through the hole in the firewall so you can find it.

Once I start mounting it along the engine bay I can cut it to the correct length to reach the sender. For this picture I’ve simply stretched the wiring out across the engine bay so it’s easy to view.

I will secure this wire to one of the looms once I’ve got everything hooked up. I’d recommend putting it in the loom if possible, but I’m out of space (and time).

I run that wiring along the sections of engine bay looking for places to zip tie it the existing loom. Once it’s pretty much mocked up I cut the wiring just a little longer than I need to reach the sender. You can take up some of the slack later, but it’s better than being too short and having to solder two wires together.

Hey, it’s a race car. It doesn’t have to be pretty, just functional!

To connect to the sender, I used a circular connector.  The sender has a threaded post that has two nuts sandwiching a crush washer.  Remove the top nut, put on the connector, replace the nut and snug it all down. Here you can see it the whole enchilada installed back into the thermostat housing.

I found this to be the most strategic spot to get the most accurate reading, but you most likely have a few different options. Just make sure you keep it away from extreme heat (like the headers) or you won’t get an accurate reading.

Now to mount the gauge.

Luckily I have a spare gauge panel left over from another project car, so this will save me some time in cutting and shaping a mounting plate for the cockpit. I choose a mounting location that is easily visible, but out of the way. In this case, the space previously occupied by the radio and HVAC controls.

This piece was left over from another project, but you can see how simply you can make your own gauge pod — even dress it up with some leather!

The spot where the radio used to be is a perfect place to mount secondary gauges.

I zip-tied the plate into the basic location and started to mount it.

For the bottom side, I mocked up a mount using soft steel bar, which I drilled and riveted into the plate on one end. The other end was already drilled and ready to be mounted on a nut and bolt that I already created for another switch plate. Full disclosure, I first tried to JB Weld it into place, but it wasn’t curing how I wanted, mostly due to lack of prep on my part. So, I went with the old rivet gun and it worked perfectly.

Taking measurements to cut and bend this metal properly to secure the pod to the dash. Race cars vibrate and shake and you want to be able to get a clear reading in the blink of an eye. Mounting gauges securely is hugely important.

Note the Vise-Grip bite – using a vice grip to keep the two pieces together during drilling made life much easier.

With the bottom mount finished,  I used an existing tab on the side of the panel to screw it into the dashboard.  The panel is tightly held in place and rattle free, so there’s no need to over engineer.

Time to do my final assembly.

For power, I used the wiring going into the start button from the ignition switch. I knew it was fused, as the main power comes from the fuse box. I routed the ground wire to the same terminal as other grounds for my helmet air and cool suit, but you just need to make sure it is secured to the body, firewall, or anything secured to them.

I’m pulling power from the right hand (top in this photo) post on the back of the ignition button.

Now that all the hard work was done, I could install the gauge. I started by running all my wires to the gauge through the gauge bracket and through the front of the panel. I connected all the wires to the back of the gauge — each electrode is marked clearly — and made sure they secure.  I installed the wired gauge into the panel and tightened everything down from the backside. The finished product is…well, usable if not gorgeous. The final step was to fire up the car and watch the needle to make sure it worked as it should.

As long as that needle stays around 180 I’ll be a happy camper!

The gauge panel will be used for other gauges such as Voltage and Oil Temp in the near future, so having the extra space is great. It’s ugly, but it works, and this made me realize I need to spend some time tidying up the interior and dash. Time to go buy more sheet metal!

About the author

Kasra Ajir

Having competed, raced, or trained with SCORE, SCCA, NASA, BMW CCA, and more, Ajir is well seasoned in the various series that pollinate the grassroots and professional auto racing world.
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