DIY CVT Temperature Gauge

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VStar650CL
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Posts: 8294
Joined: Thu Nov 12, 2020 1:25 pm
Car: 2013 Nissan Altima 2.5 SL
2004 Nissan Altima 2.5 S

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I got tired of looking at my phone to track CVT temperature in the wife's '13 SL 2.5, so here's what came of it. The problem with adding a gauge, of course, is where to put the sensor? Virtually all aftermarket trans gauges come with one that's specific to the gauge, but that kind of plumbing isn't straightforward on a CVT. It isn't like the beehive has any threaded holes to say "ahhh" for a thermometer. The answer was to use the thermometer already in the Valve Body, along with a device called a Gauge Bridge. The principle is simple, you "teach" the bridge a high and low needle position along with a sensor voltage that corresponds to each position. The bridge then plots a "slope" that interpolates everything in between or beyond. There have been products like Meter Match available for years to do that with fuel gauges, allowing people to replace unobtainable antique senders or dinosaur-age gauges with modern ones. It might be possible to use those bridges in this application too, but there's a potential problem with most of the fuel-gauge drivers that makes them unsuitable. They're all set up for "balance" operation using internal pull-up resistors, and the presence of a pullup would skew the readings from an ECU-driven circuit like the CVT temp sensor. Stealing that signal requires a "high impedance" input that simply reads the sensor voltage without affecting it. I found only one bridge that could do that, from an outfit called Widget Man:

https://www.ebay.com/itm/143709817048

Gauge Bridge.png
Gauge Bridge.png (181.43 KiB) Viewed 1983 times
Full disclosure, I know the guys and I have a small design interest in two of their other products. They make some very interesting stuff for gauge conversions and e-fans. In this instance, it was pointed out to me that the fuel gauge pullups in their product could be disabled by the user, specifically allowing it to read ECU circuits without disturbing them. So using their bridge became a no-brainer.

The gauge I picked was nothing special except for 7-color backlighting, $34 on eBay. I chose it mostly for the 80F~260F range, which is a good span and resolution for CVT's. Many of the other choices went to a useless 350F~400F, which of course makes for crappy resolution in the 130F~190F area where CVT's usually run.

https://www.ebay.com/itm/284370432320

Gauge.png
Gauge.png (314.68 KiB) Viewed 1983 times
The bridge hookup was extremely simple, just 4 wires. There was a diagram in the instructions that exactly matched what I wanted to do. The only additional need was snipping an extra resistor to get 12V output instead of 5V, since the gauge proved to want 7V to read 80F. It wouldn't go below 110F using the 5V output.

Hookup.jpg
The most difficult part of the job was actually accessing the temperature signal from the TCM. The wire runs directly from the tranny to the TCM, so those are the only two connectors where it can be tapped. The tranny connector is buried, and the TCM connector, while out in the open, has to be partially disassembled to access the wires. I do a lot of that, so for me it isn't mysterious, but here's how:

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Naturally, you also need to know which wire to tap. You'll need the SM for that:

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I strongly recommend you stay away from t-taps, scotchloks, or traditional butt crimps. Make a permanent splice for the signal wire using a solder-shrink connector, it will save you possible grief down the road. The engine compartment is a nasty place for exposed metal conductors of any sort, and all three of those other splices leave something exposed. Use a blue shrink and not red, since you'll be joining 3 wire-ends and not 2. Also, when you reassemble the backshell on the TCM connector, don't forget to replace the zip-tie. It's there to protect the bundle from vibration, and omitting it could make you sorry at some future time.

Routing isn't difficult except for pushing through the firewall grommet, which is pretty well buried on gen5 Alties. The airbox definitely needs to come out, and you'll need a metal coathanger to make a grommet-poking tool. It also helps if you slide the I-key outside-beeper off its stanchion temporarily so the grommet is easier to see:

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There aren't many good places to mount extra gauges on a gen5, so I settled for a pod attached to the fusebox cover. It had the side benefit of having all the other needed wires and signals right there. This gauge was smoked and needed the backlight on constantly, but if it needed a backlight signal, it's right there on 3 of the 4 auxiliary switches. The arrangement also provided easy access to the buttons on the gauge bridge, which mattered very much because it needed to be programmed. The gauge is now the handle for removing the cover. Scotchloks or t-taps are fine inside the car, I used the former:

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Then all that was left was programming the bridge. The instructions were pretty step-by-step for doing that, and clearing or changing the old data is pretty simple if you mess up. To get a 190F setpoint for the high temperature, I did a "torque converter stall", holding the car at 1500 RPM in drive with my foot on the brake. This brings the temp up very quickly without harming the belt or pulleys:

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That's pretty much all there was to it., now I have temps at a glance. The gauge pod is a bit tall and clunky and I'm looking for a prettier alternative, but the bridge and gauge work very well. Happy motoring!


User avatar
VStar650CL
Technical Expert
Posts: 8294
Joined: Thu Nov 12, 2020 1:25 pm
Car: 2013 Nissan Altima 2.5 SL
2004 Nissan Altima 2.5 S

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Update: I cut down a "swivel bucket" type gauge pod (which used to hold the tacho on my old Yamaha cruiser). It not only looks cleaner, but the wife doesn't bang her knee on it.

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