# Temp gauge bottoms out when brake depressed



## Frontiersman (Feb 17, 2016)

So weird thing occurred on my 2012 Nissan Frontier. I was low on fuel, and about to pull into a gas station, when it ran out a few hundred feet prior. Gas can, a couple minutes was good to go on fuel. Now, not sure of that is related, but thought worthy of mentioning. The following day, I started noticing my temp gauge was bottoming out on cold, when I depressed the brake pedal. When I release the pedal, temp goes back up to just shy of half way. Later on, I noticed the idle would rev up a couple hundred RPM also when stopped, and brake depressed. WHen I let off, to accelerate, it drops a couple hundred RPM to low, then bounces back up. Sometimes during my driving, It will be inconsistent when accelerating from a stop, and act a little jerky for a moment before going back to a somewhat normal accelerating process. 

I did do a ECU reset, Gas pedal reset/calibration. and Throttle body, I think it was reset/calibration. Did all of these in the order mentioned on another page somewhere, and although I think the resets all were good, it has not solved the issue. I had my mechanic run a diagnostic, and he got codes pointing towards the IAT or Intake Air Temp sensor. I checked all local parts stores, and they didn't have it, said dealer only. Called Nissan, they hadn't heard of an IAT by that name, and after some discussion agreed it must be the mass airflow sensor, my mechanic, and local auto parts stores were referring too. $270 for Nissan's mass airflow sensor. I discovered it can be cleaned, so bought mass airflow sensor cleaner, and followed cleaning procedure. Reinstalled sensor, did all the resets and calibrations, and it still has same issue. The frontier is a 1012 4.0 liter, and has 57k miles. Error code is P0113. I did unplug the mass airflow sensor, and tried driving it that way, and it ran terrible. I had to pull over and plug it back in. 

I have never seen or heard of a temp gauge bottoming out when I depress the brake pedal. Or associated idle issues. Is this really a bad sensor, or something else? Has anyone else experienced this? Know a solution?


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## Frontiersman (Feb 17, 2016)

Here is a video showing what is happening.
https://www.youtube.com/watch?v=NvgoJdEuJkk&feature=youtu.be


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## jdg (Aug 27, 2009)

P0113 is for an intake air temp sensor, which in this case is part of the MAF.
Thinking 2 things.
Maybe you've got an alternator/voltage issue. Get a voltmeter, put it on the battery, and watch the voltage when you press the brakes. Could be that the brakes are pulling enough current off a marginal alternator to kill off voltage enough to fool the ECM into raising the idle.
Other thing might be that you've got vacuum booster leak. You step on the brakes, leak all sorts of vacuum into the engine, idle goes up and gets all goofy.
Past that, possibly a wire rubbing up against the brake pedal arm itself...a mechanical type thing, wire shorting out, vs. a broken sensor.


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## Frontiersman (Feb 17, 2016)

Thanks for the input JDG. I no longer have a meter in my tool box, will have to run by my mechanic for that. I did notice while inspecting as much as I could think of yesterday, the ground connector on the battery, which btw the battery is a month or two old, is not super snug. Even tightening the 1omm to max, the clamp can still move with minimal effort. It still seemed tight enough to properly ground the system, but not 100%, so will replace that today. Cheap replacement, and certainly a bad ground can cause odd ball issues.


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## jdg (Aug 27, 2009)

Yes. Temperature gauges, as well as a lot of other things depend on a good power and good ground to keep the ratios of the 'top end' and 'bottom end' of the gauge indications correct. In the case of the temp gauge, say if you lose the ground, all you'll have is the power side and the signal side, needle swings down (or up, whatever the case may be).
"Back in the day", it used to be the case if your alternator was taking a dump and system voltage was dropping, your fuel gauge would like also drop, making a person think they were running out of gas, when they were actually running out of usable electricity.
Not quite the same thing, but the premise applies.


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## cj1 (Oct 21, 2014)

Is vehicle all stock no additional wiring on brake circuit for trailer etc?
Energize the brake stop light switch by hand to determine if problem mechanical/vacuum or electrical.
If temp gauge reacts when energizing the switch then need electrical diagram to explain why brake circuit would effect temp gauge.
As has been mentioned important to check for proper voltage/grounds.


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## Frontiersman (Feb 17, 2016)

All stock, no tow package.

Last night driving I noticed it also revs up and down ever so slightly, but clearly noticeable. This occurs in rhythm, when using my turn signal or hazard lights. Also, I ended up keeping the climate control totally off. No A/C or vent on. Truck ran worse with anything on, minus the radio, headlights, and tail lights (not blinking) didn't seem to make a difference.

I did try pressing and releasing the switch (interior) the brake pedal uses to initiate the brake lamps. when I push the switch button back in, as though the pedal was fully released, the needle on the gauge goes back up, and runs differently. 

Added a battery terminal shim to my ground side yesterday, as I could still move it around, with effort, at maximum tightness on the clamp. 

The battery is less than 2 months old. That said, thinking I may run by auto parts store, and have them check alternator and battery for issues. Kind of thinking the alternator could be going bad, or at least a possibility worth checking out. Fairly convinced it electrical anyway, whatever the problem is.

Also ordered a OBD2 reader, which will arrive today, so I can access error codes at will.


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## cj1 (Oct 21, 2014)

Good place to start verifying battery/alternator good.

With your OBD2 reader should be able to monitor voltage when brake stop lights energized. 

With a voltmeter you can measure if/where voltage is being dropped. For example, from the negative terminal of the battery to ground should be effectively 0 volts.


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