Heating not hot unless set to Max

Technical Area Electrics & ICE
#1
  1. My model of MX-5 is: NC 1.8 2008
  2. I’m based near: Uxbridge
  3. I’m looking for technical help or recommendations on: My A/C / Heating / climate control

I’ve noticed that recently my AC is blasting out and I’m not getting any heat unless I crank it up to 29 on the dial. Not sure where to start… I guess its probably the thermometer in the car isn’t reading correctly and always thinks it’s warm enough in the car, but wondered if anyone had any pearls of wisdom

#2

Does sound like you need a new thermostat, how does the gauge look?
It should be rock solid around the middle of the gauge if it drops when you crank the heat up then definitely new stat time.

Or pay for the genuine stat, a choice of two long or short nose. Buy the one above long nose and trim the hose on the car, same thing really at a fraction of the cost.

If that’s faulty you should be changing the longlife coolant too if it’s not been done it’s overdue now.:+1:

The genuine stat…

https://www.mx5parts.co.uk/thermostat-cover-mk3-35-p-2330.html

#3

Thanks for the reply. Took it for a quick spin… the temperature gauge is rock solid. Bang in the middle.

Interestingly the air con blasting out with no sign of heat even at 25, but as soon as it hits max the heat cranks on.

#4

Hmm…I haven’t got Aircon on mine so can’t comment on what to look at further. Maybe something in the system controls then, who knows?

#5

Just for a comparison, I have a 2008 nc 2.0 with air con. Heater adjustment works smoothly and progressively, so it does sound like you have an issue.
As the switch is electrical, I wonder if the issue is there rather than a mechanical issue.
Be really easy to swap the controls (assuming the side locking bolt for the radio is not still in place) to try someone else’s if there is anyone local willing to try to help you?

#6

Thanks. Good shout. Looks like I can pick up an eBay special for the control dials for £20ish, so might try that before I start trying to dive behind the glovebox at the seemingly inaccessible mixer flap

#7

Has the centre console been disturbed? eg Radio out or upgraded?

The ‘three knobs in a box’ assembly below the radio is the brains of the aircon/climate system, including cabin temperature sensor, and if the plug on the back is not properly connected or a wire is damaged, then there can be strange effects.
However,

  1. If you can get heat out it suggests the hot water is flowing through HVAC unit OK.
  2. If you can vary the fan speed it suggests blower and its FET control are working.
  3. Being able to change things suggests the Intake, Mix and Airflow actuators in the HVAC unit are working.
  4. This leaves just the cabin thermometer living in the three knobs box, and it could even be as simple as a bad connection inside there, such as to the actual temp sensor, or maybe within the temp knob (Servisol Super 10 contact cleaner to the rescue?)

The obvious thing to try is a different known working set of three knobs, make sure you get the Climate Control set if you have CC.

Edit. I was wrong about where the thermometer is. See the photo of the sensor in my later post below.

#8

Thanks Richard. That really useful and greatly appreciated. I’ve just ordered a set of knobs from eBay, as I don’t know anyone with such impeccable taste in cars as myself. I appreciate I won’t know if they’re working, but think it’s worth a punt for £20.

#9

I think the cabin temp sensor is separate from the 3 knobs box. From memory its located in the same area as the dsc switch to the left of the steering wheel behind a little vent. When I fitted AC to mine I jnitially had an auto 3 knobs box but because I don’t have the sensor I had no control apart from full hot or full cold. I went manual in the end with a 3 knobs from the US

#10

Yes. Here is a photo I’ve just taken.
The sensor is in the foreground with the DSC bits behind and above to the left.
The pipe connects the vent by the driver’s left knee to the air box, but in a very brief test I could not tell if the air blows out or sucks in. Logic might suggest it sucks in air from the cabin.
If these are disturbed or broken then the Min/Max problem can happen.

20201107_102505_c
20201107_102505_c1084×1500 222 KB

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#11

Thanks. Mine was pretty dusty. Removed, cleaned it and replaced the three dial control unit and STILL have the same issue. I think I’ll change the sensor now. Any idea where I could get one from?

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#13

Got a new Cabin Temperature Sensor from our mates at Mazda. Took a while to get shipped, but It worked! Thanks all.

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#14

Thanks Richard. You were spot on. New sensor did the trick

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