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Replaced the temp sensor: No heat, ambient temp display reads "--" Topic is solved

Help, Advice, Owners' Discussion and DIY Tutorials on Volvo XC90s. The XC90 proved to be very popular, and very good for Volvo's sales numbers, since its introduction in model year 2003 (North America).
bronson75
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Replaced the temp sensor: No heat, ambient temp display reads "--"

Post by bronson75 » Sun Dec 31, 2017 2:29 pm

The temp display on our has been wonky for about a year now, in the past it has read a temperature that may or may not have been correct, or typically it has read "--". The HVAC system will run the A/C or heat when it feels like it. With winter coming up and the wife needing heat, I replaced the temp sensor in the mirror with a new Volvo part from FCP, based on information I read on this and other forums. After the replacement, the display reads "--", and will not show a temperature. Figuring I received a defective part, I ordered a new one and replaced it this evening. No dice. It still reads "--" and I have no heat. This week is the coldest of the year so far and I'm looking for options. I've flushed the heater core and it flows fine, and it has newer coolant at the appropriate level. I'm convinced it's electrical, but I don't know where to go from here.

Any help would be GREATLY appreciated!

---

Follow up comment from an other forum :

(quote) which mirror? there is one in each outside mirror, with different functions.(/quote)

From what I've read, there is only one sensor, located in the driver's side mirror. I drove it to work the day after I replaced the sensor, and it showed the "--" for the first few minutes, then it showed an accurate temperature for the rest of the 30 minute drive (8*-10* F), but still no heat. The drive home and ever since, it showed "--". I think the sensor is okay, but it's getting an intermittent signal or there's some other electrical problem. I don't have a Volvo specialist or dealer close to me too yet it diagnosed, I'll take it to my regular mechanic to see if he can help, but I'm stumped at this point.
Last edited by matthew1 on Sun Dec 31, 2017 2:40 pm, edited 1 time in total.
Reason: removed "help" from title, added tags

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Re: Replaced the temp sensor: No heat, ambient temp display reads "--"

Post by oragex » Sun Dec 31, 2017 3:40 pm

From your post I get the impression that you think the sensor located in the side mirror is related with the cabin heat. Pardon me if you already know this, but if not, the mirror sensor that displays the outside temperature is only informative, not related with the coolant temperature or the cabin heating.

The coolant temperature sensor is located on the thermostat housing and the temperature of the coolant is shown with a needle gauge. Not hot enough cabin temperature can be caused by a faulty thermostat, this is a way to test it

Is there a small grid just above the right side temperature knob? A cabin temperature sensor is located there and may regulate the cabin temperature. It may be covered with dust or a small fan behind may be stuck - may blow some air but do not touch because it's fragile.

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Re: Replaced the temp sensor: No heat, ambient temp display reads "--"

Post by bronson75 » Sun Dec 31, 2017 6:12 pm

From everything I've read (and I've been dealing with this issue for a while now), the ambient temp sensor in the mirror feeds the temperature display, fuel computer and the HVAC system. If I'm wrong (and I have no problem admitting that) I'm more than open to suggestions. I do know that I had to replace the coolant temp in our old V70XC, the issue there was a misreading engine temp gauge reading), the XC90 warms up very quickly and the temp gauge gets to normal and stays there. I've flushed the heater core which had some junk in it, but it flowed fine afterwards.
I'll do the thermostat check and blow out the interior temp fan tomorrow (which I had planned on doing) and update this thread.
Thanks for the input.

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Re: Replaced the temp sensor: No heat, ambient temp display reads "--"

Post by bronson75 » Mon Jan 01, 2018 1:40 pm

Update: I'll concede being less than knowledgeable. I popped off the cover on the underside of the passenger side mirror, and lo and behold, there is a sensor in there. Something I should have checked before, I suppose.

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Re: Replaced the temp sensor: No heat, ambient temp display reads "--"

Post by bronson75 » Mon Jan 01, 2018 3:40 pm

Update #2: I swapped out the passenger side mirror sensor (temporarily) with the drivers side sensor I thought was bad. I'm getting a reliable accurate outside temp on the display now. I let the car heat up (it's -3* here now), and used my infrared thermometer and touch to check various heat points: upper radiator hose, heater core inlet and outlet at the firewall under the hood, and heater core inlet and outlet at the heater core under the dash. The engine warms up quickly (confirmed by the engine coolant temperature gauge and a hot upper radiator hose). The heater core hoses show the same result. Plenty of heat getting into the heater core, and the outlet is cooler than the input, which shows it is losing heat like it should be. But the end result is the same, no heat on any setting, auto or manual mode. I have a friend who works at a local garage who has access to Volvo diagnostic software, I'll get it to him this week and see if he can diagnose the issue. I also blew out the interior temp sensor fan on the center console with a can of compressed air.

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Re: Replaced the temp sensor: No heat, ambient temp display reads "--"

Post by oragex » Mon Jan 01, 2018 6:07 pm

It looks like the doors inside the climate unit don't open to the hot air passage. The climate unit has two passages: a cold one and a hot one. For the air to go trough the hot passage (where the heater core sits), two doors open for each the driver and the passenger. It is a bit strange because each side - driver and passenger - have each two doors actuated by individual motor and arms. Perhaps something tells the CCM the cabin temperature is already too high, this could be related to the sensor behind the grid. Just a guess, better to have it checked by a mechanic who can read the codes, before replacing parts. The heater core may still loose some heat as the coolant is passing trough, but the cabin blower only pumps air through the cold passage, see the doors at 31:20

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Re: Replaced the temp sensor: No heat, ambient temp display reads "--"

Post by bronson75 » Mon Jan 01, 2018 7:34 pm

Excellent information, thanks! I don't have VIDA/VIDAS to be able to check it, but I'll get it in and have it read ASAP.
Thanks!

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Re: Replaced the temp sensor: No heat, ambient temp display reads "--"

Post by bronson75 » Mon Jan 01, 2018 9:15 pm

Is there any way to disconnect the flaps from the motors and position them manually, at least temporarily until I can get it checked out?

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Re: Replaced the temp sensor: No heat, ambient temp display reads "--"

Post by bronson75 » Tue Jan 02, 2018 5:42 pm

So I did a bunch of poking, prodding and checking today. I checked the function of the blend doors, thinking they were bound up, had bad actuators, broken arms, or needed to be calibrated. Everything worked as expected, when I turned the temp knobs from cold to hot, the arms moved the doors fine, on both driver and passenger side. I checked the temperature of the air flow coming out of the foot vent while changing the temp on the dash, and there was only a few degree difference between hot and cold. The metal inlet and outlet tubes at the heater core were hot to the touch (I couldn't get a good reading with my infrared thermometer), but the core itself was ice cold. I have a feeling it may be an original part, I know for certain it's an OEM part, and after 180,000 miles I'm sure it's plugged up almost solid. I did a heater core flush a year ago and got what I thought was good flow, but it must not have been flowing the whole thing. I have a core ordered, and I'll update this thread with my findings.
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Re: Replaced the temp sensor: No heat, ambient temp display reads "--"

Post by oragex » Wed Jan 03, 2018 9:07 am

I cut open a heater core, show at the end of this video


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