850 Heating Problems

ryanmcgrim · Post by **ryanmcgrim** » 21 Jan 2019, 15:56

I'm having a really hard time diagnosing two separate but possibly related issues with heating our 96 850 NA with MCC. Of course I waited until it's 5 degrees out to really dig into it, and I've got more questions than answers.

Around three years ago I replaced the heater core with a sweedish parts unit, as it was recommended by RobertDYI. I had done a few of these on several other volvos, so I was comfortable with the process.

When I went to drive the car for the first time, there was a noticeable "gurgle" on hard acceleration coming from the heater core. Sounded like air was trapped in the core.

Everything that I read indicated that these are self bleeding, and if I wanted to I could take the coolant reservoir cap off, run the vehicle for 20. Then the next morning, fill it up to Max and go for a hard, longish drive.

I did all that, several times, and the gurgling never went away.

The unit seemed to provide adequate heat (but not super hot) and work, so I just went with it, since I had already spent so much time trying to burp the system, with no luck.

This winter we are finding that the air out of the vents never gets past luke warm, maybe 60-70 degrees. I have attempted to bleed the coolant system again, but there is still gurgling coming from the heater core and cool air out the vents. On top of this, the MCC mode selector knob broke off when trying to put the selector on Defrost. I assumed it was just the knob, but it is actually the MCC unit not allowing the knob to move to the defrost section. It doesn't matter if you go counter clockwise or clockwise. You can put a wrench on it and it will not allow you to select Defrost. It allows selection from 3 o clock to 9 o clock, but you cant go higher than the 3 or the 9 towards the defrost. I've pulled the lower dash, knee panels and even unhooked the cables, and I still can't move the knob to defrost.

Can anyone give me some ideas on how to fix these two issues. Could the luke warm air be selector / mcc related or is more likely that the air in the system is preventing heat and the mcc unit itself is frozen? I've been outside for 5 hours trying to figure this out and it's almost 0 degrees. I just need to get some heat going.

Both pipes to the heater core are hot. The thermostat has been replaced with a wahl, a couple years ago, and appears to be working fine. I'm fairly certain I pointed the bleeder valve to the front of the car, since I know to do that, but I guess anything is possible.

If I get a new heater coil, how can I make sure to not get air trapped in the vehicle when I install.

Thanks so much.

Post by **Ozark Lee** » 21 Jan 2019, 16:39

The damper doors can get stuck and it sounds like that is what happened in your case. Dealing with the MCC cables is beyond difficult but it sounds like you need to actually replace the MCC control head if the shaft on the knob broke off. I had similar issues on my '94 and I was actually able to repair the controller once I got it out of the dash. You can remove the cables at the damper door end and manually articulate the damper doors to get the free but care and patience is required or you can actually break the shaft to the damper itself.

There are measurements that need to be made back to the cable anchors and they are in the factory service manual. If you don't have the manual you can snag it here:

https://www.matthewsvolvosite.com/downlo ... vo_850.zip

Once you unzip it see the file named "ac heater system manual" and it goes through the procedures.

...Lee

ryanmcgrim · Post by **ryanmcgrim** » 21 Jan 2019, 17:13

Thanks for the quick reply Lee. Much appreciated. I was able to manually set the defrost arm to a position that allows airflow to the windshield, but since the mcc know won't move it's also going out the front vents. Regardless it's good that at least some air is going towards the windshield even if it's cook/ Luke warm. Just to clarify, are you saying the cool /lukewarm air is also caused by the damper doors or do you thinking related to the air in the heater core / gurgling I am hearing ? At this point my number 1 concern is getting hot air in the car then figure out the mcc / defrost problems, unless you are saying the issues are one and the same.

Post by **abscate** » 22 Jan 2019, 04:22

These cars don't typically air lock in cooling system so I would guess the air temperature dampers are the problem

Any mix of external air will over power the heat in the frigid temps wears having this week, so that's another indicator.

If you go to recirc, and that helps the heat, that would confirm the mix door diagnosis.

lcc014 · Post by **lcc014** » 16 Jan 2025, 06:23

I had the same problem with MCC mode selector got stuck and could not select defrost. I also asked this question back then in this forum. I followed the suggestion and checked the MCC cables which controlled the open/close of those vent flaps. That was no luck. Finally, I found a used MCC control panel from ebay and replaced the MCC control panel. The MCC control panel was the unit which had cables coming out of it to control the vent flaps. Then I saw the root cause of this issue. The issue was that inside MCC control panel, the mode selector knob "control"/"turn" 2 big plastic circular shape "wheels". These 2 "wheels" had some pattern which controlled the vent flaps. One of these 2 "wheels" was wrapped due to it's age and most important the exposure to heat core since day 1 of the car. The wrapped "wheel" prevented the mode selector to be turned to the defrost position.
After I replaced the MCC control panel, everything ran fine. I could select front windshield defrost again.
Hope this help.