Hot hot hot hot (HVAC)

[Jelmer]

Yipes. My car is angry with me. It all of the sudden decided to pump in hot-as-hell air into the cabin. Next week it's going to the maintenance shop, but until then I'd like to know if there's a temporary fix? Any fuse, wire, hose, etc that can be off? Or is it most likely the internal temp sensor that's shot?

 

[gerryvz]

Re: Hot hot hot hot

Not the internal temp sensor. It sounds like the compressor isn't powering up, either because it's not getting the appropriate electrical signal or becuase there's not enough pressure in the system to trigger it. Since it's going into the shop soon, I'd have them diagnose the A/C system to see why the compressor isn't firing.

Cheers,
Gerry

 

[gsxr]

Re: Hot hot hot hot

The climate control system defaults to full heat in the event of an electrical failure, and full defrost (airflow to windshield) with a loss of vacuum. Sounds like the heater control (monovalve or duovalve) might not be getting a signal. Check all fuses first, then look into the valve area. If you set the temp at max cool, and still get heat, it's probably not the temp sensor.

 

[Jelmer]

Thanks (again) for the replies, guys.

Temp is set at minimum, AC on/off makes no difference except for a gust of slightly-less-hot air. I know the fuse box, but that do you mean with "valve area"? I have the manual temp version, if that changes anything.

 

[Cavanman]

It sounds to me like the water control valve (or one side) has stuck open, as mentioned above. This used to happen on a BMW 5 series that I had that used the same set of valves.

When energised/powered up the valve is closed. If the connection fails it would default to open, hence the high heat. On the BMW the valve itself was failing and would stick open, usually when the outside temperature was warmest.

The valves are on the right hand side of the car in the same area as the control module box. When operating you can feel and hear the valve operating. If you set the temperature to cold in the car, disconnect the valves and then reconnect, you should be able to feel the valve clunk closed.

When I changed the valves on the BMW (with a second hand part) I was able to leave the valve body connected in and just changed the control part which lifted off.

 

[gerryvz]

Is your compressor actually going on when you turn on the A/C? Generally you can feel and/or hear it. If the compressor is not going on, then it's a compressor issue per what I mentioned above.

If you're not talking about using the A/C, and it's only blowing hot, then I agree with what Dave mentions -- it is an issue with the duovalve (which controls the flow of hot water through the system). Generally, as CavanMan mentions (but not 100% of the time) a bad valve will "default" to a hot-air condition. Same is true of the monovalve assemblies on earlier-generation cars.

Either way, it sounds like an electrical issue.

Cheers,
Gerry

 

[Jelmer]

Might be interesting to note for other people: if you have this happen to you, don't set the blower on 1 or higher with all the vents closed except the center (non-warmed ones). I did this today (to have *some* cool air in the car, aircon off), and when I looked under the hood I saw smoke rising up from behind the engine. Turning the blower to maximum with the vents open resolved it.

So either set the blower to 0, or leave the vents open. Scary experience.

No fuses broken. List of technical problems growing at an alarming rate, my bank account won't like the next fixup.

 

[Cavanman]

If it is the duovalve that gone, it might be worth clamping the pipes leading from it. It used to be possible to buy a tool for clamping rubber brake hoses; something like that would not damage the water hose. This might confirm if it is the valve that is gone

 

[pagodino]

I have a similar problem, fuse 7 burns out and so there is no electricity on the sender unit. I might have a cable in contact to another in this area from fuse to controll unit. Perhaps it is the rear driving light which is on the same fuse.

I had to look after... Pagodino

 

[JordiC]

I had the same problem. Refrigerant level/pressure ok, all fuses ok and compressor ok but not engaging. I became frustrated and very hot and took it to a nearby indy mechanic yesterday. He said it was the Aux fan resistor (000 158 32 45). He replaced it, evacuated and recharged the system. A/C is now, thankfully, blowing cold. I knew that the low speed aux fan turns on when the A/C system is started but didn't know that the same fans blown resistor would stop the compressor from running.

 

[gsxr]

Without the fans running, the system pressures could get high enough to trigger the over-pressure safety cutout, which would prevent the compressor from running. That's pretty unlikely though.

 

[JordiC]

Without the fans running, the system pressures could get high enough to trigger the over-pressure safety cutout, which would prevent the compressor from running.

Right but wouldn't the compressor still run before the pressures got that high, unlike in my case where the compressor did not run at all? I assume the mechanic got the compressor running first and then noticed the low fan not starting. He might have not explained it to me properly but indeed the resistor was faulty and had to be replaced. I'm not complaining as I paid for the part and 3 hours labor to have cold A/C in torrid Miami.

 

[Jelmer]

My "old" mechanic did fix this issue, and he told me it was due to to duovalve being bad. However, a few days back the same issue came back. All of the sudden the system went to full heat. The next time I started the car, it went hot again but after fiddling with the temp wheels and generally showing my frustration by hitting random things in the dash and centerconsole, it went to cold mode (AC turned off), and 5 minutes later it stabilized. This went OK for two or three more drives, and now it's permanently in full heat mode again.

I took a picture of what I believe are the valves, and they don't exactly look new.


I've been told that the temp wheels themselves can also get bad, but since replacing that is insanely expensive, I'm hoping that's not the case.

Anyone have any other idea?

 

[500ESpain]

I had the same problem so I sent my duo valve to this guy http://w140.co.uk/ and he repaired it for 20 euro + shipping. It's been working fine so far.

 

[gerryvz]

Do you have ACC? It's likely that either your duovalves are bad, but did you test your aspirator motor with a piece of tissue paper to ensure that it is pulling in air from the cabin (by the sunroof switch) to provide the system with a baseline temp to equalize from? The aspirator motor is a small little unit that is located right on top of the passenger side air duct tube (wrapped in foam), behind and to the side of the passenger airbag. You can test this by taking a 1 or 2 cm square piece of tissue paper and while the climate control system is on place this piece of paper against the small grill next to the sunroof switch. It should stay sucked against the grill if the climate control system is running and the motor is pulling air in from the cabin.

PBUs (pushbutton units) with the temp wheels go out with less frequency, but seeing as this is a standard later W124 item (whether the manual Euro system or the ACC) you should easily be able to get a used, working unit. Here in the US we can get rebuilt/refurbished PBUs as well, for considerably less than the factory new units.

I'd start by testing the duovalve, which is the most likely "wear" item.

Cheers,
Gerry

 

[Jelmer]

ACC in this case is automatic climate control, right? Then no, I have the proper manual version with two zones. I remember messing about with the temperature sensor a while back, and I'll try your steps. But, if you set the temp wheels to COLD, after the *click*, it shouldn't matter what the sensor says, right?

If the aspirator motor checks out, how can I test the duovalve system?

(Meanwhile I've already sent an email about this part to the dude 500ESpain mentioned, thanks!)

Oh, I've checked fuses already, they seem fine.

 

[samiam44]

I would like to interject on the low speed fan.

It helps provide air accross the coils for low-speed traffic. It's common for the wire to corrode up at the resistor and easy to fix. If you can go on a drive for 15 minutes @ 60 mph and the temp stabilizes 38-42 F, then this can be the problem. The high pressure safety will happen VERY quickly in 35C weather less than a minute. If your off in the ACC system- that's electronic control. For the dual valve. You should be able to use an infra red thermometer checking inlet and outlet temps on the evaporator lines to quickly identify if it's a problem. Also unless your climate is extremely dry, there should be condenstion on the return line to the compressor. or at least cold.
The key chain thermometers like that from harbor freight run $5 on sale- very handy.

 

[Miziev]

I had the same problem, and it was due to faulty dual valves too (we call them "balls")). I pulled them out, took apart and realised that they are destroyed completely. So I found a guy in Moscow, who does all kinds of repairs to MB electrical parts (including wiring) and got it exchanged for a refurbished one for about $65. Everything works great so far.

 

[Jelmer]

Valves and MAF (he had one lying around, heh) are on their way. Total cost slightly below €300. Hoping for the best!

edit: he? he who? The w140.co.uk dude.