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Having some trouble with my HVAC system and I haven't been able to find answers looking around.
My 87 turbo started blowing hot intermittently today. With no AC function on and just the fan, it was switching between blowing hot air and ambient air. With the AC on, it was switching between hot air and cold air. Really not sure where to start diagnosing this. My first guess would be one of the temperature sensors, but being that the issue persisted with no AC on, I'm not sure if its the control unit or what else it could be. Any direction would be greatly appreciated.
My guess would be the vacuum system is allowing the heater valve to open. I found out that cold AC is easily nullified by the hot water if the heater valve opens and the AC is on.
Go through the vacuum lines and test the check valve and the line to the vacuum can near the battery (LHD cars).
If the problem is not obvious, let us know if there are specific conditions where it's evident (idle, deccel, high RPM, etc.). And if you can replicate the issue when parked see if you can see if the heater valve is open of closed when the problem occurs.
Thanks for the pointer. I'll take a look at the vacuum lines. If it was a vacuum leak, wouldn't it be hot all the time? As in, if there is a vacuum leak, its not going to work intermittently.
I'm planning on cleaning out the interior temp sensor and the ambient temp sensor with MAF cleaner today or tomorrow. Hoping that solves it. If not, I'll start searching for a vacuum leak.
probably have a vacuum leak which denies vacuum to the heater valve when accelerating opening the valve and pouring heat into the cabin, you can read all about it in the 924 944 tech tips book
A cracked line or tee could cause too-little vacuum to keep the valve closed under some conditions or a bad check valve would probably do some weird stuff due to boost. Under boost the heater valve would probably open.
I've also had an issue where the solenoid valve leaks a little vacuum so the heater valve opened when vacuum is high (idle and deccel) but worked normally at cruise.
You can learn a lot from trying to correlate the issue with engine operating conditions and putting a vacuum gauge on the line to the vacuum can (post check valve) and the heater valve line. Something like a bad check valve or cracked line or fitting is a lot more common than a temp sensor or solenoid issue.
I just checked all the vacuum lines, Ts, and fittings on diagram 3 of the below. Nothing noteworthy. I didn't get to track down where the vacuum feed for the T behind the firewall is.
I also cleaned out the exterior ambient air temp sensor and interior air temp sensor with MAF cleaner and I cleaned out the contacts to the plug with electrical contact cleaner on the ambient temp sensor.
After all this, now the car only blows hot air. Nice...
Did you check for vacuum and test the check valve or just look for cracks?
If you are in the USA the Harbor Freight vacuum tester is a pretty good tool and would work as a vacuum gauge as well as a source to pull vacuum on the heater valve, reservoir, etc. for testing.
I just did a visual inspection. Apologies for the noobie approach to diagnostic. I haven't worked much on cars that use vacuum for aux functions, so this old tech is all new to me.
Going to pick up one of the harbor freight vacuum testers and a multimeter.
Testing all the vacuum lines. Besides the heater valve, the reservoir, and lines in #3, is there anything else I test?
I am hoping that one of the things I touched last night is the culprit. Ambient temp, interior temp, or vacuum leak as my problem went from intermittent to full-time after having put my fingers on them.
I would start at the check valve to the vacuum can #5 and see if it's working, and from that point see if you can pull vacuum on the can and HVAC downstream of it, and if so see if the heater valve opens and closes based on the HVAC control settings.
A small amount of vacuum loss is expected but not much. Normally going up a long hill you wouldn't run out of vacuum and get hot air in the cabin.
From there you can also see if there is a vacuum supply from the engine at the check valve.
It's also good to check and see if you have hot air in the cabin even with the heater valve closed, although this is unlikely as it would not be intermittent... unless it was failing and now has failed?
I tested the temp sensors using the fantastic guide put together here.
With an ambient temp of 75ish here were my results:
Outside temp: 16.2 ohms
Inside temp: 9.9 ohms
Mix chamber temp: 9.2 ohms.
I did the rest of the tests from the link and all my other parameters check out.
Given this 20-25 degree discrepancy, I suspect that's what's wrong. Now to find a replacement for that sensor!
Assuming those are Kilo-ohms that means the outside temp sensor is reading a little cool? Like 65F? Still seems like you could command AC and get it as the cabin can be hot from the sun.
If those readings were ohms I'd suspect you measured the wrong thing or did not disconnect the connector and were measuring in-circuit.
Apologies, you are correct, those are Kilo-ohms.
All I know is that before I handled the sensor, the problem was intermittent. Immediately after handling and cleaning out the sensor, the problem is now constant. Not to mention that the sensor reads differently from the other 2 by about 20 degrees. I did not really disconnect any of the vacuum lines—just visually inspected them (yes, I know its an amateur move). By process of elimination alone, I suspect that sensor is my most likely culprit. I ordered a new thermistor based on the specs provided by the link above. Will follow up with the results. Thanks for the help.
There is a plastic clip in the driver’s footwell to the right of the steering column. It is connected to a threaded rod that controls one of the HVAC flap doors. When that clip breaks you get full heat. Small white clip with a metal clip to hold it in place on the rod. Telltale sign is finding the metal retaining clip on your carpet.
Update: Problem resolved. I just installed the new thermistor today. After installing the thermistor, all my resistances between the 3 sensors were within 10%.
Put it all together, cranked up the car... and voila, no more heater air. AC blows cold, but more importantly, it doesn'tblowhot!
06-03-2023, 12:04 AM
Intermittent Heat in HVAC
