Weird HVAC Issue

[2000BoxS]

Hello fellow 986 owners.

Ever since I purchased my 2000 986 S, its HVAC has been behaving weirdly. By weird, I mean that the blower doesn't spin and there is no air output. I can control everything on the climate control unit, except for the AC (AC icon doesn't come on the screen when I press the snowflake button). The four electric flaps/valves (part number for one of them: 996 573 917 00) move erratically and make noises. The bar indicators on the climate control unit for fan power change on its own even in "Manuell" mode.

Here's what I did so far:

- Replaced parts with tested and functioning units:

Blower motor
Final stage ("resistor")
Climate control unit
Actuation heating relay (#18 on top of the fuse box)
Ignition switch

- Checked the voltage at one of the the electric plugs for the valve motors and the voltage continues to fluctuate rapidly. All four valves work without issues when 12v is applied to them at the motor connectors in both directions.

- Tested continuity among the electric connectors of climate control unit, the blower motor, resistor, and the four flaps. All connections are fine indicating no broken wires.

- Confirmed there are no broken fuses (under the driver side).

- Checked for foam pieces that could block something in the airpath. Surprisingly, the foam pads on the trap doors are intact.

- Noticed that the AC relay is missing so I ordered one (trunk relay box #4). This probably explains the AC not turning on, not sure if it can be the cause of all the other behaviors, especially looking at the AC wiring diagram attached.


Is there anything I missed? Anyone had a similar issue?

Thank you for your help.

[2000BoxS]

I'm assuming no one else had a similar problem?

[78F350]

It looks like you've done all of the tricks that I would have tried. How about the diagnostics that Boxster Steve suggests in this thread:
Using A/C display for oil temp???

[2000BoxS]

Originally Posted by 78F350

It looks like you've done all of the tricks that I would have tried. How about the diagnostics that Boxster Steve suggests in this thread:
Using A/C display for oil temp???

Thank you for the suggestion. I tried it when I saw the post, but it unfortunately did not work on my vehicle. I hear that the earlier MY cars have better chance of it working.

[2000BoxS]

Received the AC relay today and plugged it in.. AC now engages when I activate it using Durametric, but the snowflake icon still does not come on when I try to turn on AC on the climate control unit. No behavior changes on the HVAC unit.

[2000BoxS]

What is voltage reading at B13 supposed to be? It seems to be the idle period signal from the cluster, and it rapidly fluctuates between 2 and 9.7 volts. I think this may be related to the symptom. If anyone knows what's the reading supposed to be, I'd really appreciate it.

[2000BoxS]

..........

[blue62]

Fluctuating voltage....
I would say you have a bad ground or a broken wire.
Or possibly a faulty relay

Continuity testing is not that reliable at telling you the wire is sound.... this is why..
Wires are made up of multiple strands it only takes one strand in that wire to show continuity.
The rest of the strands in the wire can be broken and it will still show continuity.
But the circuit will not function correctly because there is to much resistance in the broken strands for the current to flow.
So if your doing continuity tests you also need to check the resistance on the wire..

You say all the fuses are good how did you check them?
did you check that they are getting power to the power side of the fuse?
In the schematic you linked it shows five fuses at the top right on the first page.
four are hot all the time one is only hot with key on.
There are also two fuses shown at the top of page two
If you have not done so....
You need to test to make sure you have power on the hot side of the fuses.

Find and test or replace associated relays.

At the bottom of page one it show the ground point for the system you need to check that and make sure things are clean and tight.

Those are the things I would start with.

[2000BoxS]

Thank you for the suggestion, blue62. I will double check the fuses, relays and wires based on your feedback.

Based on the fact the instrument cluster is sending fluctuating signal (received by the climate control at B13 wire), I guess I should expand my search to include the instrument cluster system as well.

[2000BoxS]

..........

[blue62]

Originally Posted by 2000BoxS

Thank you 78F350 for your suggestion. I tried the method but unfortunately nothing shows up on the screen of the climate control unit in my car. It doesn't look like my unit supports the feature.

Thank you for the suggestion, blue62. I will double check the fuses, relays and wires based on your feedback.

Based on the fact the instrument cluster is sending fluctuating signal (received by the climate control at B13 wire), I guess I should expand my search to include the instrument cluster system as well.

Yes make sure your ground point is good as well as the ground wire and it's connection point at terminal 31. It also looks like B13 has a connector where the wire changes color from yel/blu to orn/grn...so you need to check that connection as well as everything along that wire.