tirschler@charter.net

Self induced problem, I replaced the ignition wires, spark plugs and engine mounts on my 95, 993 started up and ran like a top. Realized I forgot to reinstall the blower resistor/switch into the air duct, upon touching it (its really HOT!!!) dropped it, touched the body sparked, shorted. Now the symptoms are:

Heater Blower runs continuously at high speed when ignition is turned on, and does not swithch off for 20min after the ignition is turned off. (engine cold or hot, or just switching the ignition on and off, still runs for 20min)

Voltage at the resistor airflow switch is now zero.

Normal functioning of A/C, heating, and defrost occurs, with appropriate airflow coming out of selected vents, but the heater blower continues to run for 20 min after ignition OFF.

If I disconnect the heater blower, the cabin fans run at 1/2 speed of selected blower switch position, unless full cold is selected, then full fan speed #4 occurs.

ToreB

Shorting the low fan speed resistor wires to GND would probably only blow the fuse. It gets very hot when the fan is running at low speed, that's why it's mounted in the air flow duct. There's a temperature fuse in it as well that might fail or needs a reset.
The CCU supervises the engine/air duct temperature with a temperature sensor, and at igntion off, the fan run in low speed if the air duct temperature is above 75degC, and shall stop when below 70degC, and about 20 minutes max.
Maybe you have a bad or disconnected temp sensor? All details on this here.
A failing rear fan will shut down the front blowers. See here for my DIY fault finding page.
Cheers,
Tore

tirschler@charter.net

Update: I removed the rear fan series resistor, and discovered infinite resistance between the bi-metal contact points. Cleaned the contacts with 2000 wet/dry paper, and now the series resistor contacts allow current flow again, resulting in the fan operating at LOW SPEED, ( with CCU fan < 2, previously high speed only) albeit still for 20min after ignition off, regardless of engine temp.
The rear fan temp sensor has a resistance of 11.5k ohms at 20C.
Rear aux blower relay replaced with oil cooler relay (known to work) and functions the same.
The rear aux fan still runs for 20min at ignition OFF, regardless.

Is this beginning to look more like a short/problem with the CCU?

Vorsicht

Double-check the temp sensor connection located on the same tube as the resistor about 10 inches higher. I have found that if this is left disconnected the fan runs continuously w/ ignition off.

tirschler@charter.net

After much head scratching and gnashing of teeth, some progress to report. Removed the CCU, to repair little snowflake switch which came flying out with springs and mini dumb bells flying everywhere. While out I serviced the CCU sampler blower, cleaned the temp sensor etc. Checked the PC boards, appeared fine, will check again though. After reconnecting the CCU harnesses, same problem. However; wiggling and pressing on the CCU harnesses resulted in the rear aux blower shutting off (coincidence?) Tried it a second time, rear blower again switched OFF. Could not duplicate a third or fourth time. That got me thinking, to try what ToreB suggests on his site. Pull the #1 fuse. This immediately shuts OFF the rear blower (cuts all power to the CCU I presume, and resets the system) Reinstalled fuse #1, and if the engine is cold rear blower switches OFF, when ignition is placed to OFF!! Yippee!! However if the engine has run for any length of time, switching the ignition OFF, the rear aux blower continues to run for 20 min, unless I pull fuse #1 again to interrupt the cycle.
Still have not isolated whether this is a CCU problem, or temp sensing problem, or both.
Incidently, the CCU sample blower also runs for 20 min along with the rear aux blower when it is running its 20 min cycle.
Is there a relay which cuts power to the CCU, which might be stuck ON?

mojorizing

I believe fuse 17 is power to the CCU and is off the ignition buss, with fuse 1 as power to the LH and RH blowers, and also inputs to the CCU and that's from the battery. That fuse 1 probably supplies power to the sample fan, processor, etc.

Does the same "reset" happen when you pull only fuse 17?

Next time you have your ccu out, it would be useful to measure the resistance of the temp. probe from the K or G connectors to eliminate wiring/connector to the probe. Unfortunately, a brief glance at the schematic doesn't tell me what pins are the temp. sensor. Will spend some time on that tomorrow....

IXLR8

Already sent him that yesterday. He is measuring the G/10 to the NTC at this moment.

mojorizing

Yep, G/10 to G/16 should be the same as the reading at the NTC sensor

IXLR8

G/10 to G/18 in my diagram. G/18 is sensor ground.

mojorizing

You're right, it's G18

mojorizing

@ Trischler, while you have the CCU out measure the resistance of the pins G/10 and G/18 coming out of the CCU using both meter polarities..there's a voltage bridge/diode circuit before the analog signal inputs to a CD4051 then into the processor. I have 5.07K ohms either polarity on a 993 CCU that has both panels separated. If it's close we can also rule that out.

ToreB

11.5k at the input to the CCU (G10 and G18)indicates a fully working air duct temp sensor.
The CCU fan is supposed to run for some time after ignition off, and 20 minutes is within spec. As long as it turns off at all you have no problem with the internal T1 transistor.
However, the rear fan operation is a mystery. Resetting the CCU by pulling fuse 1 will stop any running of the rear fan, (and CCU fan/CCU operation) so you are describing a normal behaviour.

The CCU temp sensor input could be blown, these analogue inputs are multiplexed by a common CMOS IC 4051, but also have diode protection circuitry on the input itself. I would check the diode (1N4148) and maybe change the 4051. (I would have to find which of them it is first) I would not think that the CPU analogue input is damaged, as this input probably handles several of the other sensors.
This would be a last resort though, especially in the lack of any CCU test bench equipment. I would find it more likely to be a problem in the car, not in the CCU.
Cheers,
Tore

tirschler@charter.net

OK, here is hopefully the problem. My engine harness was changed by a Porsche service buliten, and the engine bay temp probe wires are different colors than original. (Solid Red. & Red/Brn)

G-10 Blu/Grn at CCU, to temp probe harness solid red. Perfect continuity.

G-18 Brn/Blu at CCU (sensor ground) to temp probe harness, Rd/Brn.
Infinite resistance/ open circuit, break in the wire, or not grounded properly somewhere.

I am thinking to run a thin external wire directly from G-18 at the temp probe, to the back of the CCU to confirm? Thoughts?

If ops normal, then the fun of tracing the break, connector, ground contact begins!

ToreB

Interesting, good work!
I would think the culprit could be connector X3. (14-pins, on a carrier plate in engine compartment) I do not know exactly where it is, maybe someone else could chime in on this. I would guess near or inside the electric box on the left. There's only this sensor connected to the CCU back there, so you cannot use a different ground connection.
All sensor grounds are connected in a central point marked 2 in the schematics. I would guess this is under the dashboard somewhere. It would not hurt to route a wire from sensor to pin G18, but you could maybe try to find/use connection point 2, all wires going to this are brown/blue.
Cheers,
Tore

mojorizing

Sounds good, but be sure you keep the existing wire going to G/18 because that's the return also from other sensors, servo's, etc. which apparently are intact.

According to the schematic on Tore's site (http://www.bergvillfx.com/images/993hvac.gif) , there are 2 connectors on the harness that could effect the sensor connection X3 and X20...another page from the manual should have a pic of location of those connectors, I believe. I don't have that pic.