Fix your HVAC system
928 Engine Re-Re-Rebuild Specialist
Rennlist Member
Rennlist Member
Joined: Oct 2001
Posts: 7,972
Likes: 34
From: Brighton, MI
Anyone know what the negative effects would be to shorting the outside temp sensor wiring as Mr. McKenzie did? My sensor is missing, too. Are these only available as used units now?
Chronic Tool Dropper
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Joined: Oct 2001
Posts: 20,506
Likes: 565
From: Bend, Oregon
Dave--
I'm not sure that shorting the sensor is he way to go, but since it's a thermistor you can probably sub in a resistor to fool the car into thinking it's some comfortable temp outside. I don't have the specs on teh thermistor handy and I'm not under the car right now. IIWM, I would start with a 1k ohm resistor and see how the system reacts. Perhaps somebody who's metered the thermistor will chime in with a better value to start with. The good news is that resistors are cheap.
I'm not sure that shorting the sensor is he way to go, but since it's a thermistor you can probably sub in a resistor to fool the car into thinking it's some comfortable temp outside. I don't have the specs on teh thermistor handy and I'm not under the car right now. IIWM, I would start with a 1k ohm resistor and see how the system reacts. Perhaps somebody who's metered the thermistor will chime in with a better value to start with. The good news is that resistors are cheap.
Rennlist Member
Joined: Jun 2005
Posts: 3,261
Likes: 53
From: Park Ridge, IL (near Chicago)
No sensor= hot output unless temp set to 65 degrees
Sensor wires shorted=cold output unless temp set to 85 degrees
Good sensor (Used about $80)= variable vent output which is very comfortable
Instructor
Joined: Sep 2003
Posts: 217
Likes: 0
From: Roskilde, Denmark
my sensor seems to be ok, the ohm changes when i put the sensor in my hand.
It outputs arround 0.60 ohm at ca. 15(C).
But i still have HOT AIR
So next step is looking for vaccuum leaks ?
Mads
It outputs arround 0.60 ohm at ca. 15(C).
But i still have HOT AIR
So next step is looking for vaccuum leaks ?
Mads
Electron Wrangler
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Joined: Mar 2002
Posts: 13,660
Likes: 621
From: Phoenix AZ
Outside Sensor Specs:
993 ohms at 0C (32F)
588ohms at 20C (68F)
457ohms at 30C (86F)
However this suggests you still have an open circuit somewhere else...
Test the whole sensor loop: (includes temp control potentiometer)
Between Plug to HVAC Motor Console LHS (HVAC) Plug Pin 4 (YE)*
And Plug to HVAC Motor Console LHS (HVAC) Plug Pin 12 (YE)*
You should see a 1000 ohms variation between 18C - 30C HVAC setting
(65F - 86F).
Alan
Instructor
Joined: Sep 2003
Posts: 217
Likes: 0
From: Roskilde, Denmark
Maybee i didn't uset the multimeter right it proberbly read around 600 ohm.
I only know that the reading changede when i put the sensor in my hand.
I only testet the sensor, did not check the harness plug, but I put the multimeter in the plug, and it seemed like there was ressistence. but i don't remember the reading.
Last night i tryed to test the vaccuum, when i disconected the lines, i could hear that there still was vaccuum in the tank. Does that tell me anything ?
BTW flappy valve is ok, and tank/cruise (i dont have a Y connector there) line also holds vaccuum.
So do i check more with the multimeter, or do i look for vaccuum leaks ?
Thanks for your help guys
Mads
it proberbly read around 600 ohm.I only know that the reading changede when i put the sensor in my hand.
I only testet the sensor, did not check the harness plug, but I put the multimeter in the plug, and it seemed like there was ressistence. but i don't remember the reading.
Last night i tryed to test the vaccuum, when i disconected the lines, i could hear that there still was vaccuum in the tank. Does that tell me anything ?
BTW flappy valve is ok, and tank/cruise (i dont have a Y connector there) line also holds vaccuum.
So do i check more with the multimeter, or do i look for vaccuum leaks ?
Thanks for your help guys
Mads
Instructor
Joined: Sep 2003
Posts: 217
Likes: 0
From: Roskilde, Denmark
Update
Went out to test one more time.
Here is the reading from the sensor, the output changes when i warm up the sensor.
So that is ok.
The harness plug is also ok i think
This i the value i get when i connect the multimeter to the harness.
temp slider to max cold = -1554
temp slider to max hot = -1507
When i blow on the interior temp sensor the value gets lower.
So i think i have to go ahead with the vaccuum test now
Mads
Went out to test one more time.
Here is the reading from the sensor, the output changes when i warm up the sensor.
So that is ok.
The harness plug is also ok i think
This i the value i get when i connect the multimeter to the harness.
temp slider to max cold = -1554
temp slider to max hot = -1507
When i blow on the interior temp sensor the value gets lower.
So i think i have to go ahead with the vaccuum test now
Mads
Rennlist Stories
The Best Porsche Posts for Porsche Enthusiasts
9 Features and Characteristics That Only Porsche People Understand
Verdad Gallardo
I've Written 500 Rennlist Articles: Here's How Porsche Has Changed Along the Way
Joe Kucinski
10 Most Unnecessary Porsches Ever Built (And Why We Love Them)
Verdad Gallardo
Porsche 911 GT3 S/C vs 718 Spyder RS: 10 Categories, One Winner
Joe Kucinski
This Builder Is Turning Heads With Its Slantnose 911 Creation
Verdad Gallardo
Porsche 911 GT3 Artisan Edition Pays Homage to Japanese Culture
Verdad Gallardo
Porsche Reveals Coupe Variant of the Electric Cayenne With a Fresh Look
Verdad Gallardo
10 Porsche Colors That Have More Personality Than Most People
Verdad Gallardo
Guntherwerks' Final Speedster Creation Is the Ultimate Porsche Restomod
Verdad Gallardo
Rennlist Member
Rennlist Site Sponsor
Joined: Jun 2001
Posts: 6,469
Likes: 13
From: Ball Ground, GA
I have a tech paper that I wrote that describes the operation of the HVAC system. If you will send me an email message to techAT928gt.com (replace AT with @) - not a private message on this system, a regular email - I will send you the text file.
Electron Wrangler
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Joined: Mar 2002
Posts: 13,660
Likes: 621
From: Phoenix AZ
If you look back at my message - you need to remove the connection to the HVAC in the console and test the whole loop there as noted - the test you are doing is not meaningful in numeric results - except to demonstrate some connectivity.
Alan
Alan
Thread Starter
Team Owner
Joined: May 2004
Posts: 31,233
Likes: 4,220
From: Philly PA
Mads did you remove the car side of the connector to check the wire connections into the barrells? There is a good chance that this is where your problem lies just sticking in a mult meter dosnt guarantee that the sensor in the fender is still connected/. Pull the wires out of the connector(the car side of the harness) and see if they are soldered or loose like the pictures show. This may save you ripping apart the console to test the inside sensor
Addict
Rennlist Member
Rennlist Member
Joined: May 2001
Posts: 2,758
Likes: 0
From: Wilbraham, MA
I am chasing an intermittent HVAC issue, sometimes A/C works fine, other times it blows hot. The compressor always seems to engage.
Ok, I pull the headlight off, but now don't know where to find the connector you are referring to. Any chance someone has a photo of where it is located before being disconnected?
Thanks!
Ok, I pull the headlight off, but now don't know where to find the connector you are referring to. Any chance someone has a photo of where it is located before being disconnected?
Thanks!
Electron Wrangler
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Joined: Mar 2002
Posts: 13,660
Likes: 621
From: Phoenix AZ
The temp sensor is in a section of the intake tube for the alternator - it should be on the rear of the splash shield but may be lying at the bottom of the fender - trace the wire from that back to the connector.
Alan
Alan
Addict
Rennlist Member
Rennlist Member
Joined: May 2001
Posts: 2,758
Likes: 0
From: Wilbraham, MA
Good information, thanks. I had found the intake tube but didn't understand to trace the wire back. Will do so next.