I have been having problems with my blower motor not coming on for my hvac inside my 993. It has a mind of its own when it wants to work. Its been cooler at night (mid-high 60's) and it seems I have the problem mostly at nights. It doesn't come on for a few minutes after you start the car. The few times it has happened the temp **** has been set to max blue. During the days when its in the 80's, everyhting seems to work fine, so far!

After reading previous post., I looked at all the fuses, they seemd OK. On my last incident, I turned the temp to max Hot to see it the heater blower was working, "it was not blowing". About 30 seconds later everything started working fine, including the rear blower.

I was thinking I had a ccu or a final statge problem, now I am thinking the rear blower/resistor. Does the car look to the rear blower when the temp is in the 60's? It seems too warm to reach out for that.

Since the problem is intermittant, its hard to test it all the time. Once the car is warmed-up everthing seems fine, so far!

Thanks,

Fred Reyes
1996 993 C4

 

Well, Its confirmed, I only have this problem when the temp is less than 70F. Any ideas would be great.

Thanks,

Fred

 

Fred,

I wish I could help you here, but I can't quite see an obvious (or typical) cause.

-Only having the problem at "blue dot" pretty much excludes the rear blower (source of heated air) from the mix. If you had a rear blower problem, you would see the interior fan cut out when you moved the temperature control **** from blue dot to a setting where heated air (from the rear blower) is needed. Or, if you depressed the defrost button.

-On the other hand, only having the problem at "blue dot" might suggest a problem with the fresh air supply. But, that's not consistent with only having the problem when temperatures are low (<70F). In which case, heated air would (presumably) be supplied to the fans (allowing at least reduced air flow). In other words, if you have a fresh air supply problem, it would only get better with reduced temperature, not worse.

-Only having the problem in recirulation mode, or with the A/C on, might suggest a problem with the vacuum controlled bypass (recirculation) air flap. Again, your fans are losing their source of supply air.

-Completely erratic fan speed (with no rhyme or reason) might suggest a bad final stage regulator. But, you said the problem seems to be temperature dependent, and I can't see a FSR failure being temperature sensitive. Also, I personally have only heard of FSRs failures that caused the fan to continue to run when the ignition is off. Though, I'm sure there must be other modes of failure....

Looks like troubleshooting is tough, considering that the problem is intermittent. Still, I would suggest a few things:

-Following this guide for a 964, verify all your servo motors respond properly. In particular, the fresh air flap:

http://www.porsche964.co.uk/technical/servos.htm

One difference for your 993--the mixing chamber servos are in the driver and passenger footwells.

-See if running the A/C or recirculation mode has any affect. This will be a test (of sorts) for the vacuum controlled bypass air flap.

Good luck!

 

Randall G.

Thanks for your response. I do have the problem at both the blue and red dots. When I do have the problen, moving from one to the other doesn't seem to make a difference. One thing I did notice today when everything was working, I could only get a high speed on the rear blower..For some reason I am thinking the low speed resistor in the rear!

Also, I had the engine harness recall done two months ago, maybe they
cracked something or pulled something loose...

Thanks Again,

Fred

 

Hey Fred,

The rear blower resistor is easy enough to test. In the engine fuse box, simply jumper #2 (or #3) in the relay plug to #7. The fan should run in slow speed. If the resistor is failed, no fan operation (open circuit).

While you're back there, you can switch the relay with either the oil cooler or condenser fan relays. You know the slow-speed contact on the condenser fan relay is good if your condenser fan runs continuously when you turn on the A/C.

Do at least check your fresh air intake servo motor and bypass air flap.

Good luck!

 

I have a similar problem.

The fresh air fan doesn't blow anymore. However, if I push the AC button it will work after about 30 seconds. So the AC works but not fresh air. My shop thinks it's a control unit problem ($1200) so we decided to ignore it since the AC works, and when I don't need AC I drop the top.

 

Hey fluid15,

Unless your shop has actually tested the fresh air flap servo, it's unlikely your problem is with the CCU--the servos fail much more often.

Your ventilation works normally when the A/C is on, because running the A/C automatically places your ventilation in recirc. mode. So, your interior fans are drawing from the car's interior (not the outside world). I suspect your system will also work fine w/o the A/C on, but with the recirculation button depressed.

When you're not in recirculation mode, the interior fans draw air from the fresh air intake at the base of the windshield (center). If the servo motor has failed, this supply is shut off--no source of air for the fan. When you turn on the A/C (or press the recirc. button), the bypass (aka recirculation) flap opens, the fresh air flap closes, and the fans draw from the car's interior.

You can test that your fresh air servo is working properly, without even removing the flimsy black cover. Simply listen/feel for a change in the flow of air at the fresh air intake, as you push the recirculation button. If you don't hear/feel a distinct change as you move into and out of recirc. mode, your servo isn't repositioning.

Good luck, and let us know what you find.

 

where is the rear blower resistor located

 

Nevermind, I found it.

thanks

 

Great Advice!!! Thanks for the reply - I'll check that out and let you know what happens.

 

What is the function of the temperature sensor in the grill that goes to the CCU?

Randall-I tried all of your test, everything seems to work OK.

In summary, my CCU is completey dead (nothing worlks at all) when the outside temperature at night is less than 75F outisde , After about five minutes the unit will shut on and off for a few minutes eventually working fine. On a hot day everything works from the start...

Also, how do you remove the connectors on the CCU. My hands will not fit behind it. I noticed that there isn't enough slack on the wiring to remove the
connectors after the CCU is pulled out.

Thanks,

Fred
96 993 C4

 

Fred,

The temperature sensor in the front of the car is an input to your CCU. Unless you have an OBC, in which case you'll have another of the exact same sensor (for displaying temperature).

The CCU-input outside temperature sensor is not described at all in the shop manual (that I could find). Other than showing up in the wiring diagram, that is. Near as I can tell, it's part of an algorithm for controlling the mixing chamber flaps. Yet, the 964 does without an outside temperature sensor input for its CCU. Which is, to be honest, a good thing. When the outside temperature sensor fails, it causes weird problems with the temperature control. For example, cool air only on blue dot, hot air anywhere off of blue dot.

How to Remove the CCU

Hope this helps, and keep us posted on what you find.

 

Randall - I followed your "check" procedure and you were correct! Aside from the AC, air WILL blow if using the Recirculation - the only function not working is the fresh air, which must be a faulty flap from the outside.

Many thanks for sharing your advice - that's the great thing about this board. I may not fix it since I never use the fresh air much (I do have a cabriolet - my REAL fresh air fix!)

 

Guy's........Very, Very interesting thread, living in Texas I read all the post relating to AC performance.

Could someone please post the PN for the outside temperature sensor for a '98 C2S and aprox. price? Looks like mines dead and I didn't even notice .

TIA

 

Does anybody have the pinouts for the connectors on a CCU?