Notices

HVAC COMPRESSOR RELAY REPAIR PROCEDURE w/PICS

 
 
Old 01-01-2011, 08:03 PM
  #16  
dr bob
Chronic Tool Dropper
Lifetime Rennlist
Member
 
dr bob's Avatar
 
Join Date: Oct 2001
Location: Bend, Oregon
Posts: 17,732
Default

Greg--

The circuit appears to be protected by fuse 17 on my car (1989), in the supply side between the 30 bus and the "fresh air blower relay X". Fuse at H37 protects everything at common power connection 8 at G33. I agree completely that the protection for the clutch circuit is marginal at best though; expecting the 30A fuse to protect the board traces and the 1.0 mm^2 clutch wiring is not realistic.

Options for downstream protection include a 10A or maybe 15A fuse in the clucth coil circuit. Easy to do while the control head is apart. Otherwise, you can protect everything between the freeze switch and the coil by adding a fuse at the freeze switch. But--- This gets into a too-common area where undocumented "features" like a fuse installed in a very unexpected place will cause you or your future electrical diagnostic person to pull out large patches of their own hair if/when that fuse blows. The document that I drew a dozen years ago is in my cars log book for that future "victim", and a fuse could be added to the drawing. The "victim" would still need to know to look there though.


Originally Posted by GregBBRD View Post
A couple of thoughts....

Note that the stock relay and wiring to the compressor, is an unfused circuit, in the 928 vehicle. The relative "weakness" of the original relay saves the relay board and wiring, in the vehicle, from severe electrical damage (fire).

Adding a huge relay, such as this, will certainly eliminate the problem with the relay failing...but if anything ever goes wrong....plan on a fire, instead of relay failure.

Most of the relay failures are actually a result of the resistance in the circuit increasing, with age. Any place that either the loom deterorates or if the compressor clutch resistance increases will increase the current through the relay and cause failure....of the stock unit. Significantly increasing the amperage rating of the relay, without regard to this circuit, will significantly increase the likelyhood of a fire.

We rebuilt many of these control units (928 International has them). Although we do use a higher amperage relay, we do add a protection fuse to this circuit....which has reduced the "comeback" of rebuilt units to almost zero (0). We also found that the A/C control units had "other issues" than just the failure of the relay. We built "test boards" to completely test each unit, before and after repairs. Prior to the addition of the new fuse, many of the rebuilt control units came back with the new relays melted....the result of a problem on the circuit to the compressor. Now that the circuit is fused, when there is a problem, the fuse will blow, instead of the relay failing.
dr bob is offline  
Old 01-01-2011, 09:52 PM
  #17  
GregBBRD

Rennlist
Basic Site Sponsor

 
GregBBRD's Avatar
 
Join Date: Feb 2005
Location: Anaheim
Posts: 9,696
Default

Originally Posted by dr bob View Post
Greg--

The circuit appears to be protected by fuse 17 on my car (1989), in the supply side between the 30 bus and the "fresh air blower relay X". Fuse at H37 protects everything at common power connection 8 at G33. I agree completely that the protection for the clutch circuit is marginal at best though; expecting the 30A fuse to protect the board traces and the 1.0 mm^2 clutch wiring is not realistic.

Options for downstream protection include a 10A or maybe 15A fuse in the clucth coil circuit. Easy to do while the control head is apart. Otherwise, you can protect everything between the freeze switch and the coil by adding a fuse at the freeze switch. But--- This gets into a too-common area where undocumented "features" like a fuse installed in a very unexpected place will cause you or your future electrical diagnostic person to pull out large patches of their own hair if/when that fuse blows. The document that I drew a dozen years ago is in my cars log book for that future "victim", and a fuse could be added to the drawing. The "victim" would still need to know to look there though.
That 30 amp fuse is indeed worthless protection for the relay board and the wiring to the A/C compressor....I've seen relay boards with smoke pouring out of them that didn't even get that fuse warm.

We supply a 3 amp fuse that gets installed at the de-icing switch with every rebuilt relay board. I think that Sean saw this and thought it to be such a great idea, that he is retrofitting vehicles with this fuse...even when the relay isn't burned up.

Not too worried about future diagnostics problems....if there isn't power at the "overheat" switch, even the dumbest diagnostic person is going to look at the de-icing switch next. Anybody that spends over a minute or two finding that fuse needs to turn in their volt/ohm meter...they shouldn't be allowed to buy the battery needed to power it up!
__________________
greg brown




714 879 9072
[email protected]

If the kids keep breaking a crowbar in the sandbox, sometimes it is easier to just give them a stronger crowbar, instead of trying to figure out what they are doing in the sandbox.
GregBBRD is offline  
Old 01-02-2011, 11:09 AM
  #18  
Dwayne
Addict
Rennlist Member
Thread Starter
 
Dwayne's Avatar
 
Join Date: Sep 2007
Location: Ridgecrest, California
Posts: 1,351
Default

Originally Posted by the flyin' scotsman View Post
Another great write up Dwayne...........well done.

If I may some tips on soldering:

the PCB used in the HVAC head is very basic through hole technology and when the assembly was built at factory the components were mounted on one side of the board then ran through a solder bath at such a level that only the bottom side of the board and the legs of the through hole components touched the molten solder. The solder is then drawn up the legs of the components with capillary action leaving all perfectly soldered.

This action can be replicated by the home mechanic by soldering from the bottom side of the PCB allowing the solder to be drawn up each individual wire by using Dwaynes method above and bending the tinned wire to secure it to the PCB. Once all wires are soldered then, if required, clip off any excess.
Hello Malcolm,
THANKS for the tip and the info on original manufacturing process - very interesting for me. THANKS for the feedback!


Originally Posted by borland View Post
Dwayne,

Glad you got it done.

As you know, internal relay is a DPDT, but only used as a DPST. It is switching two signals at the same time. You can see this from the WSM wiring diagrams, those two signals go externally on a smaller (0.5) wire for one switch and larger (1.0) wire the other. So one side of the relay is switching low current, and the other high current.

Based on this, another approach would be to install two SPST minature relays (one high current, the other low current). These are small enough that they could be both mounted inside the control unit.

From Radio Shack .... here's a minature 12 volt SPST rated at 10 amps...
http://www.radioshack.com/product/in...lterValue=SPDT

And this SPDT minature rated at 1 amp...

http://www.radioshack.com/product/in...lterValue=SPDT

Or, you could be able to get by with a single 5 amp minature like this...

http://www.radioshack.com/product/in...lterValue=DPDT
Hi Borland,
GREAT info on the low current/high current switching and the other available options for relays I did not know about - THANKS!

Originally Posted by SharkSkin View Post
Excellent writeup!

One thing to note, on OBs the AC compressor clutch will never engage unless the engine is running, assuming the wiring has not been altered. These cars have a special "speed relay"(XI on CE panel) that won't engage unless it sees a tach signal. So, to troubleshoot AC on an OB you need to jumper this relay then go through steps in the writeup above. 1980 and later work essentially as described in Dwayne's writeup.

For my OB I added a power relay to the CE panel to serve the purpose of the relay that Dwayne added above(also to take the load off of the speed relay), details here.
THANKS, Dave! I checked out your link and it was amazing! Outstanding quality documentation and very educational. Your site has a wealth of insightful and valuable repair information! THANKS for the feedback and the additional comments.

Originally Posted by GregBBRD View Post
A couple of thoughts....

Note that the stock relay and wiring to the compressor, is an unfused circuit, in the 928 vehicle. The relative "weakness" of the original relay saves the relay board and wiring, in the vehicle, from severe electrical damage (fire).

Adding a huge relay, such as this, will certainly eliminate the problem with the relay failing...but if anything ever goes wrong....plan on a fire, instead of relay failure.

Most of the relay failures are actually a result of the resistance in the circuit increasing, with age. Any place that either the loom deterorates or if the compressor clutch resistance increases will increase the current through the relay and cause failure....of the stock unit. Significantly increasing the amperage rating of the relay, without regard to this circuit, will significantly increase the likelyhood of a fire.

We rebuilt many of these control units (928 International has them). Although we do use a higher amperage relay, we do add a protection fuse to this circuit....which has reduced the "comeback" of rebuilt units to almost zero (0). We also found that the A/C control units had "other issues" than just the failure of the relay. We built "test boards" to completely test each unit, before and after repairs. Prior to the addition of the new fuse, many of the rebuilt control units came back with the new relays melted....the result of a problem on the circuit to the compressor. Now that the circuit is fused, when there is a problem, the fuse will blow, instead of the relay failing.
THANKS for chiming in here, Greg. I had not considered the fusing aspects you point out but agree some protection is needed in the circuit. Do you recommend placing an in-line 3A or 5A fuse on the power line into the freeze switch or on the power line out of the freeze switch?

THANK YOU for your comments and recommendation, it is very educational for me, helps improve the quality of this post, and may help prevent and unfortunate side effect.

Originally Posted by dr bob View Post
Greg--

The circuit appears to be protected by fuse 17 on my car (1989), in the supply side between the 30 bus and the "fresh air blower relay X". Fuse at H37 protects everything at common power connection 8 at G33. I agree completely that the protection for the clutch circuit is marginal at best though; expecting the 30A fuse to protect the board traces and the 1.0 mm^2 clutch wiring is not realistic.

Options for downstream protection include a 10A or maybe 15A fuse in the clucth coil circuit. Easy to do while the control head is apart. Otherwise, you can protect everything between the freeze switch and the coil by adding a fuse at the freeze switch. But--- This gets into a too-common area where undocumented "features" like a fuse installed in a very unexpected place will cause you or your future electrical diagnostic person to pull out large patches of their own hair if/when that fuse blows. The document that I drew a dozen years ago is in my cars log book for that future "victim", and a fuse could be added to the drawing. The "victim" would still need to know to look there though.
Hello Dr Bob - THANKS again for your work in this area and your original guide. You're very conscientious about documenting "modifications" and I wish some of the previous owners of 3 of our 928s had been as considerate - would have made troubleshooting much easier!

THANKS to all for your positive comments and recommendations for improving this post!
Dwayne is offline  
Old 01-02-2011, 11:13 PM
  #19  
GregBBRD

Rennlist
Basic Site Sponsor

 
GregBBRD's Avatar
 
Join Date: Feb 2005
Location: Anaheim
Posts: 9,696
Default

Dwayne:

I've never seen any issues with the wiring inside the vehicle, but I have seen one freeze switch with a problem. I, therefore, install my fuses before the freeze switch.

I originally started with 5 amp fuses (and never had any issues) but recently reduced this to 3 amp fuses, again without any issues.
__________________
greg brown




714 879 9072
[email protected]

If the kids keep breaking a crowbar in the sandbox, sometimes it is easier to just give them a stronger crowbar, instead of trying to figure out what they are doing in the sandbox.
GregBBRD is offline  
Old 01-03-2011, 12:28 AM
  #20  
Dwayne
Addict
Rennlist Member
Thread Starter
 
Dwayne's Avatar
 
Join Date: Sep 2007
Location: Ridgecrest, California
Posts: 1,351
Default

Originally Posted by GregBBRD View Post
Dwayne:

I've never seen any issues with the wiring inside the vehicle, but I have seen one freeze switch with a problem. I, therefore, install my fuses before the freeze switch.

I originally started with 5 amp fuses (and never had any issues) but recently reduced this to 3 amp fuses, again without any issues.
Sounds great. I'll research what kind of external in-line 3A fuses are available and pick one up. THANKS for the help.
Dwayne is offline  
Old 01-03-2011, 10:13 PM
  #21  
Dwayne
Addict
Rennlist Member
Thread Starter
 
Dwayne's Avatar
 
Join Date: Sep 2007
Location: Ridgecrest, California
Posts: 1,351
Default

The post has been updated to add the inline fuse to the compressor circuit. THANKS Greg and to ALL for helping to improve this post.
Dwayne is offline  
Old 01-04-2011, 06:10 PM
  #22  
86'928S MeteorGrey
User
 
86'928S MeteorGrey's Avatar
 
Join Date: Jun 2008
Location: Surprise, Arizona
Posts: 1,914
Default

Another amazing write-up Dwayne!

Every time I try and do a write-up, I get impatient half way through with the photos and just finish the install and go drive. I commend your patience and eye for detail!

I've actually been putting off replacing the HVAC relay, and now I can do it with these excellent instructions. Glad I waited!

One comment though... Probably a non-issue, as the relay probably won't wear out in my lifetime since it's actually rated for the current going through it, but does Radio Shack sell the socket that matches that relay? Sure would be cool to solder those leads to a socket and route the wires and socket to an accessable location for super easy relay replacement.

You Da Man Dwayne!
86'928S MeteorGrey is offline  
Old 01-04-2011, 11:37 PM
  #23  
Dwayne
Addict
Rennlist Member
Thread Starter
 
Dwayne's Avatar
 
Join Date: Sep 2007
Location: Ridgecrest, California
Posts: 1,351
Default

Originally Posted by 86'928S MeteorGrey View Post
Another amazing write-up Dwayne!

Every time I try and do a write-up, I get impatient half way through with the photos and just finish the install and go drive. I commend your patience and eye for detail!

I've actually been putting off replacing the HVAC relay, and now I can do it with these excellent instructions. Glad I waited!

One comment though... Probably a non-issue, as the relay probably won't wear out in my lifetime since it's actually rated for the current going through it, but does Radio Shack sell the socket that matches that relay? Sure would be cool to solder those leads to a socket and route the wires and socket to an accessable location for super easy relay replacement.

You Da Man Dwayne!
Hello Mike,
Thanks for the comments! I was thinking the same as you and asked the cashier at Radio Shack the same question about the socket. They were not aware of any they sold. They basically said "what you see here is all we have." So, I don't have confidence that there isn't one available but it would require a little research which I have not done, yet.
Dwayne is offline  
Old 01-05-2011, 01:04 PM
  #24  
Lorenfb
Super User
 
Lorenfb's Avatar
 
Join Date: May 2003
Location: SoCal
Posts: 3,535
Default

'I originally started with 5 amp fuses (and never had any issues) but recently reduced this to 3 amp fuses,'

To many/all on this thread, the use of a fuse in series with the compressor clutch will solve the
problem of a damaged climate control unit. Well, that's not really the case.

Given that the output relay is on a circuit board with thin/small circuit tracks, which were not designed
to carry much current beyond the current of the clutch itself, the issue of using a fuse is not as simple
as some might suggest. If the fuse is too small, a surge may blow the fuse. If the fuse is too big,
the circuit track will be damaged. So use of a fuse becomes questionable as to its utility, i.e.
Has a proper fuse amperage been selected for adequate protection and reliability in all cases,
most likely not? And furthermore, is a fuse of any value at all? But for some, they think
the problem is solved.

To properly use a fuse, the relay's main power source must not go thru the CCU, as is the case
with all applications noted on this thread using a fuse. The weakest point in a fused circuit
must the be the fuse which is not the case here. That's most likely why Porsche didn't use a fuse.
If Porsche had designed the CCU system properly, they would have placed the compressor
relay in the 928 relay panel and fused the circuit there with a 5-10 amp fuse.

Side note: With the exception of the relay failure on the CCU, most/all other 928 climate
control issues are unrelated to the CCU, i.e. Most/all mis-diagnose these problems including
dealer Porsche techs.

Bottom line: Attempted fixes/mods may not always work as first (simplicity) thought!
Lorenfb is online now  
Old 01-05-2011, 01:17 PM
  #25  
Jfrahm
Addict
Rennlist Member

 
Jfrahm's Avatar
 
Join Date: Mar 2002
Location: Boulder, CO
Posts: 5,732
Default

Great writeup!

If your OEM AC head relay is still working, you can, I believe, protect it by mounting an underhood-grade relay near the AC compressor and powering the AC clutch with a fused power wire from the jump post. I used a relay with a mounting tab from a fog light wiring kit for this project. Now the relay in my climate control head only energizes a small relay which requires it to pass very little current. I hope it lasts a long time! Just be sure to fuse the new power wire very close to the jump post in case it gets grounded, and use insulated spade connectors to prevent accidental shorts.

-Joel.
Jfrahm is offline  
Old 01-05-2011, 02:43 PM
  #26  
Lorenfb
Super User
 
Lorenfb's Avatar
 
Join Date: May 2003
Location: SoCal
Posts: 3,535
Default

"I believe, protect it by mounting an underhood-grade relay near the AC compressor and powering the AC clutch with a fused power wire from the jump post. Now the relay in my climate control head only energizes a small relay which requires it to pass very little current."

The best and the easiest solution to REALLY protect the CCU!!
Lorenfb is online now  
Old 01-06-2011, 12:05 AM
  #27  
Dwayne
Addict
Rennlist Member
Thread Starter
 
Dwayne's Avatar
 
Join Date: Sep 2007
Location: Ridgecrest, California
Posts: 1,351
Default

Originally Posted by Jfrahm View Post
Great writeup!

If your OEM AC head relay is still working, you can, I believe, protect it by mounting an underhood-grade relay near the AC compressor and powering the AC clutch with a fused power wire from the jump post. I used a relay with a mounting tab from a fog light wiring kit for this project. Now the relay in my climate control head only energizes a small relay which requires it to pass very little current. I hope it lasts a long time! Just be sure to fuse the new power wire very close to the jump post in case it gets grounded, and use insulated spade connectors to prevent accidental shorts.

-Joel.
Hello Joel,
I would be interested in finding out more about your installation. Do you have additional details (e.g., model of the relay and specs on the relay, recommended fuse amperage, wiring diagram or pictures of the install)?
Dwayne is offline  
Old 01-06-2011, 09:24 PM
  #28  
86'928S MeteorGrey
User
 
86'928S MeteorGrey's Avatar
 
Join Date: Jun 2008
Location: Surprise, Arizona
Posts: 1,914
Default

Originally Posted by Dwayne View Post
Hello Mike,
Thanks for the comments! I was thinking the same as you and asked the cashier at Radio Shack the same question about the socket. They were not aware of any they sold. They basically said "what you see here is all we have." So, I don't have confidence that there isn't one available but it would require a little research which I have not done, yet.
I believe this is the matching socket.

http://www.mouser.com/search/Product...ADaJYdXA%3d%3d

Going to the underhood would probably be better, but this should theoretically be easier...
86'928S MeteorGrey is offline  
Old 01-06-2011, 09:34 PM
  #29  
86'928S MeteorGrey
User
 
86'928S MeteorGrey's Avatar
 
Join Date: Jun 2008
Location: Surprise, Arizona
Posts: 1,914
Default

Here's the Radio Shack socket for it. It's a little more compact, but would be harder to mount I think.

http://www.radioshack.com/product/in...lickid=prod_cs
86'928S MeteorGrey is offline  
Old 01-06-2011, 11:26 PM
  #30  
Dwayne
Addict
Rennlist Member
Thread Starter
 
Dwayne's Avatar
 
Join Date: Sep 2007
Location: Ridgecrest, California
Posts: 1,351
Default

Originally Posted by 86'928S MeteorGrey View Post
Here's the Radio Shack socket for it. It's a little more compact, but would be harder to mount I think.

http://www.radioshack.com/product/in...lickid=prod_cs
That was easy! THANKS, Mike!!

I'm liking the idea Joel has for preventative failure of head unit relays that are still working. For those relays that have died, as in my case, I'm liking the idea of somehow fixing the relay to the CEB and fusing the compressor circuit there as Loren suggests. I wonder if this has been done already?
Dwayne is offline  

Thread Tools
Search this Thread
Quick Reply: HVAC COMPRESSOR RELAY REPAIR PROCEDURE w/PICS


Contact Us - About Us - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.