1999 C5

I'm planning on pulling the dash top soon but can anyone give me some starter pointers including maybe why I have no comms? I'll list what I've done so far to try and check this out. I have comms with all other modules in this car which was unused for many years. I'm presently happy with electrical function generally after much ground and connector cleaning.

My (dual climate) HVAC operation is as follows

-Air flow direction (via vacuum) working fine
-Set to cold (not AC) both sides get unheated air
-I'm not getting any DRIVERS side heat blowing when set to deliver hot air
-I SEEM TO ONLY GET HEAT AT THE PASSENGERS SIDE IF SET TO HOT ON DRIVERS SIDE (see above, no actual hot air, just set to hot). Presumably you should be able to get hot air at passengers side without having to have hot air at the drivers side? Once hot, I can't cool the passengers side independently.

-Today I pulled fuse 27 and on replacing it I was able to hear whirring of motors. I think it sounded as though the passenger side operated first then silence for a few seconds and then more whirring.
-I pulled the HVAC unit to check inside and it's circuit boards and connectors look perfect. It's functions APPEAR ok.
-After doing this, still NO COMMS

My AC isn't functioning but no doubt needs charged. You can understand I'd like have COMMS to get fault info. Obviously my HVAC isn't working perfectly but doesn't seem a million miles away from functionality. Seems pointless to charge the AC just now.

This one is a bit "out there" but in all the time I've had and have been working on this car I have never got the check tire pressure message (no sensors in my wheels). Today when I pulled fuse 27 the tire pressure warning came on. It has gone away again after re-fitting the fuse. Surely very strange but quite a specific occurence!

I have developed an annoying dash rattle which sounds like from the top of the HVAC box. This is another reason I want to pull the dash but of course I'd prefer to go in armed with some information from my DIC!

 

For any module “no comm” you would check that modules power and ground and go from there !!

 

I’ll check voltage on the HVAC fuse tomorrow. Any specific way to test the HVAC ground for it? I’ve exhaustively cleaned all grounds and splice packs though so I suspect grounds are fine.

I checked all the ignition switch related fuses today and readings were ok showing battery voltage.

Are the connector blocks beside the BCM called star blocks and they would affect code gathering, is that right? I may have stumbled upon my HVAC no comms issue. I had either one or two damaged wires leading to one of those blocks. I thought the blocks did nothing (I thought they were just safe housings for unused wiring loom items) so just insulated the cut wire going to those blocks. I have just read that the blocks short wires together. I’ll look tomorrow for which (one or two, maybe three) wires which were cut just short of those blocks.

 

Yes, next to the BCM are 2 "Star Connectors" where the serial data wires for each module meet...the purple wire on each Star Connector goes to the DLC where you hook up your scan tool...when you hook up your scan tool its like another module joining the party as it were. The serial data wire for the HVAC module is a white colored wire...I use a scope to test those data lines which produce a 0-7 volt square wave when that module is "talking" on the network...you can see if you have continuity between the white terminal at the Star Connector and the HVAC module end...it looks like terminal D12 at the HVAC module. It's best to check for power AT the module...if you had a break in the wire for instance between the fuse and HVAC module you would show good power at the fuse...it would never get to the module !!...below is the pinout for my 2001...yours should be the same...C5 (brown) and C12 (orange) are your power "hot at all times and "key on" !!...C1 and D1 are your grounds ...I like to "load test" my grounds with an old headlight bulb...you can hook up a 12 volt test light to battery positive or any 12 volt source and when you probe the ground the test light should be bright...another thing is pop off the splice pack "comb" (pictured) and see if the white wire terminal is corrosion free and also on the comb...the comb ties in all the comm wires so modules can communicate between each other. Hope this helps !!

 

Thanks v much C5 Diag. Can't in good conscience say I understand how the system works (the video was well above my head..) but lo and behold here we have my white wire not connected (I had just wrongly thought the wires terminated in those boxes for safety rather than joined for any function).

My no comms is through my DIC not a scan tool, but I imagine it doesn't take guesswork to conclude this is very likely my HVAC no comms problem. Not had any time today to fix it. Feels like great news though.

 

That white wire should be going to the splice pack connector...after that it should work !!...electrical diagram below !!...you can see the serial data wire (purple) going to both splice packs...that purple wire goes to the DLC pin 2 where your scan tool hooks up to.

 

Great.
I mentioned I wasn’t using a scan tool, just the DIC. I presume the white wire is still very likely to be my problem?