P0645 "A/C relay signal missing"

deano1 · Post by **deano1** » 01 Apr 2022, 18:08

Hey there
I know some of you hard core Volvo peops have been thru A/C troubles before,, I know because I have been searching and read a lot about it.

I Just never really found my particular issue.
Working on my 1999 s70 AWD and found that air conditioning doesn't kick in.
I get the ECC lights flashing at start up and tried using the panel control buttons to retrieve codes but I didn't have success with pushing re circulation button 3 times in 3 seconds or 4 times for that matter.
So from there i checked refrigerant level and that seems ok.
Pull codes with my Autel and get P0645 "ac relay signal missing" Hmmm
I tried swapping relay and still not kicking in. Hmmm
Watched one of Roberts Diy vids and he says to "jumper the bastard and that'll fix her". So i do,,, and bingo! The comprenday kicks on and pipe leading to cabin gets a nice layer of frost,, WoooHaa! I'm thinking this is fixed.
Pull the jumper and replace with relay and she goes dead again.

What gives? And where did my friggin relay signal go?
cheers Dean

Post by **ZionXIX** » 02 Apr 2022, 11:36

Is your replacement relay a known working one?

deano1 · Post by **deano1** » 02 Apr 2022, 18:26

Just a little update.
So i was testing the car today and took it for a drive to get fluids and a/c moving.
Noticed a/c wasn't working but that centre ventilation was blowing ambient outside temp but passenger and driving window vents position seemed like flowing heat.
So probably some directional flaps are not moving when they're suppose to and when ECM doesn't get proper position signal then fails to send compressor "on" message??? Just guessing.
But I persisted to test and decided to jumper compressor again and noticed system pressure was bit low so I added can of system leak seal to top it off.
Left it jumpered for a bit then turned off the car removed jumper then swapped relay back in. Amazing enough the compessor continued to cycle!.
So maybe low system the whole time?

Post by **abscate** » 02 Apr 2022, 18:54

Do you have the left hand knob center dash showing blue or red?

deano1 · Post by **deano1** » 03 Apr 2022, 05:30

I realize there are situations where compressor will be commanded to operate like switching cabin temp to max blue or switching too defrost.
What I do when I'm testing is turn switch to blue area then look for compressor engagement if I don't hear or see it cycle on then I check codes and that when i see ECM code "ac signal missing"
But to get off topic a bit here, something tells me there may be a ghost or two lurking in this machine as some of the other codes I'm seeing are a little unsettling but to my amazement there is no CEL illuminated. For instance, apparently both rear wheel sensor signals have also gone AWOL. No CEL.
Maybe I forgot to plug them back in after rear sub frame drop? Yeah that's possible, But you think it would now turn on CEL after realizing signal is missing?
And yes, CEL works in pos 2 pre start phase of ignition.
So, otherwise this project car was saved from scrapper only to remind and torment future generations that 1999 S70 AWD is a dark force to be reckoned with. Yes,, PITA type dark force.
cheers Dean

Post by **abscate** » 03 Apr 2022, 05:44

I think missing sensors may only trigger an ABS code , not a CEL like later models? Make sure your ABS light works!

In 1999, the low pressure switch no longer is a series switch between the ECC control unit command signal and the compressor. It provides a logic signal between the ECU and the ECC control unit

deano1 · Post by **deano1** » 03 Apr 2022, 06:11

ABS Light is functioning as it was flashing when i was accessing codes and was present during key pos 2 check.