AC compressor not working 98 S70 continued......

pb49415 · Post by **pb49415** » 07 Jul 2018, 17:07

I appreciate all the assistance with my AC issues. I have an idea of a possible option to get it going. I’ve exhausted all troubleshooting. Last on the list is to replace ECM. Instead of going through the trouble to find a used replacement, I thinking to jump across the #3 and #5 terminals under the compressor relay and install a 12v temperature control switch. Please refer to link below.

https://www.amazon.com/gp/aw/d/B076M9JS ... Y5G6&psc=1

This is one of multiple switch options. I would run wiring into the interior and program it to cycle every few minutes. Is this a viable option? Or would it potentially cause extensive damage to the system. Compressor does work when relay is manually tripped.

dj_v70 · Post by **dj_v70** » 07 Jul 2018, 21:14

Your plan is possible, however, several failure modes could end up destroying your compressor if you have other issues.

Based on your other post, I believe your interior electronics work. You can just re-wire that voltage to control your ac relay instead. Just cut wires to pin 1 and 2 of ac relay, run input of low pressure switch (interior electronics) to pin 1 and pin 2 to ground (I think those are right pins, this is off top of my head).

However, that is still not optimal. You are removing low and high pressure switches from ac that protect the compressor from damage due to to some failure modes.

So, you could use your interior electronics and leave that connected to low pressure switch and re-wire low pressure switch output to control your ac relay instead. Then you would be covered so that low pressure (due to leak) won’t destroy your system.

Of course, high pressure failure mode (ie blockage) could still destroy your system. Your Volvo high pressure switch is not suitable for directly controlling a relay without ECM (computer). Older Volvo’s use high pressure switch that is suitable for controlling a relay. I don’t know if you could retrofit an older sensor. If so, you would have the most pertinent functionality without the complicated electronics. Or you can just risk destroying your system if high pressure failure mode occurs.

pb49415 · Post by **pb49415** » 07 Jul 2018, 21:42

So, the input to low pressure switch is the 12v output from interior control to pin 1 (bypass switch). Or the output from the switch itself? Or either or?

My buddy that’s previously sent me the other parts actually has a couple of ECM’s for the car. I was in disbelief. He’s got a treasure trove of parts for it. He’s going to send both out. That would be the most convenient way to start. Before going through the pains of rigging the system. I’m not concerned about damage. If it breaks anymore, it wouldn’t matter because it doesn’t work now anyways and won’t unless I maybe do something like that. I’ll make a note of the wiring you suggested. In case the ECM’s have no effect. I did try jumping across the low pressure sensor harness to see if that did anything. It did not.

dj_v70 · Post by **dj_v70** » 08 Jul 2018, 18:02

“So, the input to low pressure switch is the 12v output from interior control to pin 1 (bypass switch). Or the output from the switch itself? Or either or?”

The interior control goes to the input of low pressure switch. If you mod the ac, you want to use output of switch though so that you are keeping the low pressure system shutdown also.

“ I did try jumping across the low pressure sensor harness to see if that did anything. It did not.

”

That is the expected answer since you previously stated that you got 0ohms on your meter. That proved that your switch is not your particular issue.

My guess is that there is at least 50% chance that it is your wiring, not your ECM that is your issue. You want to check your wiring at ECM connector.

pb49415 · Post by **pb49415** » 12 Jul 2018, 02:37

Question, I’ve got a replacement ECM that I would like to swap out. I need to know if when I pull the one currently in place if it will kill the 12v to the ignition or whole car in general? I can’t disconnect the battery for a specific reason or unhook the 12v current. Please advise,
Thanks,
Paul

Post by **abscate** » 12 Jul 2018, 06:42

Pulling any electrical component with voltage applied is bad practice, and an ECU really bad practice.

If its the radio code you are worried about, there are workarounds for that.

pb49415 · Post by **pb49415** » 12 Jul 2018, 07:26

Nope a bigger fish that I don’t want to fry or mess with. I’ve got an interlock on my ignition. Any power loss is a MAJOR violation.

dj_v70 · Post by **dj_v70** » 12 Jul 2018, 15:38

Paul,

I noticed that some posts suggest that ECM is not exchangable, it is coded to your car. I don't know much about that, you might want to research it first.

Pulling the ECM will not disturb the electricity at the ignition. However, it is possible that leaving the battery connected while pulling the ECM could ruin the ECM. I have read that some people did it successfully. I don't know much about that either, you might want to research it first.

I will be troubleshooting my AC this weekend (same symptoms as yours). I'll post anything significant that I find in a new post.

pb49415 · Post by **pb49415** » 12 Jul 2018, 20:30

Sounds good. I wish you the best on your troubleshooting. Looking forward to hearing your results.

Post by **abscate** » 13 Jul 2018, 04:42

Let the gurus confirm, this but I believe the ECU pinned to car started with 99 models, not -1998. You do have To match software but not VIN on a 98, to be confirmed.

Specifically ,it’s Bosch ME7 software that makes ECU units non-swappable

Sorry using acronym ECU as swappable for ECM , too.