2000 S70 GLT Demented Electronics

Oysterman · Post by **Oysterman** » 13 Aug 2026, 16:40

New to the forum, thanks for any help. 2000 S70 GLT with just over 100k miles. Sometimes AC cold cuts out, control head still has power, fan blows strong. Reputable AC shop ensured system was properly charged with gas, relay was good, AC system generally given a clean bill of health. AC always initially comes on fine and blows cold. Realized that when the engine got warmed up, the cooling fan was cutting out---no call was being sent to the fan to come on when the engine was really needing it (although it always would come on on start up just fine). Car was running a little hot, coolant was boiling out of the reservoir. Thinking the AC problem might be related to the poor engine temp control, I bypassed the car's brain and wired the cooling fan so that it will come on high whenever car is running. Made sense, I live in the deep south. But AC is still cutting out intermittently. Sometimes the control head also goes dark and no fan at all. So the AC is failing in two ways. AC shop sent me to another shop that will work on old Volvos from time to time. That shop told me the correct messages weren't being sent out by the car's brain at the correct times, that it had dementia and I just needed to get a different car. Any ideas about what's going on here? Is there a way to get the computer checked over and renewed? The car looks good and has a good engine and otherwise seems to run really well. It's fun. But I need the AC.

aeg · Post by **aeg** » 14 Aug 2026, 01:33

You need to fix the engine cooling system properly first, because the ECU will disengage the AC compressor clutch if the engine is getting too hot. Is the check engine light on? Is the check engine light burnt out?

Post by **abscate** » 14 Aug 2026, 06:32

The classic AC works/stops failure is the clutch gap being worn out over time - you are just about due at 100k miles
the gap gets too big, and when the compressor gets warm from working it heats, and the clutch drops out.

Search 'bread clip fix' and 'zip tie fix' in this forum for the fast cure - the Volvo cure would be to adjust the shims in the clutch to close the gap back to specs (0.1-0.3mm I think?)

YOu do want your AC and fan to be working correctly for both engine and system performance. The fan should go on when the AC is engaged, but not on engine start up alone.

If the ECC control unit is going dark , then you need to find a junkyard replacement, from another 1999 (special year)

Should be cheap on ebay , ping me here if you have trouble finding one , I do have one off a 1999 in my stash

Your ac control unit has numbers and the word AUT on it , correct?

Oysterman · Post by **Oysterman** » 15 Aug 2026, 06:14

aeg wrote: 14 Aug 2026, 01:33 You need to fix the engine cooling system properly first, because the ECU will disengage the AC compressor clutch if the engine is getting too hot. Is the check engine light on? Is the check engine light burnt out?

It was getting hot, but not hot enough for the check engine light to come on. Now that I wired it so that the fan runs on high speed whenever the car is running, the engine temp problem seems to be solved. No more coolant boiling out of the reservoir thankfully.

Oysterman · Post by **Oysterman** » 15 Aug 2026, 06:23

Thanks to both of you. Abscate, two more things to mention. I did have the AC shop I'd taken it to adjust the gap, so that should be okay. And the previous owner (my brother, who wasn't the first owner) told me that he also experienced intermittent AC issues and had swapped out the ECC control unit for one he got on ebay. Now it seems like the AC fails in two ways. Sometimes ECC goes dark and complete shutdown, ie, no blower, no lights on the ECC. And sometimes lights on the ECC and blowing fine, just no cold. But then it might just cut back in fine. Since the ECC has been swapped once before, it makes me wonder if the problem is upstream of it? Are there reputable services I can send it off to for testing and repair?