Help me diagnose my AC trouble

[15Michaeljoseph]

If you jumper the coolant temp sensor you should hear the low speed and high speed relays click. If both click and only the high speed fan turns on, you need a new low speed fan relay. Auxiliary cooling fan that is.
If you can only get your heater fan to run on high, you need a heater fan motor resistor pack. See write UPS in the DIY section or in your Bentley Manual. Once you Eliminated these two, you can move on to your AC. Too many things wrong at the same time is not good

I believe I need a new aux fan low speed relay, because no matter how I jump the pins in the sensor, I can only get it to run on high. Also, there's nothing wrong with the blower motor (knock on wood) and it works on all four speeds. I also think the system might not be charged after all after checking it out with a friend who has a gaugeset. There aren't any other R134a parts on the system aside from the screw-on connectors at the high and low-side ports

I'm not too smart, but by looking at the wiring diagrams, I believe my issue to either be the Evaporator temperature sensor, or the evaporator temperature regulator.

Is it possible to jump the evap temp sensor momentarily to see if the compressor kicks on? Do I need refrigerant?


--Mike

[15Michaeljoseph]

Well, I figure after about 5 years I ought to update this, inspired by another forum member who PM'd me asking for advice.

Long story short: my e30 now has working A/C to keep me cool in the recent MA heat waves. The problem? Worn out HVAC sliders, user error, and a system that had never actually been converted to R134a. I was an absolute moron in 2014!

When I started trying to get the A/C in my car to work 5 years ago, I was a dumb kid who really didn't know much about what I was messing with. Feeling much more confident with automotive electrical systems after 5 years of wrenching and refined with other mechanical projects, I elected to revisit the e30's A/C. Growth is truly comforting, and the positive feedback loop of learning and honing in the practices related to a practical skill is something to reflect on.

...plus it gives me an excuse to further delay my impending rear subframe refresh.

Anyway:
My last post to this thread indicated that I had somehow figured out that my low speed aux fan resistor was bad, which I guess was correct, as I was able to jump it and get it to run after soldering in a new resistor. After this, I replaced the receiver drier, converting to the 'single switch' updated version, which I'd seen in the comprehensive R134a conversion A/C thread. I borrowed a vacuum pump and bought a manifold gauge set, which I used to pull a vacuum on the system. After confirming with a 12v+ jumper that the compressor clutch was good, I attempted to load some refrigerant into the system... unsuccessfully.

At this point, I gave up, content that whatever refrigerant or electrical issue was going on was too great for me to handle... Until July 2019.

The key to success is reading and understanding the A/C section of the e30 ETM, hosted at wedophones.com. If you're planning on doing any electronic troubleshooting on your car, you absolutely need to check it out, it's a life-changing resource and totally FREE.

Following the ETM using various jumper leads and my Fluke multimeter, I was able to confirm power to fuse 20, and continuity between fuse 20 and the HVAC controls it provides power to. Ohm testing confirmed that my evaporator temperature regulator was functional, and another continuity test verified my snowflake switch itself was good.
After a few hours, I had figured out two weaknesses: the ohm readings for the A/C pressure switches were incorrect, and more importantly, there was no continuity between the ETR and the 'power input' to the snowflake switch... Which is why it wasn't lighting up or triggering the aux fan to turn on or idle bump via the ECU.

Finally, I narrowed down the 'open' in the circuit to the HVAC slider buttons. As you may know, the A/C system is designed not to operate unless the panel and/or floor vents are open all the way to prevent the evaporator from icing up. I pulled the radio and looked down onto the slider panel. No matter where the sliders were on their wiper track, I still had no continuity. There is a female plug that slides into the ends of the wiper contacts on the rear left side, and mine seemed to be a bit melted. Cleaning and re-affixing didn't help, so I elected to delete the HVAC slider 'fail-safe' feature by jumpering the female plug with a short length of 10 gauge household wire that I tinned with solder on the ends. If you Google "e30 HVAC slider switch jumper" or something to that effect, there's another thread with pictures where the guy did exactly what I describe here.

Now, with car running, the button lit up when pressed, the aux fan kicked on to low, and the idle bumped up a bit to ~800rpm. Clearly, my jumper solved that piece of the puzzle. But what about the pressure switch reading issue?

I retraced my steps from 2014, when I had replaced the receiver drier and converted to the single pressure switch. To convert to the single switch, one must short out or jumper the original leads for the low pressure switch, and attach a new pigtail to the high pressure switch leads, in order to accept the new high/low pressure combination single switch. That's all well and good, and I actually did do that right... However the switch I had purchased was for R134a, which runs at a higher operating pressure than R12 and can't be used in my application. Simply, I reinstalled my old drier (which I had kept capped up in my parts stash) and reconnected the original dual switches to their provisions on the car harness. I muttered some prayers and got that vacuum pump out (good to have friends who are willing to let you borrow their specialty tools!).

With the system vacc'd down to 29 inches of Mercury, the pump was run for an hour, then the gauge valves were closed and left to sit overnight. No change in pressure in the morning meant no leaks in the system (presumably). At this point, it is time to fill the system with freon.

With the car idling and the A/C on, I connected the first can of vintage DuPont R12, bled the line, and opened the low side valve.

Nothing. Everything working, but compressor not engaging and the can not getting cold or emptying out.

A friend (Das Delphin, on r3v) had suggested jumping the compressor until it had built up enough downstream pressure to trigger the pressure switches... Turns out that's exactly what I needed to do!

After jumping the compressor clutch lead and letting it spin up for about 45 seconds with the can still attached and open, I reconnected the clutch wire to the car harness. It immediately clicked on by itself. From this point I was able to inject ~2.5 cans of R12 and a few ounces of A/C mineral oil into the system. Surprise, surprise, after an hour of idling and freon can swapping, I had coldish air at the vents.
Once the car was properly taken out for a drive with good airflow over the condenser, dash vent temps dropped further. This is exactly what I've been missing every summer since I've owned the car!

I've elected to keep the system original with R12 because it works just fine (knock on wood!) and I don't want to spend the big bucks on parts for a proper conversion at this point. Plus, with an EPA 609 certification, getting freon isn't an issue, aside from the higher initial cost... Hopefully the system won't need to be topped off for a couple years. Either way, I don't mind; it adds to the originality of the car am I right!? ;)



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