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I keep smoking Denso AC Amplifier, any ideas as to cause?

 
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austinporsche  



Joined: 26 Apr 2018
Posts: 43
Location: Austin, TX

PostPosted: Thu Sep 12, 2019 9:01 am    Post subject: I keep smoking Denso AC Amplifier, any ideas as to cause? Reply with quote

Hi.

I have an 1988 924S (red, manual, 87K miles) that I bought a couple of years ago but which I am only now becoming familiar with.

Live in Texas so AC is important.

My problem/issue seems to be with what is called the "Amplifier" on the AC schematic (lower left hand corner of schematic, "AMPLIFIER" in all caps). Among other things, it's where the current for the AC compressor clutch coil comes from (black/white wire).

I've fried two of these amplifiers so far and before I fry a third, I would like to find the root cause.

So here's the timeline/steps so far.

1. No AC
2. Checked system with gauges, no refrigerant
3. Pumped in positive pressure air, located hiss/leak (o-ring at compressor)
4. Replaced o-ring, pumped system down, held vacuum
5. Compressor clutch wouldn't engage even with 12v directly on it; ohm'ed it, bad coil
6. Replaced compressor from "donor" (have 4 944 carcasses, 3 being pre-1986); pumped down again, confirmed it still held vacuum
7. Confirmed compressor clutch engaged, filled system with R12a
8. System worked, produced cold air for about a week then just stopped
9. On advice from this forum, removed AC amplifier (under glovebox, fixed to firewall, same general area as the relays) and popped the top; board was fried at a solder joint to a transformer winding on the board)
10. Replaced the amplifier with one from one of the donors (still have 2 good amplifiers left)
11. AC again started working, stopped after 5 min
12.Same solder failure on the AC amplifier board. But this time much, much hotter, more extensive burn on the board.

Before I smoke a 3rd AC amplifier, I thought I would ask whether anyone has seen this problem before, and if so, what might be the source of what appears to be a lot of current being drawn -- enough to vaporize a solder connection and in the 2nd attempt brown a burn a sizable portion of the board -- and how I might fix it.

Anyone with experience with this would be greatly appreciated for advice.
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