924Board.org

[austinporsche]

I've have a 1988 red/black 924S with about 87K miles: my favorite Porsche to drive (I also have a longhood 911, 944(s), 928 and a 986).

I'm in Texas so AC is kinda important.

The system functions save for the fact the it keeps "frying" what in the climate control system schematic in the 924/944 Haynes manual is referred as the "AMPLIFIER".

It's a Nippon Denso part number 07 7300 - 0320 (also marked as 477 959 531).

I've smoked 3 of these so far, all at the same wire route on the PCB.

The AC Amplifier doesn't fail/fry immediately, but usually after 1-6 months, the AC will stop working, and popping the top on the amplifier box show the same burned out spot on the PCB inside.

I find it unusual and perplexing that it fails over time and not the first time that current is applied to it by turnign the AC on.

Here's a link to a post on Rennlist I made a year ago with both the wiring diagram and pics of a good and a toasted AC Amplifier (I fried the good amplifier in the picture about six months ago).

https://rennlist.com/forums/924-931-944-951-968-forum/1181230-i-keep-smoking-up-the-ac-amplifier-any-idea-as-to-the-root-cause.html

I know some other people have "fixed" this problem by putting a big dollop of solder on the PCB route where it fails, and I myself am thinking about putting a heavy gauge wire jumper between the two pins on the PCB in order to bypass the bad conduction route on the PCB.

But I would first like to see if I can figure out what the root cause is.

My smartest EE friend (I'm an EE as well but more an RF propagation guy than a circuits guy) tells me it has all the makings of a current short that is allowing a bigger than designed for current draw thru the transformer in the AC Amplifier, probably the result of a failed winding in a motor or maybe the compressor clutch.

Basically, V=I*R. If resistance decreases, then the current (I) increases to maintain the same potential (voltage).

Both the fresh air blower and AC blower seem to work fine with the AC amplifier removed, implying me me at least, that neither motor is in the AC amplifier circuit.

So maybe it's the compressor clutch winding that has a short in the winding, which would reduce the overall resistance of the winding. Does anyone know what the "typical" resistance for the clutch should be?

The only other thing to mention about my particular car is that the big, multi-blade resistor that sits on top of the fresh blower is starting to break down/disintegrate and has apparently gotten so hot that it has melted the cowling covering that blower/air intake in the engine compartment behind the firewall. (I can't seem to find anywhere to buy a replacement resistor -- new or used).

Has anyone else had this problem and tracked it down to the root cause? Motor? Resistor? Switch? something else?

Any thoughts/help would be most appreciated.


[img][/img]

[KDJones2000]

My guess is that your A/C electromagnetic clutch is drawing too much current, causing this to fail.

Try a new A/C compressor/clutch and see if that fixes it.
_________________
1987 924S SPEC car
Team Toofah Racing
1988 944 Turbo S

[austinporsche]

Agree. Using V=I*R, if resistance is down, current is up if V is kept constant.

And if the windings are shorting, that creates a shorter path -- and less resistance.

It's easy to measure the resistance of the clutch winding at the black wire on the compressor side of the refrigerant pressure switch just behind the left headlight.

Does anyone know what a nominal resistance range should be for the clutch winding as measured at that wire (to ground)?