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I keep smoking up the AC "Amplifier"; Any Idea As to the Root Cause
I've got 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". (see pic)
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. (see pic)
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.
However, 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.
The only other thing to mention is that the big, multi-blade resistor 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?
A cause of your frustration may a dirty brush contact or corrosion/rust on the motor commutator. This can create additional resistance within the circuit while running due to a poor connection with the brushes. Or... You may have worn/damaged brushes. A peak inside wouldn't hurt. Here's a before & after picture of a windshield wiper motor I took apart years ago for reference.
wow, nice job on renewing that wiper motor: it looks new.
my challenge is that there is a switch, several potentiometers, resistors and four (4) motors (fresh air blower, evaporator blower, 2 condenser fans -- fans run, but they are still in the system/circuit, albeit pretty far electrically from the "Amplifier") somewhere in the climate control system, and I can't detangle it from the schematics, so was hoping someone has been here before and might be able to point me to which one of those is the highest probability candidate before I start attacking them one by one.
Check ground point G7 (black/purple). It supplies ground for most of the relay components in that schematic. The joint in the board appears to be a grounding lug, no?
02-08-2020, 06:10 PM
I keep smoking up the AC "Amplifier"; Any Idea As to the Root Cause
