dr bob

merchauser

thanks......assuming this paste helps wick away heat. can a lack of thermal paste cause an intermittent issue like mine?

dr bob

No.

There's no self-protection built into the actual mosfet drivers. The paste is certainly needed, and the amount is usually a lot less than what most folks think. You need just enough to fill the surface pores in the metal bits. The mica isolator means two very thin film applications are needed, plus the contact between each block and the case becomes critical.

merchauser

UPDATE, PSA, FWIW: in addition to the absence of thermal paste there was a good bit of oxidation on
one side of the board. a thorough cleaning, replacement of the 4 MOSFETs, and new thermal paste seems
to have solved the problem. in addition, the car is running a noticeable few degrees cooler (yay!) I think that might be
because the old bits were not allowing the fans at optimum speed?

many thanks, again, to the group!

merchauser

Very happy with results as I mentioned above; car runs cooler. Relatively easy and inexpensive

Wondering if I should get into the cooling fan control unit.

would like to know if there are any relays in that box?

Captain_Slow

Good job on troubleshooting the amp issues. Sounds like the seal may have been compromised. I've opened a used one I picked up cheap and found the paste and everything else was in good shape. No oxidation.

merchauser

BUMP: OP amplifier issue resolved; on to possible bad AC high pressure switch: new questions:

in post #4, Rob Edwards posted an excellent reference for cooling fan operation that is worth reposting:

https://webfiles.uci.edu/redwards/pu...%204-19-18.pdf

that, along with Jake's (ducman) explanation helped me understand how these fans work. but here is where I get a little muddy.

the manual states that when the AC pressure switch is above 10 bar (145 psi) , the fans will get 12v, and thus fans will be on high or FULL fans.
to me, that means with the AC on, fans will ALWAYS be on high? according to WSM, the only time pressure might be at <145, would be with an ambient
at <60'. not applicable to my climate.

with a recent recharge, my high pressures were too high, and I think this might have been due to my fans not operating at full/high speed. as a test, I
removed one fan fuse at a time, and the remaining fan would go to high, as it should in case one fan is down. thinking I have a bad AC high pressure switch?

dr bob

The refrigerant pressure is sensed by a transducer, not a switch. The amount of current allowed to flow through the transducer is proportional to pressure. Unfortunately there is no published pressure -vs- current chart published.

In my limited experience (15 yrs black S4 DD in Los Angeles) the high-speed mode happened only while sitting at idle in stop-and-stop traffic. At that same time, the car is under-producing power for the fans and the system voltage is dropping towards 12 from 14 or so. While the fan controller is calling for full speed, the dropping voltage prevents that from actually happening.

Know that the fan controller and the "final stages" module use pulsed DC to control the fan speed. The FET drivers switch on and off rapidly, with the amount of "on" time adjusted to get the desired fan speed. The controller pays no direct attention to voltage except for the protection mode, and even that's a selective process that monitors fan current rather than input voltage. The system is way (too much?) smarter than a progressive pressure switch, thanks to the multiple inputs used for control.

merchauser

thanks dr bob
if you can, go to page 9-07, of the cooling fans manual

https://webfiles.uci.edu/redwards/pu...%204-19-18.pdf

you will see that full 12v is assigned to fans with 15 bar AC pressure. so wouldn't that mean fans are nearly always at high with AC on?