mirousse

Just want to share this:

Driver side cooling fan had been inoperative for a few weeks now. Engine was not cooling well in traffic. I followed the diagnosic steps of the service manual and it led to a faulty fan output module. I cracked it opened and replaced all 4 transistors with NTE2389 transistors along with new heatsink compound. Everything is just fine and working.

This was a 24$ fix!

I'm unable to upload the pictures...I'll try later.


Michel
89 S4 ROW

oups59

Good Job Michel - Thanks for sharing

fraggle

Where did you get the transistors? I think I need to do this fix.

mirousse

I got them at a local electronic part store. The type of store that sells TV, radio, computer parts. This is a common type of transistor, not specific to the928 but generic for many applications.

I took pictures of the transistor swap but was unable to upload them yesterday. I'll try again later.

Michel.

dprantl

http://www.newark.com/nte-electronic...11?Ntt=NTE2389

Dan
'91 928GT S/C 475hp/460lb.ft

Earl Gillstrom

I had the exact same problem on my '88. One fan not running. There was a cold solder joint on the output transistor. Re-soldered and all is good. You can fix a lot of control units with a re-solder.

mirousse

Here are the pictures...

fraggle

@#%$@#^% I goofed and only ordered two transistors for some reason. Which side is passenger and which is driver?

Lorenfb

"I cracked it opened and replaced all 4 transistors with NTE2389 transistors along with new heatsink compound."

This solution (using generic N-Channel MOSFETs) has the same problem as those mentioned
on other threads, i.e. The Vgs signal of the original design does NOT provide adequate drive
to develop the needed motor current without a special selected N-Channel MOSFET. This is
because the marginal original design only provided about 12 volts to the MOSFET gate.
The proper circuit design requires a charge-pump to develop an adequate Vgs when turning-on
a MOSFET to achieve a low Rds, i.e. About 4.0 to 5.0 volts above the battery voltage (a gate
voltage of 16 - 17 volts). Without an adequate Vgs signal to achieve a low Rds, the MOSFET
with operate at a destructive high temperature.

A simpler original design should have had the motors connected to 12 volts and had the
controller switch the ground. That way the MOSFET could have easily been provided
an adequate Vgs to achieve very low Rds, and thus reducing the MOSFET power dissipation,
reducing the likelihood of failure.

Bottom line: The original controller design was prone to fail, but now more so with improper
replacement parts. If the fan controller has not yet failed with a generic part, you've been lucky!

dprantl

So what's the solution? Do you know where to source these special selected N-Channel MOSFETs?

Dan
'91 928GT S/C 475hp/460lb.ft

928porschemangreg

The most important thing is that heatsink sticking to the mylar, and the mylar to the case. It has to dissipate heat all the way to the aluminum case.
Whatever the factory heatsink compound was, it dried up and the bond between these parts loosened up, causing the Mosfets to retain too much heat in themselves.