Engine cooling fan gremlin - solved
05-10-2011, 11:42 PM
#16
Chronic Tool Dropper
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05-11-2011, 12:34 AM
#17
Burning Brakes
Join Date: Oct 2008
Location: Palo Alto, CA
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I am also considering the membership thing, first I want to make sure I survive the 928 experience, it was in doubt for a bit but things are sorting out now and I may just be around here a while if things keep improving. Must confess, I was ready to resell early in the experience.
Thanks,
Diehard
Thanks,
Diehard
05-11-2011, 02:17 PM
#18
Instructor
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Join Date: Apr 2008
Location: Boisbriand, Québec, Canada
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Unfortunatly I was at the end of my patience when I re-soldered and did not take any pictures. 
Here's the link to my previous post about the output stage transistor replacement. There are some pictures of my work. What I did was heat/melt/cool all circuit board solders (back of what you see on the pictures) with a pen like soldering iron. No need to remove the varnish nor get rid of it - just melt and cool.
https://rennlist.com/forums/928-foru...ive-fixed.html
Hope it helps...
Michel
89 928 S4
Here's the link to my previous post about the output stage transistor replacement. There are some pictures of my work. What I did was heat/melt/cool all circuit board solders (back of what you see on the pictures) with a pen like soldering iron. No need to remove the varnish nor get rid of it - just melt and cool.
https://rennlist.com/forums/928-foru...ive-fixed.html
Hope it helps...
Michel
89 928 S4
08-02-2012, 11:52 AM
#19
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Follow-up...
Got back into a one-fan operation last week
I did all the diagnosis process again and it led back to a faulty output stage again.
This time I took it appart, removed the plastic plug from the circuit board, baked the circuit board in the oven at 385F for 8 min for reflowing (don't breath the fumes and vent the kitchen!) followed by a half hour cool down, reconnected the plug and the 2 fans are now back to life!
Michel 928 S4
Got back into a one-fan operation last week
I did all the diagnosis process again and it led back to a faulty output stage again.
This time I took it appart, removed the plastic plug from the circuit board, baked the circuit board in the oven at 385F for 8 min for reflowing (don't breath the fumes and vent the kitchen!) followed by a half hour cool down, reconnected the plug and the 2 fans are now back to life!
Michel 928 S4
09-25-2012, 12:57 AM
#20
From the other thread (https://rennlist.com/forums/928-foru...ssion-4.html):
"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 about 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!
"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 about 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!
11-06-2018, 08:46 AM
#21
From the other thread (https://rennlist.com/forums/928-foru...ssion-4.html):
"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 about 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!
"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 about 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!
Is there any news on this topic?
- Did the new transistors work for a longer time properly or did they fail?
- is there a better solution to repair? Or different components to use for a proper repair?
11-06-2018, 12:35 PM
#22
I've the same problem now and don't want to spent hundreds of $ for an exchange parts...
Is there any news on this topic?
Is there any news on this topic?
- Did the new transistors work for a longer time properly or did they fail?
- is there a better solution to repair? Or different components to use for a proper repair?
These 928 fan modules are very reliable and rarely if ever fail, i.e. many mis-diagnose the module as being a problem.