Heater Control
Discussion
Hi guys,
Last major thing to sort out. The heater blower did run but then packed up a short time later. Upon removal of the fan we found it was seized, put 12V across it and lubricating the bearings it came back to life. Great. Fitted it into the car only to find that fan speed pot has zero effect, nothing. Removed the control box (the one with the ribbon cable and various other things attached then opened it up. Looks clean enough? There is a MOSFET in there which looks like what controls the speed?
I'm loathed to shell out for a new box so was wondering if anyone had a spare to test?
Also am I right in thinking that the lower dash panel hasn't got another control box of some sort?
Shorting the pins means the fan runs flat out, therefore the MOSFET or other isn't doing what it's supposed to do? It was running then seized, could this blow the MOSFET?
We'all solder all the joints to make sure it's not a dry joint issue, then replace the component. Any help in the meantime appreciated!
Last major thing to sort out. The heater blower did run but then packed up a short time later. Upon removal of the fan we found it was seized, put 12V across it and lubricating the bearings it came back to life. Great. Fitted it into the car only to find that fan speed pot has zero effect, nothing. Removed the control box (the one with the ribbon cable and various other things attached then opened it up. Looks clean enough? There is a MOSFET in there which looks like what controls the speed?
I'm loathed to shell out for a new box so was wondering if anyone had a spare to test?
Also am I right in thinking that the lower dash panel hasn't got another control box of some sort?
Shorting the pins means the fan runs flat out, therefore the MOSFET or other isn't doing what it's supposed to do? It was running then seized, could this blow the MOSFET?
We'all solder all the joints to make sure it's not a dry joint issue, then replace the component. Any help in the meantime appreciated!
The lower dash doesn’t have any control logic. It has a simple pcb to connect the various control potentiometers and also to connect resistors to drop voltage for the LEDs in the panel. If it’s any help - when the above pcb is disconnected the heater fan runs at full speed. If the fan pot has failed and gone open circuit then that could be a possible cause of the fan running at max.
Considering how much of a pain access is, I replaced my fan unit even though I freed it up with lube. £100 new from Germany off ebay. Done on the fan motor part number. Just the blower motor and fan, retaining the original housing.
Hope it's the mosfet, nice cheap repair :-)
BTW. The guy who originally made the heater control circuits still repairs them.
Hope it's the mosfet, nice cheap repair :-)
BTW. The guy who originally made the heater control circuits still repairs them.
Yonex, your on the right lines
As the fans get older they draw more current, the original MOSFET was only designed to sinc a max of around 10a depending on which one was used.
In the ones I repair for everyone, I use a different to original MOSFET, just need to watch the output from the LM324 which creates a crude PWM for running the MOSFET as they can also get killed in the process and its worth putting in a pair of transistors to amplify the signal and protect the chip. Its also worth putting the unit across a scope to show what the waveform is doing as I've seen the waveform drag with old components due to age, with can also create extra heat with the MOSFET never truly turning off and thus causes a big runaway problem over time.
Jody
As the fans get older they draw more current, the original MOSFET was only designed to sinc a max of around 10a depending on which one was used.
In the ones I repair for everyone, I use a different to original MOSFET, just need to watch the output from the LM324 which creates a crude PWM for running the MOSFET as they can also get killed in the process and its worth putting in a pair of transistors to amplify the signal and protect the chip. Its also worth putting the unit across a scope to show what the waveform is doing as I've seen the waveform drag with old components due to age, with can also create extra heat with the MOSFET never truly turning off and thus causes a big runaway problem over time.
Jody
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