NC Blower MOSFET replacement

In the end I went for a flexible drill extension. With an 8mm nut driver I was able to push/hold this on with one hard and power an electric screwdriver with the other.

I’m please to report that I’ve changed the MOSFET and that was indeed the cause on my not running fan. Now all I need to do is get an A/C re-gas.

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As a slight aside but relevant part of the ‘fan not working’ story I will update this thread with the continuing result. With of word of caution.

Recap:- fan not working, as the dashboard was out for loom change, original MOSFET controller repaired as the fan was not working.

On reassembly initially working and tested fan stopped working again very soon after.

Cable detached from MOSFET controller and repaired unit left in foot well.

This unit also failed not long afterwards.

For me clearly the MOSFET replacements were not the total solution, the first failure was no random incident and the two replacements were not due to faulty repair or faulty components or for lack of air flow to cool it in the open air.

I had tested the motor historically and had it running at full speed direct from a battery, I was initially concerned about the wires feeding the test heating up but could not measure the current as it was greater than my meter could measure at over 10 Amps, I also measured the armature resistance of the motor but found no immediate cause for concern. The motor had been completely dissembled and cleaned, lubricated and refitted not obvious signs or distress.

MOSFET replaced and blew again under observed conditions in the foot well.

With the repeated failure of the MOSFET, I repeated all the testing and evaluated the results.

I was left with three possible fault conditions, the heater controls were feeding erroneous signals to the MOSFET controller and was causing the failure, the motor was defective and causing high current to destroy the MOSFET or three the MOSFET’s were not fit for for the job and failing.

I was looking at higher rated MOSFET’s and reviewing the replacement of all three components as a solution.

The testing I under took replaced the motor with a load resistor of 10 ohms and measured the voltage output of the heater controls to the MOSFET and this appeared as expected, it varied as you increased the fan speed and the auto was full on as you would expect.

This proved the thrice repaired MOSFET controller and wiring was sound, what then was causing the issue.

The motor was inspected more closely and found that one of the armature segments was short circuit causing short circuit, high current, to flow once every revolution. Missed on the initial inspections as the very low resistance (2-3 ohms) of a heathy segment and the normal irregular readings of a brush armature motor masked the shorted segment. Add to that that the motor actual worked when connected to an unlimited current supply.

Moral of the story for me is take more notice of your instincts and double check when is appears right but smells wrong.

A replacement motor and the system appears to be working fine with the MOSFET controller in the foot well not over heating but getting very warm.

Next job… test some of these theories that the MOSFET controller can be changed without removing the gas from the A/C and the dash from the car.

This was an unusual fault that consumed a lot of time fortunately not a lot of cash as I have the capability to repair the MOSFET myself.

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…and the damming evidence to look for…

Even though i have not been on site for the last four months, i feel i need to respond to (eaa53 John) regarding his P.M. on how i got on with my MOSFET problem.

Blower and radio not working.
I ‘removed’ the inner fuse box, replaced every fuse and checked every terminal, disconnected every multiplug and again tested/cleaned every pin and socket.
I removed the fuse box in the engine bay, replaced every fuse and again cleaned every connection, removed and bench tested the blower and starter relay, then i removed and bench tested all eight or nine relays in the engine bay under the fuse box.
I removed the two earthing blocks to the nearside inner wing and the one to the offside.
I also found an earth block high up above the fuse box in the footwell.
Now i am getting pretty desperate and it’s time to really get stuck in.
Out comes the seat carpet and foot rest, out comes the glove box.
One bolt about 4" long behind the front of the door opening, two further bolts to the left of the glove box and one more in the footwell.
This now allows you to pull the dash back towards yourself and gives you 1/2" more space, with the dexterity of a dwarf monkey working upside down in the footwell i managed to get the nearest self tapper out and unplug the MOSFET, i must have spent two hours trying to remove the second self tapper without success, this is when i thought if i get this screw out how the hell am i going to get it back in.
So without going any further and MOSFET still in situ i ordered a replacement known working MOSFET for £26 +£6pp (Thankyou MX5HEAVEN) which was delivered within 24 hours.
Back to the car, MOSFET plugged into the loom and everything working as it should on all speeds with it dangling in the foot well.
This is were i decide not to fit it in the blower housing in case it blows and needs to come out again, so i fabricate an angled ‘metal’ bracket and bolt it through the second bolt up by the side of the glove box, recheck the blower and disaster the blower is running at full speed and will not turn down, it’s then i realise what i’ve done by securing it with a metal bracket, i’ve created a dead short.
Out with the MOSFET and fabricate a new bracket out of plastic, now everything working as it should and that’s where it is staying for the time being.

Next job radio, and thanks to Barrie (bettabuilda) i can confirm an NC2 radio will fit an NC1.

Next job brakes, front and rear calipers stripped and rebuilt along with new rear pads.

Next job suspect heat shields rattling at certain revs, off with the back box and check all the heat shields from the exhaust manifold to above the back box, everything seems to be secure so refit the exhaust with new hangers and back the car into the garage, damn thing is still rattling and i am just about getting hissed off when my youngest comes out and i ask him if he could hear that sodding rattle, “yes Dad it’s the up and over garage door vibrating”…begger (spell checker).

So apart from a few other jobs carried out that’s where i an now and the car has not seen a road for the last five months.

MOT booked for tomorrow, please wish me luck as i’ve had enough of it.

Dave.

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Morning Dave! The trials and tribulations of mx5 ownership! Good luck with MOT. I love the launch edition and I’m sure you’ll get it how you want it :+1: Glad the head unit was of use too.
Barrie