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Electric fans causing voltage drop and stalling megasquirt

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    I just installed an E34 alternator in mine. You need a longer bolt on the bottom (about 5.5") which is a little annoying if you don't have it but otherwise it dropped right in. Note that I do not have the factory airbox, however, which I think might interfere with it normally. There is a post floating around here somewhere detailing it but really it was fairly painless.


      You could also use a PWM fan controller module from basically any newer car. I have one from the E46 that I intend to use with my N52. Send it a PWM signal from Megasquirt to ramp up the speed so you don't get a voltage spike, the high current itself is handled by the PWM module so they're easy to control.

      Personally I put the clutch fan back on the M20, even though I had the lower temp/lower speed switch from an M42. It probably costs 1-2hp, but it's simpler and probably lighter, and it doesn't make my idle drop when the fan comes on. I wasn't seeing voltage drops, but the fan does cause a torque load spike on the engine when it turns on. Modern DMEs account for this in their models, and automatically increase engine torque to compensate (the same reason the DME needs to know if the A/C is turned on), but I don't think Megasquirt can do that. Normally it wasn't an issue, until I was stuck in traffic for 3 hours on a hot day with the fan cycling ever 2 minutes..

      Also, if you're seeing voltage drops at the Megasquirt itself, your grounds are probably not good enough. There's a plated ground in the engine bay that you should use on the passenger strut tower (late model), and I put in way more grounds than the 1 or 2 that most guides recommend in the connector. I then ground it to the firewall inside the glovebox, as well as making sure my engine grounds are good. I also have a ground to one of the studs on my valve cover. You do NOT want ground offsets especially when running aftermarket wideband sensors.

      I really doubt you need a bigger alternator just to run an electric fan, unless you have a huge stereo system or something. The E30 electrical system is already sized to run the electric fan, and it probably wasn't as efficient as modern ones you can get now.

      BTW, my E30 only runs at 13.5v. This voltage is acceptable and has never been a problem. The alternator is stock and original, but I did replace the voltage regulator about 10 years ago.
      Last edited by nando; 07-12-2021, 09:15 AM.
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