Controlling rad fan w/ PDM15 and M400

[eric_s]

Hi,

Currently we have a radiator fan powered from one 20A output of PDM15 but switched low by M400 (Figure 1). Our previous electrical staff did the wiring and personally, I do not feel comfortable feeding high current (~7A) to M400 -- we suspect this setup caused the PDM to reset while the car was running at high revs.

So we are looking at developing a solution similar to Motec DHB, but made in-house due to budget limit. The idea (shown in Figure 2) is to have a microcontroller continuously polling the CAN bus for specific messages, ET for instance, and output 5kHz (or 10kHz) PWM with varying duty cycle depending on the parameters (ET, WT, OP, RPM etc). As I understand it the output of PDM is limited to 10Hz, and seeing it'll be switching at high frequency, I was wondering whether PDM will get damaged from providing power to mosfet which will be driving the radiator fan?

Cheers.
Eric S.

[Holmz]

I thought most people just switched them off and on with hysteresis.
Is there an advantage in using a PWM and having the speed variable?

One could also power the fan on and off with some combination of speed, brake and throttle position.

[eric_s]

Hi Holmz,

The main reason to use PWM is to soft-start the radiator fan. We previously had the fan wired and configured in PDM and M400 to simply turn on and off at certain engine temp, but as soon as the fan turned on the car died (and start again by itself with rolling motion) due to in-rush current. We are considering acquiring a new set of battery, we're using Shorai LFX19A4-BS12 battery at the moment.

Cheers.
Eric S.

[Holmz]

^In that case then^
Having an elegant fan solution is masking the real issue some some flaw in the design of the wiring/implementation of the fan system.

Some combination of a large capacitor, alternator, battery or wiring would be an easier solution.

If I was wanting to understand it, then I would start by measuring the current draw over the short time frame when the fan starts up, and voltage at the battery at that time.

But if I was lazy, then I would just put a 1/x-Ohm resister in line with the fan to limit the current and that would probably solve the issue?

[pftuning]

Some of the BMW and Nissan fans have built in PWM controllers. You could feed hot side with PDM then control speed with MX00 pwm output at signal level.
I'm controlling BMW E46 fan that way (although with another brand of ECU). Had to fiddle on the bench to figure out PWM frequency but my ecu doesn't have to sink a large amount of current, which I'm happier with.

We operate a Nissan VQ37HR fan wide at full speed, but it does seem to ramp up speed when powered (we just grounded PWM input).

[Herrubermensch]

This actually raises a question the answer to which would be beneficial for everyone who reviews this board. When controlling an engine fan with a PWM output from an ECU or dash and a PDM, it would seem to be just as simple as Peter (Pftuning) has described it, viz., run a 20A output from the PDM to the high side of the fan, then connect the low side (ground) to the PWM output of the of the ECU or dash. That way, the low side is switched by the ECU or dash, the switching frequency is controlled by the ECU or dash rather than the PDM (which is slow for these purposes), and the high current connection runs directly from PDM to fan. So, whence the necessity of a DHB in this design? DHBs are intended to be used in precisely this circumstance, but the question is why they are necessary? Holmz hints at a possible reason, viz., that the low side is still too high, as it were, for the dash or ECU to handle safely, so the DHB provides a kind of current buffer. Obviously, the DHB also allows one the wiring flexibility of switching the high side of the fan. Any thoughts on this point I think would benefit this thread.

--Peter

[SprinterTRD]

The smple answer is: a thermofan will exceede the ECU/ADL's maximum current rating. Also none of the AUX outputs can sink anywhere near 15A.

That is why a DHB must be used with a ECU/ADL as it is designed to switch high currents.

[pftuning]

The smple answer is: a thermofan will exceede the ECU/ADL's maximum current rating. Also none of the AUX outputs can sink anywhere near 15A.

That is why a DHB must be used with a ECU/ADL as it is designed to switch high currents.

^This.
I prefer to minimize the amount of current an ECU has to sink.


Which is why I like the convenience of fans with built in PWM controllers. I'm told later model BMW's (E92?) use some lightweight fans with built-in controllers.

[Herrubermensch]

Thanks. Very helpful.

--Peter

[TKManx]

ok I am confused by why controlling the Rad Fan with the PDM from an input channel from the M400 I an issue ?

If a Rad Fan needs say a 20 Amp circuit the PDM will have one and once the fan is going then its draw will be well below 20 Amps and the signal from the ECU doesn't need to be anything special and can even get it via the can bus based on the ECU inputs such as temp etc.

Looking at a lot of Rad Fans you could even run them on an 8 Amp circuit as the draw after start up is under 8 Amps.

Can someone please explain why this setup is a bad idea and a PWM is required ? I see all this mention about ECU current etc and just cant get my head around what the ECU has to do with the current required to run a Rad Fan hooked to a PDM, it is isolated from the Rad Fan Current draw from what I can see.