PWM Fan Controller Turns Off Alternator Current ???

I recently upgraded my Ford E350 with dual electric fans (Dorman 620131) that together at high speed draw 37.5 amps at 12.6V and flow 4,450 cfm for cooling. I wanted to use a variable speed controller monitoring the coolant outflow temperature from the radiator to minimize noise and power requirements from the alternator. I chose a PWM controller rated at 150 amps which requires battery voltage applied directly to the fans and the controller switches the ground leads to control rpm. I also have a Powermaster 200 amp output rating 3-wire Ford 3G alternator supplying the 12V battery system. The alternator exciter is wired to the ignition switch through a 500 ohm resistor. The PWM controller has two modes: a normal mode in which fan speed is PWM controlled and a fail-safe mode in which full battery power is directly applied to the fans to give full high speed.

To my surprise and confusion, whenever the controller is operating in the PWM mode, the output from the alternator is shut off (measured 0.0 amps - alternator output is directly connected to battery positive with 2 AWG cable). In this mode, the fans are powered by the battery and the voltage steady drops with the average 20 amp load to the level of 10V resulting in engine overheating and ignition shutdown. If, however, while the controller is in the PWM mode, I switch modes to the fail-safe mode (terminating the PWM operation) the battery voltage immediately recovers to 13.8V and the alternator output current jumps to 50 to 70 amps depending on the state of the battery discharge and the current requirements of the ignition system. Turning off the fail-safe mode has the alternator current drop to zero in less that 10 seconds. Switching back to fail-safe mode re-establishes the full alternator current in the same less than 10 seconds.

When the engine is initially started (i.e. cold), the alternator output current varies from 10 to 20 amps before dropping to a steady 10 amps with an alternator/battery voltage of 14.5V and a steady exciter current of 6.66 ma. When the fans turn on with the PWM mode, the exciter current jumps to 27.22 ma. and the alternator output goes to zero as the system voltage drops from 12.6V to 11.1, then 10.8 and then 10.1 (continuously changing) in about 30 minutes. All the time in the PWM mode, the exciter current is 27.22 ma. +/- 0.06 ma. Switching to the fail-safe mode, the exciter current drops to a steady 6.66 ma. regardless of output current which immediately returns to the 50 to 70 amps cited above. This situation can be repeated for hours on end switching back and forth between modes. PWM on - alternator seemingly disconnected, PWM off - alternator functioning perfectly!

PWM controller manufacturer says it must be EMI interference but offers no corrective approach except filter capacitors which so far have been ineffective. Alternator manufacturer says it can't happen, a refuses to reply to email!

My PhD in Chemistry and 40 years in Aerospace and Astrobiology have done nothing to help me either understand this PWM interference with the alternator control system or come up with any plausible solutions other than giving up on the PWM controller. Searching the internet, I can find no mention of a similar or identical problem. I would desperately appreciate any explanation and suggested remedies that anyone on the forum can provide!

It sounds like there is something messed up with the alternator regulator, or the rectifiers that feed that.

What you call the "exiter" is normally the ignition/alternator warning lamp.
The external feed should only be needed until the alternator is producing enough voltage to supply it's own excitation current.

It provides an external feeds to the output of the auxiliary rectifier which powers the main regulator unit in the alternator.
Once the alternator is running, there should be no external current and the regulator / field are then powered directly from their own rectifiers.

Example: https://www.ratwell.com/technical/ChargingSystem/AlternatorSchematic.jpg

This one gets me and I am like you just change the chemistry for physics. Have you tried a diode to the PWM switch? I wouldn't think there would be two different ampere signs but it certainly acts 0 or 1.

I'm about to remeasure the system, but the current in the idiot light circuit (I am using a Led with a parallel 500 ohm resistor as suggested by the manufacturer) is steady in both configurations (PWM on and off). When I first installed the alternator, I used the 12V LED only and the alternator wouldn't turn on. Then I used just the 500 ohm resistor in the exciter (their name also) wire line and the alternator ran perfectly (this using the mechanical fan system before I added the electrical fans). I then added the LED light - the alternator continued to function but remained dim when engine running.

I'm aware that the alternator needs the 12V input to provide the starting magnetic field and that the regulator controls the field wire current to vary the output current/voltage. I have seen discussions of a pilot field current and a main field current and thought these might describe the "exciter" and field currents but this is a (chemical culture) hypothesis! If it's the voltage regulator problem, then how is the regulator (which is internal and presumably shielded by the metal bulk of the alternator) affected by the PWM signal or generated EFI??? I also see that the regulator has a shutdown mode and a battery sense system, but I can't find any info about how the disconnection occurs. If it's just generated by ignition off, this can't explain at the observed zero current in PWM mode since the exciter circuit is still drawing current though at 6 times the proper operating mode level.If the disconnect is triggered at low to zero alternator rpm, this also doesn't describe my problem. Utterly confused!

BTW, I'm measuring currents to the fans and from the alternator to the battery with a DC clamp-on meter. The currents in the exciter line are measured with a regular ammeter. Also, the fan motors are brushed, not brushless.

Just a few clarifying measurements taken today to hopefully add to someone's understanding:

1) Resistance from exciter wire (disconnected from alt. light) to ground is 1.48 K ohms.
2) Current drawn with exciter wire connected to ignition via alternator light and parallel resistor, with ignition on and the engine not running is 27.88 ma.
3) First engine start, Batt. is at 14.4V, exciter current is 6.64 ma., and alternator current to battery is 32.6A DC and 3.2A AC as measured by clamp-on meter.
4) Disconnect exciter wire with engine running, alternator current to battery goes to zero both DC and AC.
5) Engine running at 175F (before fans come on), Battery at 14.2V, exciter current 6.68 ma. alternator current to battery is 28.9 A DC 3.5 A AC.
6) Fans on (PWM), battery at 11.2V, exciter current is 28.88 ma., current draw on battery is 21.3 A DC 0.0A AC, current alternator to battery is 0.0A DC 0.0A AC.
7) Turn on Fail Safe (PWM off), Battery is at 13.5V, exciter current is 6.62 ma., fan current is 27A, alternator to battery current is 58.3 A DC and 4.0 AC.

The key new information is basically three points:

a) Curren drawn by alternator exciter wire, ignition on but engine not running is the same (27.88 ma.) as the exciter current draw when PWM is on and no current is flowing from alternator to battery.
b) During regular operation (no PWM), alternator current has a AC component of about 4.0 amps.
c) With PWM on both the DC and AC currents from the alternator to the battery are zero amps. (all current measurements from alternator to battery and to the fans from controller are measured with an AC/DC clamp-on meter).

Everything else reproduced as above.

FrankGRUN said:

Disconnect exciter wire with engine running, alternator current to battery goes to zero both DC and AC.

Click to expand...

That is a key fact; the alternator appears to have an internal fault.
Once running it should not need any external feed to keep working.,

A full wave rectified three phase power source never drops to zero so the regulator should never lose its supply and shut down, if everything is correct.

I suspect some of the rectifiers in the alternator have failed.

That's what I have been told before including discussions of how to get the alternator started in an emergency mode - stating that once started it would continue to output current until the engine shut down.

Is the AC current component in the non PWM mode testing of the alternator output to the battery consistent with rectifier failure?

I have drawn up to 145 amps from this alternator during these tests - would that be consistent with some rectifier failure?

But, I don't understand why turning on the PWM controller would turn off the alternator. It can't be current draw as the current drawn from the battery by the controller driving the fans at low (PWM) speed is less than half the current drawn from the alternator in non-PWM operation (fans at full high speed).

Put a scope on the output. That'll confirm that one or more of your diodes is bad.

I've just pulled the wire to the fans from the controller and tomorrow will shield the wires with copper tape with conductive adhesive. As soon as that EMI test is done, I'll move the vehicle to put a scope on the output.

FrankGRUN said:

I'll move the vehicle to put a scope on the output.

Click to expand...

Also check the voltage on the lamp/exciter terminal, to see what the auxiliary rectifiers are giving?

You didn't mention the model year of the E 350 but many late model vehicles charge output has a ECM or PCM component to it. Alternators used to control the field current and output voltage solely through the alternators internal voltage regulator. I would have to do some research as to how the vehicles powertrain control module might influence charge system outputs. There is a chance of alternator failure but I am leaning towards EMI from the fan controller in PWM mode.

I should have indicated that my E350 is of 1982 vintage, no PCM. The charging circuit is just the alternator and its internal regulator. For the last several days, I have unwired and dismounted the PWM controller and have been wrapping the ground leads from the motors to the PWM controller in copper foil tape with conductive adhesive to shield them as suggested by the PWM controller manufacturer. A messy job at best! Should be finished today and then be in some position to report on the EMI possibility. As I have mentioned, I originally bought into the EMI possibility because of the ability to turn the alternator on and off by switching the PWM function off and on, and the high currents that are being switched on and off so quickly. I hope the shielding does the trick, but the lack of any internet mention of this problem suggests the such a successful outcome is rather unlikely!

I have seen poor grounds cause some really strange behavior. I would do a voltage drop test on both the positive and negative sides of the charge system. While the cooling fans are operational non pwm mode and with pwm mode. Should be less than .5 volts under load on either + or - sides. Positive lead of volt meter at the positive lead of the alternator and negative test lead to the positive battery post. Then positive test lead at the alternator case and negative lead to the negative battery post. The system must be under a load when testing voltage drops.

I indicated in my last reply that I was frustrated in not finding any internet reference to this problem. Well, that should have been referenced to the "popular" version of internet resources. With this frustration, I triggered Engineering Citations search and found the following paper which should be accessible by triggering this link:
This paper is followed by more recent publications addressing ASIC component susceptibility to EMI - all stressing that there is an intensity threshold that must be reached before the regulator shuts down. At least this is my simple reading of the text and experiments!

rfranzk, thanks much for your comments and I will measure the voltage drops when I have the system reassembled tomorrow (weather permitting). In my previous voltage drop tests (executed before adding the PWM controller and the electric fans, and done to cleanup and upgrade the 1982 Ford wiring harness) I was able to show no voltage drop in the power system (to battery plus and ground from alternator) of more than 0.15 V with 82 amp draw. This with 2 AWG cable.

Did you follow this from Powermaster? https://www.powermastermotorsports.com/faq-alternators_a.html

My dash light does not work after I installed my one wire alternator. How do I get my dash light to work?

Some Powermaster alternators have an indicator light drive. The indicator light wire from the stock wiring harness has to be connected to this terminal of the one wire alternator. If the you had an OE externally regulated alternator, then use a conversion wiring harness (part # 150). If you had an internally regulated alternator with the two spade wiring harness connector, simply remove the black rubber cover on the side of the Powermaster alternator and plug the harness in. (PLEASE NOTE: This applies only to part #s 17294, 37294, etc. and not to part # 178021, etc.)

shortbus, I had not seen that comment on the Powermaster FAQ list, but this alternator (their number 47759) is a 3 wire based on the Ford 3G body and rated at 200 amps with an internal regulator. Of the three wires, one goes to the battery output terminal on the alternator, one to the electric choke on the carburetor and the last (which I'm calling the exciter wire) goes to the ignition through a 500 ohm resistor with a 12V LED wired in parallel for an idiot light. I've asked Powermaster to review the exciter currents I have measured and to tell me if they are the values anticipated.

With a 3 wire, what external regulator are you using? Most people when going aftermarket choose a single wire alternator with internal regulator.

Just downloaded the PDF for that part number and it says it is a one wire?

Their documentation leaves much to be desired! If you go back to their catalog you will see that the 47759 is a 3-wire with internal regulator . A similar unit is available with an internal regulator that turns it into a one wire unit. Power master tech has suggested that if the shielding is ineffective, I should switch it to a one wire which can be done in the vehicle. The harness adapter included with the 47759 to facilitate 3-wire connections is their part number 131. The pdf info for that part number has no info on the harness adapter! Frustrating!

Gentlemen, I have finished shielding the ground wires from the PWM fan controller to the fan motors. The shielding was done by wrapping the wires with copper shielding tape with conductive adhesive. The wires were wrapped to 1/4" of the connectors to the controller and the fan. The shield was grounded to the chassis at the end by the controller. Then on the advice of the PWM controller manufacturer, I connected two electrolytic capacitors (100 uf and 1000 uf) to each of the +12V leads at the fan motor (positive side of the electrolytic to the fan motor, negative side to chassis ground).

The net result - NO ALTERNATOR SHUTOFF AS THE PWM CONTROLLER MANIPULATED THE FAN SPEED! As first suggested by Darryl at AutoCoolGuy.com (manufacturer of the PWM controller), the problem was EMI shutting off the voltage regulator on the alternator.

For some other measurements:

After first startup (PWM controller not engaged): Battery voltage is 14.3V, exciter current is 6.66 ma., exciter voltage is 9.25V, Alternator current is 24.2A DC 2.9A AC, engine rpm is 1100 in neutral

Just prior to fan startup, battery is at14.3V and alternator current is at 12.8 A, exciter current is 6.66 ma. and exciter voltage is steady at 9.3V

After fan turn on, alternator current slowly increases to 14A, then 16, then 18 and on up to 33.2A, then holds steady and slowly drops to 28.2A. Battery is steady at 13.8V , exciter current and voltage steady at previous readings. Engine temperature steady at 180 F for the duration of the 1 hr. test.

So, experimentally established: Variable speed PWM controller generates sufficient EMI to shut down the internal voltage regulator on a Powermaster 47759 alternator based on a Ford 3G unit with 3-wire exciter circuit and rated at 200 amp. I suspect the susceptible component is the op amp used in most modern alternator voltage regulators.

Thanks for your interest and support,

Frank Grunthaner