How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Juan Crespo Your Own Question
Juan Crespo
Juan Crespo, Tech Trainer
Category: Mazda
Satisfied Customers: 1523
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
19298667
Type Your Mazda Question Here...
Juan Crespo is online now
A new question is answered every 9 seconds

No cooling fan operation, new fan has power and ground

Customer Question

no cooling fan operation
JA: Tell me a bit more about what's going on so he can help you best.
Customer: new fan has power and ground appears no signal from PCM
JA: Are you hoping to fix this yourself?
Customer: trying to figure PCM logic / how it controls it
JA: Is there anything else important you think the Mazda Mechanic should know about your Mazda?
Customer: yes
JA: What else should I tell the Mazda Mechanic?
Customer: I have only a snap on solus no way of controlling circuit thru my scanner
JA: Our top Mazda Mechanic is ready to take your case. Just pay the $5 fully refundable deposit and I'll fill the Mazda Mechanic in on everything we've discussed. You can go back and forth with the Mazda Mechanic until you're 100% satisfied. We guarantee it.
Submitted: 11 months ago.
Category: Mazda
Expert:  Juan Crespo replied 11 months ago.

Hi there. For some reason there is no vehicle information on my screen. Would you mind telling me what year, model, trim level, engine size, trans type, drive type is that Mazda you're working on?

Customer: replied 11 months ago.
2008 mazda 3 2.0 4cyl 4 door sedan
Customer: replied 11 months ago.
auto trans
Customer: replied 11 months ago.
fwd
Expert:  Juan Crespo replied 11 months ago.

Thank you.

PCM monitors signals from ECT, A/C control switch-pressure switch-compressor clutch and VSS to control fan operation. PCM will not command the fan on if A/C is off and engine temp is below 98 degrees Fahrenheit. Power to the fan comes from the fan module.

Does the fan work with engine at idle and A/C on?

Customer: replied 11 months ago.
no it doesnt
Customer: replied 11 months ago.
ac pressure drives up to 400psi the switch doesn't cut it out either
Customer: replied 11 months ago.
ect runs about 200 far
Expert:  Juan Crespo replied 11 months ago.

Then chances are your fan control module is bad. Mazda sells it as part of the shroud/fan assembly.

Customer: replied 11 months ago.
brand new fan/module
Expert:  Juan Crespo replied 11 months ago.

Can your scan tool read "fan command" PID?

Customer: replied 11 months ago.
how does PCM control fan on blue wire
Customer: replied 11 months ago.
no
Customer: replied 11 months ago.
just duty
Customer: replied 11 months ago.
always says 0
Customer: replied 11 months ago.
my ac switch also reads off
Customer: replied 11 months ago.
if I back probe black/green wire at ac switch with a ground wire then reads on
Customer: replied 11 months ago.
but no fan operation
Customer: replied 11 months ago.
I'm not sure if I'm following correct path with an ac issue for fan not working
Expert:  Juan Crespo replied 11 months ago.

You're going too fast for me to follow - I'm an old mechanic who can't type very fast. Please wait until I respond before replying.

Customer: replied 11 months ago.
sry
Expert:  Juan Crespo replied 11 months ago.

Please do not backprobe anything unless you're doing with the probe from a scope.

The fan module is a computer in it of itself that interprets the serial data it gets from the PCM on the Blue wire to then turn the fan motor at whatever speed it deems necessary.

In this case, it looks like the original fan module went bad and took the PCM along with it. Doesn't your Solus have scope capability?

Expert:  Juan Crespo replied 11 months ago.

Also, did you ever have code P0128 pop-up in the past?

Customer: replied 11 months ago.
I have a scope but think maybe found issue. The blue wire has 11.5 vts on it, disconnect harness plug voltage gone. should the blue wire have voltage on it? If PCM controls fan on that circuit?
Expert:  Juan Crespo replied 11 months ago.

This is not about voltage; as I just posted, the Blue wire is used for communication between the PCM and the fan module. Instead o a regular voltmeter, use the scope to see if there is a serial data being transmitted on that Blue wire.

Expert:  Juan Crespo replied 11 months ago.

How do you figure you found the issue?

Expert:  Juan Crespo replied 11 months ago.

The reason I ask is because you might have found another easier way to diagnose this issue - all we go by is what Mazda tells us to do, which is not always right or even easy to understand ;-)

Expert:  Juan Crespo replied 11 months ago.

Whatever you do, don't put a jumper from Battery+ to that Blue wire - the amperage might burn the new fan module.

Customer: replied 11 months ago.
I think my new fan is junk, I scoped the blue wire and have a wave form a/c commanded on PCM is reading 82% duty cycle but no fan operation also I may be wrong but thought data wires only ran on 5vt
Expert:  Juan Crespo replied 11 months ago.

You're correct in the sense that CAN network runs on 5 volts serial data signal. In this case, the 12 volt duty-cycle signal should be from the module to the fan motor; varying that duty-cycle is what allows the module to control fan speed.

I haven't worked with a Solus before, but I hear they're pretty good. I don't know if it is just the Solus Ultra, but it should have an actuator test function that would allow to turn the fan on. Does yours have that function?

Customer: replied 11 months ago.
No mine is an older Solus pro but have a Vantage Ultra I tested with had a signal. If I understand correctly yes should 12 vts module to motor but have just as much voltage on blue wire from module to PCM with a/c switched off. The voltage is back feeding from fan module if I disconnect harness plug for fan module voltage goes away
Expert:  Juan Crespo replied 11 months ago.

I have to apologize. You were absolutely right. The strategy Mazda uses to run the fan is based on voltage signals the PCM sends through the Blue wire, not serial data as I thought. For example, if the PCM sends close to 0 volts, the fan module will turn the fan on at full speed. Full battery voltage would make the module turn the fan at medium speed.

Once again, you're right; if there are 12volts on the Blue wire with the A/C on, the fan should run at medium speed. Therefore I agree with you in thinking that new fan module is junk.

Customer: replied 11 months ago.
Its ok so backprobing PCM fan module disconnected running about 3.5 vts duty about 60% so testing module should have continuity to ground wire not power wire? 12vts going to run fan at medium, pcm decreases voltage fan increases speed and to turn fan off does pcm cut voltage all together?
Expert:  Juan Crespo replied 11 months ago.

The fan module is basically a double electronic relay (see diagram attached). In instances when fan is not necessary at all, the PCM will turn the control circuit (Blue wire) off/open. For example, if vehicle is cruising at 45 MPH on a flat road, on a cool morning (ECT less than 98 degrees), with no A/C on, the PCM will turn the fan off completely.