Help with this Fan Circuit

[wickedweed]

Hi. The above partially completed schematics is part of my car's (VOLVO) fan speed controller. This controller basically switches the fan in 3 States: OFF, LOW SPEED, HIGH SPEED. At this moment, the HIGH speed is not working (fan doesn't turn at all).

The Drain and Source of the BUK7908 are paralleled together and the gates are controlled by the same output on the logic board. The gate of the BUK7508 is controlled by another i/o pin on the logic board.

I have no clue what is connected to the point marked 'X' and also what is connected to the other pin of the inductor (definitely not the supply/ground pins). Are there anyone out there who can possibly guess what the mosfet/caps and inductor connected on the +12V rail of this circuit do?

I do suppose that PWM pulses on the BUK7908 gates are used to determine the fan speed?

Thanks!

 

[Pommie]

I would guess that it's PWM'd and therefore the most likely source of the problem is the high/low switch.

Edit, BTW your drawing is very hard to read. Is that coil on the right hand side anything to do with the fan?

Mike.

 

[wickedweed]

Hi. The coil I guess is a wirewound inductor. No idea what's the function though. The motor is on the left of the schematic beside the BUK7508.

 

[alec_t]

Looks like the 7508 switches 12V to point X via a choke, to provide power to some auxiliary unit.
Since your fan works ok at the low setting the fault is probably in the speed-selection switch itself, as Pommie said.

 

[wickedweed]

Looks like the 7508 switches 12V to point X via a choke, to provide power to some auxiliary unit.
Since your fan works ok at low and medium settings the fault is probably in the speed-selection switch itself, as Pommie said.

hi. does 'speed-selection switch' mean the two BUK7908?

 

[alec_t]

No. It's the switch on the dashboard.

 

[dr pepper]

There are diffrent types, in fully electronic versions the cars ecu provides a signal, pwm but not the same pwm the motor is driven with, but a reference signal, the fan controller uses this to generate the actual pwm that goes to the motor.
The fact yours is low and high speed suggest that it similar to a peugeot system where there is a transistor or relay switch and a power resistor which drops the voltage in low speed, the latter is the one that usually fails, the resistor burns out.
I would see if you can power the fan directly at full speed, if it runs full speed then its either a duff switch unit or possibly the wiring has broken, being burnt by the exhaust is common, or worn through on something moving.
I cant see your schem.

 

[wickedweed]

There are diffrent types, in fully electronic versions the cars ecu provides a signal, pwm but not the same pwm the motor is driven with, but a reference signal, the fan controller uses this to generate the actual pwm that goes to the motor.
The fact yours is low and high speed suggest that it similar to a peugeot system where there is a transistor or relay switch and a power resistor which drops the voltage in low speed, the latter is the one that usually fails, the resistor burns out.
I would see if you can power the fan directly at full speed, if it runs full speed then its either a duff switch unit or possibly the wiring has broken, being burnt by the exhaust is common, or worn through on something moving.
I cant see your schem.

hi. mine is a 2008 volvo v50 and there is no relay leading to the fan unit. This fan speed control module I am trying to troubleshoot has a 3-pin connector which are + and gnd from the car battery and a signal wire (some serial protocol) coming from the ECU.
Here's are pics of the schematics of the module itself.

I have not desoldered the transistors yet to test but onboard testing for continuity and shorts with a DMM seems ok so far, except that I have never dealt with a 5-pin mosfet before.

The BUK7908 comes in a 5 pin package (**broken link removed**) and looking at the datasheet, the 2 additional pins are Isense and Kelvin Source pins. With a DMM, it is telling me that these two pins are shorted with the SOURCE pin. I am gonna desolder them to test again when they are out of the board.

Any heads-up with these info?

 

[dr pepper]

If the circuit was just th etop half then I'd say that point x was switched 12v low speed and what you think is a choke is actually a power resistor, and +12v on your drawing would be high speed switched 12v.

However the lower half grounding the fan motor is classic pwm control.

It doesnt make sense to have both, maybe the upper section is come sort of noise filter.

The current sense and kelvin source may well read a short to the source, esp if there is some charge on the gate for the current sense, are you usre the drain and source are linked together, that really doesnt make sense.

At this point I'd be using a current probe and a 'scope to look at the current changes in the circuit, bad sectors on the motor comm can be easily spotted with this trick, you could also t pin the control lines to see whats going on, maybe the ecu doesnt even know high speed is demanded.

 

[alec_t]

a signal wire (some serial protocol) coming from the ECU.

Could that be the PWM signal?

 

[dr pepper]

Most likely, the most common serial protocol by far in cars in can bus, but that has 2 wires.