Start with the basics, check if the diodes are failed short (so allowing reverse current), check the surface mount capacitors (I doubt the electrolytic would have failed short), and check the large transistor (probably a FET) to see if it’s shorted.

You could always inject a low voltage at a fixed current and see what gets warm with the pinky test if you can’t find the shorted component through poking around with the multimeter.

paradigital wrote on 2025-10-13, 17:22:

Start with the basics, check if the diodes are failed short (so allowing reverse current), check the surface mount capacitors (I doubt the electrolytic would have failed short), and check the large transistor (probably a FET) to see if it’s shorted.

You could always inject a low voltage at a fixed current and see what gets warm with the pinky test if you can’t find the shorted component through poking around with the multimeter.

But how do I check diodes or caps if shorted? Just with beep test?

Update:

Diode 5 beeps, while others don't. Could that be the issue, since everything else is not shorted? That means that this diode allows reverse current, plus it is next to motor wires, so makes sense, right?

boby wrote on 2025-10-13, 19:09:

But how do I check diodes or caps if shorted? Just with beep test?

The beep test is sometimes not enough. If it does not beep, there clearly is no short. But if it beeps, take a look at the display of your meter. Depending on the model, it will display a voltage (at a test current of typically 1mA to 10mA) given in millivolts or a resistance in ohms. If the value on the meter is nearly as low as the value you see when you just touch one probe with the other probe, there clearly is a short. If the value is much higher (like 20 ohms or 80 millivolts), there still is a way for current to flow "easily", but current flow is still limited. Depending on the design of your device, this might be OK.

mkarcher wrote on 2025-10-13, 19:22:

boby wrote on 2025-10-13, 19:09:

But how do I check diodes or caps if shorted? Just with beep test?

The beep test is sometimes not enough. If it does not beep, there clearly is no short. But if it beeps, take a look at the display of your meter. Depending on the model, it will display a voltage (at a test current of typically 1mA to 10mA) given in millivolts or a resistance in ohms. If the value on the meter is nearly as low as the value you see when you just touch one probe with the other probe, there clearly is a short. If the value is much higher (like 20 ohms or 80 millivolts), there still is a way for current to flow "easily", but current flow is still limited. Depending on the design of your device, this might be OK.

It closes to 1 I think, same as when I connect the probes. So most likely it is shorted. Problem is that I have no spare diodes.

That diode is likely across the motor. If this is a low-voltage device (runs off some AA cells) and a powerful motor, the motor might look nearly like a short circuit, even if it is still OK. Why do you think the motor is OK, and there is no short inside the motor?

To properly check components like diodes and caps, just use you soldering iron to lift one end - diodes should conduct on only one direction,
caps should not conduct - big caps might show a pulse but will not maintain DC conduction (if electrolytics, be sure your tester polarity is right
- some multimeters are negative on the plus lead when testing conductance/ohms - best to confirm with another multimeter)

Also be aware that a stalled DC motor (ie: not running) can show a very low resistance, it's "effective" running resistance comes from
the back-emf of the rotatation of it's coils. Unless you ohmmeter can show very low resistance, it might read 0 causing you to thing it's
shorted. - With battery in and measuring across the actual leads to the motor, does it read voltage - the stalled motor will draw a lot,
but you should see some voltage - truly shorted will read 0 (and draw excessively from the battery - don't do this for long)

If you can measure current on your meter, with the motor disconnected, see if it draws a lot when you try to activate it - if no,
the electronics are probably ok (note that most cheap meters won't fair well if you try and measure a lot more current then
they are designed to handle - like driving a dead short - best to use a current limiting resistor - sized to draw the max
your meter can handle if it were the only load on the battery)

If the motor is actually shorted, likely some internal brushes or other parts have come loose... you may be able
to disassemble and repair it.

mkarcher wrote on 2025-10-13, 19:46:

That diode is likely across the motor. If this is a low-voltage device (runs off some AA cells) and a powerful motor, the motor might look nearly like a short circuit, even if it is still OK. Why do you think the motor is OK, and there is no short inside the motor?

I have disconnected the motor and test in on external battery - runs fine.
With the motor disconnected, there is no short on motor points on the PCB or on the diode next to the motor.
Still, with connected power there is no voltage on the PCB where motor should be connected.

The device is steam window cleaner and runs on single 3.7V cell battery

DaveDDS wrote on 2025-10-14, 03:00:

With battery in and measuring across the actual leads to the motor, does it read voltage - the stalled motor will draw a lot,
but you should see some voltage - truly shorted will read 0 (and draw excessively from the battery - don't do this for long)

With battery connected there is no voltage on the motor lead points at all

Looks like the motor is ok. I have disconnected it from the board and it runs fine on external battery

DaveDDS wrote on 2025-10-14, 03:00:

If you can measure current on your meter, with the motor disconnected, see if it draws a lot when you try to activate it - if no,
the electronics are probably ok

Doesn't draw anything, simply no voltage on motor leads, connected or not

Should the motor beep? When disconnected from PCB, and tested it's leads with meter?

Then your problem is somewhere else, not the motor or the diode. There is a transistor near the motor, this is probably used to switch it on and off. Assuming it's a MOSFET, you can test the voltage between the gate (left pin) and the source (right pin) during motor activation. Does the voltage change? If it doesn't, then you have a problem there.

If the IC near the transistor is a gate driver, you can look up its datasheet and then test the voltage at its input pin during motor activation. It's probably connected to a microcontroller that tells the circuit when to turn the motor on and off.

asdf53 wrote on 2025-10-15, 16:29:

Then your problem is somewhere else, not the motor or the diode. There is a transistor near the motor, this is probably used to switch it on and off. Assuming it's a MOSFET, you can test the voltage between the gate (left pin) and the source (right pin) during motor activation. Does the voltage change? If it doesn't, then you have a problem there.

Ok, that might be it (maybe). There is no voltage on the transistor at all. However, probing it's left and right pins together it beeps (shorted?)
Also, when I try any of the pins against ground with connected power, It shows voltage.
I mean, left or right pin from transistor + black wire where the motor connects, shows around 4.2V (battery is full)

The problem is that I can't see a lot on the photos, sorry. Can you make one with good lighting that shows the whole board where we can see the letters on the components? What is the model number on the transistor?

The most common configuration here is an N-MOS that connects the negative terminal of the motor to ground, with pin 1 being the gate, pin 2 being the drain and pin 3 being the source. With the power on, but the motor not running, the voltage at pin 2 should be high. With the power off, the resistance between pin 1 and pin 3 should be very large. Don't use the multimeter in beep mode, use it in resistance mode.

The transistor turns on when the gate voltage (pin 1) is higher than the source voltage (pin 3). Test the voltage between these pins during the time where you expect the motor to turn on. It should change from 0V to a positive voltage.

asdf53 wrote on 2025-10-15, 18:17:

The problem is that I can't see a lot on the photos, sorry. Can you make one with good lighting that shows the whole board where we can see the letters on the components? What is the model number on the transistor?

The most common configuration here is an N-MOS that connects the negative terminal of the motor to ground, with pin 1 being the gate, pin 2 being the drain and pin 3 being the source. With the power on, but the motor not running, the voltage at pin 2 should be high. With the power off, the resistance between pin 1 and pin 3 should be very large. Don't use the multimeter in beep mode, use it in resistance mode.

The transistor turns on when the gate voltage (pin 1) is higher than the source voltage (pin 3). Test the voltage between these pins during the time where you expect the motor to turn on. It should change from 0V to a positive voltage.

This is the best I could. Unfortunately transistor was dirty and when I tried to clean, I pilled off the numbers of it.
There are some numbers on the PCB it self so maybe that can help

Okay, thanks for the photos. It's helpful, but it's going to be a bit difficult without knowing the pinouts of the ICs. Do the tests I suggested from above with your multimeter and post the results.

Is it like this?

If that is correct numbers for pins then I have ~ 140 kOhm between 1 & 3, and ~ 100 kOhm between 1 & 2
Power is off in this case

Power connected but the motor switch off:
Pin 1: 4.12V & Pin2: 4.12 V (battery is 4.19V )

Again, these are the measurements if the pins are numbered as I marked on the image

Almost, it should be 1-2-3 from top to bottom. As we don't know the model number, we need to guess the most likely configuration.

Looking at your photo this is probably the configuration:

The mosfet will be an N-Channel FET that is used to switch on / off the Motor. It uses between Ground and Source to measure the current going through the FET (and the motor). R9 is to clean up the signal a bit. D5 is to prevent reverse current from the motor, as it is inductive.

The FET has the pinout 1,2,3 - with 2 being the tab at the top. It is Gate(1), Drain(2) and Source(3).

If the motor should be turned on, you should be able to measure a voltage of at least 5V or so at the gate (pin 1) against ground (0V, negative input coming from the battery.)

Not sure if I missed, but can you state what the individual connectors are going to?

Two pin connector is power switch.
Three pins is some diode (I believe for charging)
Second (horizontal) 2 pins connector is power source
Battery is 3.7V since cell

P.S. As I said before, this is some cheep (25$) steam window cleaner. It is not necessary that I fix it. But that is not the point. I like these stuff, plus I always learn something, so would be nice to fix.
But if I don't manage, it is fine too

boby wrote on 2025-10-17, 10:31:

Is it like this? […]
Show full quote

Is it like this?

If that is correct numbers for pins then I have ~ 140 kOhm between 1 & 3, and ~ 100 kOhm between 1 & 2
Power is off in this case

Power connected but the motor switch off:
Pin 1: 4.12V & Pin2: 4.12 V (battery is 4.19V )

Again, these are the measurements if the pins are numbered as I marked on the image

Sorry, I didn't see your reply at first.

Is the motor connected or disconnected when you do the measurements? It needs to be connected.

Did you really measure 4.12V from the source pin of the transistor (pin 2 in your photo) to the negative pin of the battery connector? It should be 0V. If it is really 4V, it means that the white resistor (R010) is broken. Measure the resistance across it with the power off.

Also measure the voltage from the drain (pin 3 in your photo) to the negative pin of the battery connector.