How do I diagnose a problem with electronic bricks and motors?

Question

My children and I recently assembled the 42030 RC VOLVO L350F Wheel Loader (aside: with the 1,600 pieces and time investment, this set will likely survive to the heat death of the universe intact because nobody wants to take a day or two to reassemble it into something else).

As soon as we took it out of the box we knew it likely had a problem because none of the motors worked when we plugged them together and put batteries in the battery pack and remote controls. We built it anyway (that sucker took around 7 hours to build between the two of us that stayed awake), knowing that if a part is defective we could probably cross-ship a replacement from LEGO.

Now that it is assembled, it still does not work. We have double-checked everything according to the instructions. When we use the remote, nothing happens on the assembled set. Asking the Internet brings up plenty of positive reviews, but nothing about problems with this set.

How do we troubleshoot which part is defective, or if there is an error on our part? Ideally, such advice would be applicable to any set that has motors, battery packs, and remote controls.

What we have verified so far:

• The battery pack lights up when we turn it on. The motors beep and spin for a brief moment, indicating they are receiving power through the receivers.
• Unplugged and replugged everything.
• I tested both remotes and the battery pack with a multimeter, verifying correct voltage and amperage across the two contacts on the end. The batteries are not the problem: these are brand-new batteries that expire in eight years, and they output nominal volts and amps.
• We tried different communication channels between the remotes and receivers.

I have not been able to verify function without the battery pack or receivers because I am not sure how much current to apply to an individual motor, nor do I wish to experiment and make the situation worse.

What else can I try before I contact LEGO and spend a week or more waiting for replacement parts?

Answer 1

I haven't ever used Power Functions remote-control elements, so I can't give you a complete answer from experience, but here are two simple tests:

• Connect the battery box directly to each motor, leaving out the remote control receiver entirely. The motors should spin when the light on the battery box is lit, and the slide switch should allow reversing the direction.

  Caveat: Looking at the parts list for 42030, one of the motors is a “servo” motor. I don't know exactly how to expect it to behave when connected directly to a battery box, but LEGO's description suggests it should at least move.

• Use any camera with a live view or video recording (not an optical viewfinder), such as that on any smartphone, to look at the transmitter while you press buttons. The infrared signal should be visible as a purplish-white dot.

  (This occurs because most cameras are more sensitive to near-infrared light than our eyes are, unless they have unusual filtering. This also can be seen with any infrared TV remote control.)

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On electrical testing:

I have not been able to verify function without the battery pack or receivers because I am not sure how much current to apply to an individual motor,

This suggests that you are confused about how electric circuits work. Outside of specific electronic circuits, you do not choose how much current to apply; the load (here the motor) expects a specific voltage and then draws current according to its needs. (For a motor, the current depends on how much load is on the shaft: a motor not connected to anything will draw much less current than one which is stalled.)

(If you're actually referring to a current-limited power supply, then never mind.)

The voltage for Power Functions is 9 V (or somewhat less), as indicated by the 6 cells in the battery box.

You should be able to apply 7-9 V to the two closer to the middle contacts of the motor's plug (for the simple motors, not the servo motor) and get it to spin. However, it will be tricky to make a connection without the special plug, so if you attempt this be very careful not to short out your power supply. I would suggest not trying it; the battery box should be a reliable power source.

Instead, check the outputs of the battery box with a voltmeter. Between the outer two contacts you should find around 9 volts (for alkaline batteries) or 7.2 volts (for NiMH rechargeable batteries). Between the middle two contacts should be the same voltage, but it will reverse polarity depending on the slide switch.

(Try not to short out the contacts by letting your meter probes touch each other, but if you do, it won't be disastrous: LEGO battery boxes have overcurrent protection. If you do trip the protection then the output will be low or zero for a while before it resets.)

Answer 2

The issue turned out to be a known problem with the battery terminals in the remotes. Certain batteries would not make enough contact with the positive terminals due to a plastic piece in front of them. I did the following to fix it:

1. Removed the battery covers from the remotes.
2. Removed all of the AAA batteries.
3. Inserted a small flathead screwdriver behind each of the positive terminals and bent them toward the battery cavity slightly.
4. Put everything back together.
5. Chased the cats around the living room with the now-functioning LEGO set.

I did this after reading this question and its answers on Amazon.