Harbor Freight Bauer 18V water pump only starts when cool, ZD1 is heating - Page 1
Forum section: electronics / repair

author: squoril
author_rank: Newbie
timestamp: October 15, 2025, 11:26:35 pm
content: Starts and runs for as long as you want. Will not restart when warm ~>90F OAT, OAT ~<70F reliable restarts.Icing down the control board guarantees a restart and reveals ZD1 as a heating component.Assuming its ZenerDiode1 what would i replace it with?Are all ZDs the same?1.7x2.7mm -ish dimensions, SOD-123 package?ZD1 is 3rd component above outside yellow wire on the right. TIA for any ideas.

author: Mr.B
author_rank: Unknown
timestamp: October 15, 2025, 11:35:41 pm
content: Quote from: squoril on October 15, 2025, 11:26:35 pmNo.Voltage and Power rating are the important parameters.What are the markings on the device so that we can try to find out these two parameters.

author: squoril
author_rank: Newbie
timestamp: October 15, 2025, 11:41:02 pm
content: There may be a marking but i will have to get my 10x loupe from work to make it out.Would reading voltage and current through ZD1 in operation provide enough information to make an educated guess?

author: floobydust
author_rank: Super Contributor
timestamp: October 15, 2025, 11:47:43 pm
content: I would expect U1 to be overheating and go into thermal shutdown. It really doesn't have much if any PCB copper heatsinking.Check the tab is not too hot on it, like a finger burner? The IC is close to the zener you are after, but zeners getting hot still work.

author: Mr.B
author_rank: Unknown
timestamp: October 16, 2025, 12:00:55 am
content: U1 looks like it could be a voltage regulator supplying U2 and the rest of the circuit.Although U3 could also be a voltage regulator.If it was a voltage regulator going into thermal shutdown then the unit most likely would not continue to run “as long as you want” after starting. Quoted bit from the OP.Just thinking out loud...

author: floobydust
author_rank: Super Contributor
timestamp: October 16, 2025, 01:23:31 am
content: The main voltage regulator has a lot of heat to dissipate, 18V down to maybe 5V for MCU U2.I thought the zener could be helping that, but instead notice ZD1 cathode going to U1 tab. That puts it as absorbing transientsI dunno, a person would have to sketch the power supply section schematic.

author: squoril
author_rank: Newbie
timestamp: October 16, 2025, 02:43:00 am
content: Oh i am loving the discussion, thank you so much!I might be able to scare up a real thermal imager at work and get a better look at what's getting hotThe MCU soft starts the motor, RPM sensing (jammed impeller protection id guess) and motor over temp (what i had originally thought was going on.)Ill see about getting IC markings if the half assed conformal coating flakes off nicely. If not I'm sure an MEK Q-tip will take care of it.

author: Mr.B
author_rank: Unknown
timestamp: October 16, 2025, 02:48:36 am
content: A thermal imager will definitely give us all a much better insight into what is going on...

author: Mr.B
author_rank: Unknown
timestamp: October 16, 2025, 02:52:05 am
content: Quote from: squoril on October 15, 2025, 11:41:02 pmYou can easily measure the voltage across it with a decent quality multimeter.Measuring the current through it is probably not necessary and would definitely challenge your SMD soldering skills.However, if it is being used for transient suppression then measuring the voltage across it will be pointless.

author: squoril
author_rank: Newbie
timestamp: October 16, 2025, 03:11:40 am
content: Im no expert with an iron but i could tie my meter in for amps if needed. My meter is an Agilent U1242A in cal.IIRC when powering on with the ZD1 area of the board frosted it was the first to melt and when cooling to allow a restart immediately after shutting off it was the last to frost (i took this to mean it was constantly heating and was the problem component)Ill try and get some thermal images posted Monday.

author: Mr.B
author_rank: Unknown
timestamp: October 16, 2025, 03:43:52 am
content: All diodes in proper operation will dissipate some heat. Just how much depends upon how much current is passing through them. The frosting/thawing that you are observing may be a red herring and may well be expected behaviour.At the moment we don’t know what the purpose of the diode is and need to better understand that.If you can get clearer photos of both sides of the PCB and then as@floobydustsuggested, try to draw out the circuit (reverse engineer), particularly in the ZD1 and U1 area and where they connect to with regards to the power in wires.The thermal imaging may tell us nothing about the diode, which is fine, but more importantly what else is going on elsewhere in the circuit.

author: floobydust
author_rank: Super Contributor
timestamp: October 16, 2025, 05:03:04 pm
content: The blue wire appears to lead to a temperature sensor (thermistor). Where is it? Battery, Motor etc. ? PCB is "B_NTC" which implies the battery NTC.From the way the control board is behaving, it might think that is overheated and go into some shutdown/lockout mode.

author: squoril
author_rank: Newbie
timestamp: October 16, 2025, 06:02:22 pm
content: B_NTC goes to the battery jack for external battery temperature monitoringThere is a second thermistor out of frame that is mounted on the motor case. I separated and chilled motor thermistor at first with no improvement.Ill have better photos and hopefully thermal images monday.