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Joined: Fri May 13, 2022 2:29 pm
Location: Netherlands

Coolant pump makes continuous noise

I have my third i-MiEV/C-Zero (2018) (1st: i-MiEV 2011, 2nd: C-Zero 2013, 3rd: C-Zero 2018), and I notice that the coolant pump near the engine runs continuously when the car is ON. Coolant is continuously pumped around. It makes a constant (annoying in the interior) humming noise until the car is turned off.

Anyone recognize this? Has anything been changed in the pump control or DC/DC converter in the later years of construction?
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Location: Silicon Valley, California

Re: Coolant pump makes continuous noise

macowi, welcome to the forum. Sorry to hear about your problem. I don't recall this ever being discussed on this forum.

I was hoping that someone experienced with this would chime in.

I cannot imagine that continuous running of the coolant pump is normal, nor that Mitsu would have made such a design change so late in the car's production run.

Looking at the Mitsu workshop manual, they devote a full section (14) to the cooling system. Sadly, the only two faults they identify are a coolant leak and overheating, although they do offer a procedure for testing the water pump relay which is located in what Mitsu calls the "Relay Box". I think it's the fusebox under the bonnet below the brake vacuum booster (LHD cars). A stuck relay could result in continuous running of the pump.

There is a temperature sensor located externally in the forward right corner of he MCU inverter. It has a connector that can be disconnected. Their testing procedure involves removing the sensor, immersing it in water and they offer the following readings:
0°C 5.1kΩ-6.5kΩ
20°C 2.1kΩ-2.7kΩ
40°C 0.9kΩ-1.3kΩ
60°C 0.48kΩ-0.68kΩ
80°C 0.26kΩ-0.36kΩ

I wouldn't bother going to the trouble of removing the sensor but would simply try to probe it in situ. I think that a shorted sensor would result in continuous running of the pump.

The cooling pump running has always mystified me, as, even when the car is cold, it cycles when the car is charging and before the coolant has even had a chance to warm up. So, speculating, I think there also a separate timer for the pump, independent of temperature. Anyone?

So, may I suggest that you get out your multimeter, and good hunting (and good luck)!
EVs: Two '12 Wht/Blu SE Prem., '13 TeslaMS85, three 156v CorbinSparrowsLi(NMC), 24v EcoScoot(LiFePO4)
EV Conversions: 156v '86 Ram PU, 144v '65 Saab96, 48v1kW bike
RIP(2021) ICE: Orig.Owner '67 Saab96V4, '88 IsuzuTrooper; '76 MBZ L206D RHD RV
Posts: 2
Joined: Fri May 13, 2022 2:29 pm
Location: Netherlands

Re: Coolant pump makes continuous noise

Thanks JoeS for thinking along! :)
As first I tried to replace the coolant temperature sensor with a new one, but that was unfortunately not the solution.

I did discover something else when I connected Car Scanner: the temperature of de heater ('Water temperature HVAC') is consistently about 20 degrees Celsius higher than the ambient temperature. I compared the values with my brother's 2011-Ion on the same moment (screenshot below).
I wouldn't expect the heater to have anything to do with the water pump, but you can't be sure, so as first I will try to replace the sensor.
If anyone has a suggestion let me know :)


I found another topic with a problem like mine: viewtopic.php?p=42657#p42657

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