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Oct 06

Choosing a water temperature sensor for MicroSquirt




Another of the challenges associated with fuel injecting an old motorbike is that none of the existing sensors were ever intended for connection to a modern engine management system. In particular, the coolant temperature sensor in the radiator has a single signal wire coming out of it and the ground return is through the chassis of the bike.

The problem with having the sensor grounded to the chassis is that it will have lots of induced noise from other high-current parts such as the fuel injectors, ignition coils, starter motor, etc due to voltage drop in the ground path. I think this diagram borrowed from the MicroSquirt hardware manual, page 13 is a good illustration of this problem:

MicroSquirt Sensor Grounding

MicroSquirt Sensor Grounding

The solution is to install a “two-wire” sensor with a signal wire and a ground wire that can be connected directly back to the ECU and avoid effectively all of that voltage drop and noise.

I settled on a GM LS1/2 coolant temperature sensor since it (almost) fits nicely in the existing location and the MicroSquirt engine management system already knows the calibration values for this sensor so it’s pretty much plug and play.

The small problem with this sensor is that it has a larger thread than the original sensor so here is what i did about it:

I set up the new GM sensor in my metal lathe and turned down the thread to the major diameter of the old single wire sensor and then cut the correct thread onto the GM sensor. It was a metric M10x1.25 thread, and now it screws nicely into the radiator and talks to the MicroSquirt once I selected the correct sensor in the configuration.

Here are a few pics or the work in progress:

CLT Sensor Before

CLT Sensor Before

CLT Sensor in lathe

CLT Sensor in lathe

CLT Sensor After

CLT Sensor After

CLT Sensor Installed

CLT Sensor Installed

Oh and while I was working on the lathe, I realised that one of the radiator mount bushings was missing so i quickly made another one, gotta say a metal lathe has quickly become the most useful machine in my workshop…

Radiator Bushing

Radiator Bushing

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