M20/M40 Coolant temp sensor (M50 swap help)

[slybma 38]

The temp gauge on my E30 has been stuck on cold since doing my M50 engine swap and i realised that i forgot to swap the sensor from my old engine over to the new one so im looking for a sensor or if anyone knows the part number?

The engine is running quite rich and im not sure if its due to it thinking its cold or something else, do the M50 ECUs read engine temp from the same sensor as it uses to set the gauge or do they run two sensors?

Would appreciate any help.

[e30ftw 410]

Use the brown temp sensor from an m20, It's a sensor dedicated to the temp gauge, blue one on m20 is for the ECU. I'm not 100% on the m50, but on my M52 there was a screw in the head I removed and it let me fit the brown sensor in next to the ECU one.

[slybma 38]

Was the plug the same or did you need to change it over from the m20?

[e30ftw 410]

You need to wire it to the C101 plug, there will be the temp sensor (unless you wired it to the M50 wire).

I put the brown sensor in, cut the plug too (any 2 prong plug fits) then run the wire to the C101 which goes to the temp gauge. IIRC the M50 has 2 separate sensors, so you may have one you can swap out and plug in. I don't have an m50 however so can't comment 100% how it fits..

[slybma 38]

I just went and had a look it has two sensors a blue and a black one, i wired it in when i did the conversion harness it reads just not correctly so id say itll be due to it being the wrong sensor. im not sure if its the blue or black one i need to swap out though so might be trial and error just need to get one of those brown m20 sensors

[hotwire 352]

The temp gauge & ECU sensors are completely different in operation. Can't recall colours on the M50 engine but the temp gauge will be a single wire unit whereas the ECU sender is dual - a signal reference back to the ECU. The temp gauge simply earths through the sender.

[slybma 38]

I think that they use the two pin sensors and use a common earth point as apposed to earthing through the sensor because it still gets a reading itll just be that the resistance values given out by the M50 sensors are in a different range to that of the old m40 engine.

[hotwire 352]

Cant recall specifically on this engine but typically gauge senders are single wire. Anyway, check the resistance of each, the ECU sender will be around the 3k ohm when cold & down to 250 ish ohms at operating temp. Temp gauge unit will start at 180 ish & come down from that with rise in temp

[slybma 38]

I have a rough idea of which one it is but will do some testing later to confirm, now i just need one of those brown M20 sensors