Coolant sensor replacement / retrofit

[Duncmasterflex]

Guys after some help here again...
So i fired up the old M20B25 yesterday after a complete rebuild and happy to say she went first turn of the key. Idling a little lumpy but it may be connected.
I replaced both temp sensors”‹, blue and brown with genuine BMW ones, when I click the key the temp gauge goes from rest to the bottom of blue but that's it. I ran the engine for like 5 mins heaters on full and was getting warm air but the temp gauge didn't budge. I disconnected the brown sensor and had the old man put a jumper wire in and ground it to the engine,but it never budged, si I'm guessing the gauge is toast or there's a break in the loom.
So I've googled retrofit temp gauges on E30's and read a load of posts that bang on about 1/8 npt sensors, m12 and m14 adapters, new housings spliced into radiator top hoses etc etc...
Are they over complicating this? Could i not get an aftermarket gauge and plug into the new BMW sensor? It's only a resistor isn't it? Surely the ohm values can't be that different on the aftermarket sensors?
Also, does the brown sensor wire go through the ECU and give it info on running for idle etc or go straight to the gauge?
I'm not that keen on taking the clocks apart and messing with the gauge, was thinking of maybe running the multimeter from the brown plug to the multiplug on the back of the plugs to see if the sender wire is ok or broken in the loom, not sure which one it is but wonder if Bentley manual might say?
Any constructive help would really be appreciated here chaps, I'm obsessed with coolant temperature after the whole engine rebuild was due to the last 885 head cracking, and I'll have a complete breakdown if i cook another head!!!
Thanks guys

[Brianmoooore]

The brown sensor connects to the gauge and only the gauge. No effect on anything else whatsoever.
I certainly wouldn't recommend adding an aftermarket gauge. The BMW one is reliable, accurate, and the important part of the dial is expanded, giving a better indication when it matters than any add on gauge.
It's extremely unlikely the gauge itself has failed - it would be a first, although duff SI board batteries (failed while the main battery was disconnected?) can affect the gauge, but almost always affect the tacho. first.
Are the SI LEDS and bulbs behaving?
Disconnect the C101 engine loom plug, and use a piece of wire to earth pin 4 of the body side with the ignition on. This should make the temp. gauge read full hot. If it does, your problems are confined to the engine loom, and the first place I'd be looking is under the rubber boots of the round plug and socket under the inlet manifold, where the injector loom connects to the main engine loom.

[Duncmasterflex]

Hello again Bri, you've come to my aid once more!

Not sure what the SI LEDS are but all the bulbs”‹, tach and fuel gauge are working.
As for the C101 i don't think mine has that as it looks like the loom is a splice job with the old M20B23 loom, there doesn't seem to be any king of plug to separate the looms apart from the one under the glovebox
The injector loom all just feeds into the rest which is a bummer as I've got a spare injector loom with that round plug and a nice neat rail that houses all the wires and would have been a lot neater job, but unfortunately I'm working with what the previous monkeys did

Either that or did the earlier 6 cylinders ( mine is an '85) not have the C101 plug?
Am I able to test the brown plug wire to the back of the blue cluster multiplug see if it's a broken wire problem?
Thanks again for your input mate

[Brianmoooore]

Early cars had a rectangular engine loom plug and socket, used up until 1985, IIRC, plus a 3 pin plug and socket behind the dash. Pre face lift M20 looms didn't have the plug and socket under the inlet manifold, and the lack of it is no bad thing, since it's not particularly useful, and has a fatal flaw in its design.
Now that you've told us the history and state of your loom, I can't make any meaningful comment on where the break in the gauge wire might be.
The gauge wire is brown/purple all the way, and goes to pin 26 of the blue plug to the instrument cluster.
How many pins did your original coolant sensor have? Unlikely, but some very early stuff used 2 pin ones, and it's just possible your loom could have been connected to the wrong one.

[Duncmasterflex]

That makes sense then, the pre-facelift not having the plug. The original sensor only had one pin too, so by the looks of it i need to keep tracing the brown/purple wire back to find the break, or alternatively run a completely”‹ new wire from pin 26 to the sensor??
Thanks again for your help and knowledge Bri