Burst heater hose now MS thinks coolant is 77C all the time
 

I blew the heater return hose but was carrying water so I got home with no overheating.

I've replaced the hose and my gauge cluster temp still works correctly (always did) but now MT and TS both show coolant at 77C all the time.

You can imagine that this makes starting very hard even with cranking tied to MAT but once the car is warm there are no problems unless you shut it off and expect it to start again. The hot restarts are much easier than the cold starts but not good.

My question is, is my sensor done or did the absence of water around the sensor screw my therm files?

If the sensor is bad is there more than 1 coolant sensor on a 97 engine? Dealer says 1 but wasn't sure since they thought the one back in the heater pipe was a sender for the gauge (that still is working correctly).

I ask because my major experience with bp cooling electronics is on the old style wax pellet sender + water temp sensor setup on a 94 protege bp.

Please help as this is delaying my turbo install :vash:

The files will be OK, check the engine.

The gauge and ECU are on separate sensors.

I replaced the coolant temp sensor after confirming that unplugging it results in -40C in MT and TS. After the new $65 sensor I get the same thing, 77 all the time and -40 when unplugged.

I unplugged the small middle plug from the MS adapter harness (I've had issues with certain pins) and turned the ignition to on and found the coolant temp went slowly down to -8C. The actual temp was probably 40-50C as it was partially warm. I plugged that plug back in with the ignition on and the coolant went right to 77C.

Any idea's? The sensor wiring didn't appear damaged and I don't have any broken connections in the MS adapter harness.

Like I said earlier, all this came up immediately after blowing the heater hose leading to the coolant sensor.

Measure resistance at the ECM and see what your therm file says the temp should be. If they match then it is not your ECM and is wiring or sensor issue. If they don't match reload everything?

My therm files are on the computer I'm typing on and they say that static 77C = sensor failure.

I'll have to check out the car tomorrow. I didn't know these files were legible until now, I'd assumed there were hex code or something.

I'd check the wires. I would guess that "sensor failure" is some resistance reading that it outside what it is expecting to see. But I'm a MS newb.

So here's the scoop, both clt sensors read correctly (correct range and ohm vs temp) and they read the same at the sensor or at the yellow clt wire going into the MS.

So it's software or hardware in the MS.

Side bar: the clt reading goes from static 77C to static 102C with the multi meter connected to the wire.

Attached is my therm factor INC file...

Mine did the same thing after installing my coolant reroute. Mine was stuck on 170F, which equates to 77C. It turns out while trying to unplug sensor I managed pull the wire partially out of the plug. Maybe you bumped the connector while installing the new heater hose. I'm not sure how it is on a 1.8 but on my 1.6 I was able to squeeze a finger or two under the coil and wiggle the connector to see the reading jump around like crazy.

Today I went so far as to warm up the car, shut it off, remove the case from my MS, and compare reading off the clt sensor and where the clt temp signal enters the MS PCB.

Readings were dead even and resistance went up as the engine cooled. The values were consistent with the therm factor file.

I couldn't trace the CLT circuit as it goes under the map sensor and I have no desire to remove it, also I don't know what the value changes should be. There must be something wrong hardware / software wise but I don't know enough about this unit to diagnose it (I bought it pre-built).

At Scot's suggestion I jumped the sensor pigtail I got 77C but when I removed the jumper it went to 45C and slowly went down from there. This is all with the senor unplugged.

The temp settled at 24C with a cold engine (not run for 24hrs) in 15C ambient.

I plugged the sensor back in and got 77C again.

Again the old sensor gives the same reading from it's pins as the brand new sensor, within ~2.3%, at any given temp.

WTF the car magicaly fixed it's self?!
 

After reassembling the MS I started the car up and checked sensor readings with TS, same old 77C. I then went on a 50km drive on some twisty country roads.

When I got home I heard my fan on so I got the lap top and checked it again....93C...... After confirming readings went up and down with temp and that they were accurate so I went for another drive and the car ran flawlessly.

For some reason it's working now and I hate that I don't know why. I didn't touch anything in the MS it's got the 13yr old CLT sensor in it EXACTLY as it was prior to having a fit and it's working perfectly. Unless the MS just wanted out of it's case for fresh air and this was it's way of saying so I simply don't get it.

My guess is a loose connection somewhere.

This may be a long shot (and I don't know exactly where the sensors are on a Miata I haven't had mine for long enough to figure it out) but is it possible that the sensor wasn't getting submerged? Some sort of blockage or low coolant level?

I bring it up only because I owned an FC that someone else put a Haltech in and they put the coolant temp sender in such a place that unless the coolant was absolutely full it wouldn't get fully submerged and constantly gave weird readings.