MK2 Cooling Fan / Temp Guage Issue

[Shane]

More testing.

Everything seems to be pointing to either the clock or the temp sensor being faulty (although new)

I've ordered new second hand clocks so will try them first and report back.

[snowbird1]

Jul 20, 2017 7:47:05 GMT Shane said:

Jul 20, 2017 6:12:58 GMT snowbird1 said:

Check that you are getting flow through the little 8 mm pipe from the rear of the cylinder head, it was blocked on mine.

Thanks mate, sounds like a stupid question but how can you check if there is flow? Cheers

Just pull off the line from the intake manifold side of the engine (when the engine is cool!), you should get a steady stream with the engine running. Coolant flows from the rear of the cylinder head through the oil filter heat exchanger and idle control.

[Shane]

Ok, so the new second hand cluster came today, plugged it in and all worked well apart from the bloody temperature gauge, still reading Hot.

The next thing on the list is the coolant temperature sensor. All though it is new this is the thing that determines what the gauge actually reads. When I wiggle it the temp gauge does jump about, is that normal?

An OEM sensor is £60+ so I'm going to try another aftermarket sensor at £25 first.

Failing that what's next?

[Shane]

Thanks snowbird1 , i just disconnected the line into the throttle body and water flows fine, like you said it goes from the rear of the cylinder head through the oil filter heat exchange and then into the idle control this suggest that there is no blockage and the water pump is working, this however is what I expected as the car is not actually overheating.

[gwnwar]

Shane.. Pull the connectors from the sensor and the cluster unit.. Then check for any ground OHM reading in the wire run to frame/engine for the wire that goes to the gauge. If you have any reading at all wire is grounded and should not be.
You are wigging the connector to the sensor to make it jump.. Hold the harness firm about an inch or two from the connector so it doesn't move wiggle connector see if it moves. if not I would look at harness.. have you looked at the connector good and sensor for bent contacts..

[Shane]

gwnwar , thanks, when you say "check for any ground OHM" do you mean place one of the multimeter probes in the VIOLET/WHITE wire in the sensor plug and then the other probe onto the chassis/engine and if i get a continuity reading that is bad i.e there is a short somewhere?

And then do the same the other end where the wire connects to the plug on the cluster? each end grounded should show no read?

Contacts on the connector look fine

Thanks mate

[gwnwar]

All you want to check is the wire its self. both ends of wire disconnected.. and check either end to frame/engine. you do not want a reading. What you are doing is checking to see if wire is pinched/grounded to frame. All the sender does is add a ground signal to the gauge to make it swing toward hot. The more ground the higher the gauge goes. Hope this is clearer then mud..

[Shane]

Tested and there are no shorts.

I have also replaced the temp sensor and still no joy.

I have spoken to a local MX5 specialist and he has seen this before. He advised to wire in a 12v Light bulb or a resistor in the wire from the sensor to the gauge which should keep the needle down as its restricting the current going to it. If that works I will bare with it until its on the road and then may take it to him to investigate further.

Still open to suggestions in the mean time!

Thanks

[gwnwar]

Time to go have a tea. I'll take mine cold and sweet.. Southern USA thing and it is 95deg outside..
When you say hot you do mean a full swing to hot end of the gauge swing..
The fan is coming on to cool the engine right.. It does not boil right..
The gauge should run show around around 1/2 way on gauge when normal temp.
The gauge is a kissing cousin to a dummy gauge. it shows cold or hot not degrees of hot. When engine starts to get warm the sensor grounds the gauge to make it swing. It is not like the older NA gauge that mover hotter/high as as engine ran to normal or hot..
Oil pressure gauge is the same it swings up when engine started IF there is pressure not how much pressure.

[Shane]

Jul 25, 2017 16:42:12 GMT gwnwar said:

Time to go have a tea. I'll take mine cold and sweet.. Southern USA thing and it is 95deg outside..
When you say hot you do mean a full swing to hot end of the gauge swing..
The fan is coming on to cool the engine right.. It does not boil right..
The gauge should run show around around 1/2 way on gauge when normal temp.
The gauge is a kissing cousin to a dummy gauge. it shows cold or hot not degrees of hot. When engine starts to get warm the sensor grounds the gauge to make it swing. It is not like the older NA gauge that mover hotter/high as as engine ran to normal or hot..
Oil pressure gauge is the same it swings up when engine started IF there is pressure not how much pressure.

I dont mind a cold tea

Yes, it swings off the gauge towards the 'H'

The water doesn't boil, the fan is coming on.

I appreciate its not an accurate measure of the heat I just wish it would stay half way and not swing off the gauge

Thanks gwnwar 

[gwnwar]

You said if you wiggle the sender connector the gauge jumps. Is this just the connector or wires too by connector..
With engine running hot and gauge full hot can you genteelly remove the connector from sensor with out making it jump.
if so is the gauge still at full hot. May have to try it a few times.. Will may take a few seconds for gauge to drop..
If gauge doesn't drop there is a problem in the harness.

[Shane]

Jul 26, 2017 16:58:23 GMT gwnwar said:

You said if you wiggle the sender connector the gauge jumps. Is this just the connector or wires too by connector..
With engine running hot and gauge full hot can you genteelly remove the connector from sensor with out making it jump.
if so is the gauge still at full hot. May have to try it a few times.. Will may take a few seconds for gauge to drop..
If gauge doesn't drop there is a problem in the harness.

As soon as the plug is removed while engine is running at hot the gauge drops to cold I.e stops sending a signal, has this one stumped you gwnwar?

[gwnwar]

Yes some what.. The wiggling of the wire changing the gauge swing.. If you move the wires around a little. and hold connector and twist wires by connector does it move.. Your heater loop hose is not kinked is it..

HAVE good 12v to B/Y wire at gauge cluster..
Good ground to B/L L=blue at gauge cluster. 0.00 OHM reading..
This wire goes from cluster to multi ground connector number 3 under dash 5 wires in 1 strip..
it also jumps to the number 1 multi ground strip in engine compartment..

[Shane]

Ok, so I took the advice from the local "mx5 specialist" which kind of worked!

The advice was to wire in a 12v bulb into the wire that goes from the temp gauge to the sensor. This acts as a resistor which holds the gauge half way.

But all is not perfect.

As the gauge creeps upto about 55-60% the fan turns on and as the relay "clicks" on the gauge jumps to 100% HOT.



The fan runs for about 3 minutes and the gauge creeps down from 100% HOT to about 80% then the fan turns off and when the relay "clicks again" to turn the fan off the temp gauge instantly drops down to 45-50% which is perfect.

It seems when the fan turns on it increases the current to the temp gauge.

What could that be?

My head is officially hurting gwnwar!

[gwnwar]

OK back again.. You put a lamp in the V/W wire??..
The V/W wire is a ground wire not a power wire.. Adding bulb adds resistance sent to gauge. more resistance high gauge goes..
Sender ads and pulls resistance to gauge..
Bulb adds resistance to wire it goes up. then the sender resistance is added in, gauge goes up and fan on..
As engine cools down with fan, sender sends less resistance to gauge and it goes down..
Can you see flow in radiator with cap off when hot..
What is the temp of the water..
Can't remember did you change the thermostat with OEM unit..
Did you check to see if the loop hose for heater tubes is not kinked..
My head is still spinning..