[ugly mx6 guy]

Trying to weed out another gauge issue on my 91 MX6 GT project car. This time it's the coolant gauge and its odd behavior. The gauge itself now works (it originally didn't) but as the coolant temp rises, the needle will settle at exactly halfway between C and H and not budge even a fraction of an inch until I turn the key off. I have been driving GD cars for over 20 years now and I have never seen one that do that. I would expect the needle to move just a bit with whatever driving conditions I am experiencing, but not this one. I also am trying to relate this issue to when my fan is supposed to kick on. I know the temperature range the fan is supposed run, but I never hear it kick on in this car. Of course the fan runs fine when the A/C is on but that's a different scenario. I ordered an infrared temp gun to check what temp the car is running just sititng there...more info to follow based on those results.

 

[Mazda Carnage]

Trying to weed out another gauge issue on my 91 MX6 GT project car. This time it's the coolant gauge and its odd behavior. The gauge itself now works (it originally didn't) but as the coolant temp rises, the needle will settle at exactly halfway between C and H and not budge even a fraction of an inch until I turn the key off. I have been driving GD cars for over 20 years now and I have never seen one that do that. I would expect the needle to move just a bit with whatever driving conditions I am experiencing, but not this one. I also am trying to relate this issue to when my fan is supposed to kick on. I know the temperature range the fan is supposed run, but I never hear it kick on in this car. Of course the fan runs fine when the A/C is on but that's a different scenario. I ordered an infrared temp gun to check what temp the car is running just sititng there...more info to follow based on those results.

Why is every car related F2T issue you have with these cars never simple to figure out. Or is it that everything I can figure out, you can to and solve all the other issues yourself and never post them.

I have been thinking about what could cause the gauge not to sweep properly and what would cause the fan not to kick on.

At first I though maybe a ground somewhere was affecting them both but.

The cooling fan and temperature gauge are two separate systems.

Info and tests.
The cooling fan is triggered by a water thermo switch on the thermostat housing.
On the A-spec the thermo switch cuts ground at and above 97*C (177*F), On the B-spec the thermo switch supplies at and above 97*C.
The A-spec runs a Normally Closed fan relay and the ground signal from the thermo switch is used to keep the relay coil energized and the fan off. To test the cooling fan and relay, turn the key on and disconnect the fan thermo switch, the cooling fan should come on. To test the thermo switch, with engine temp at or above 177*F there should be no ground signal at the male spade connector on the thermo switch.
The B-spec runs a Normally Opened fan relay and the ground signal from the thermo switch is used to energize the relay coil and turn the fan on. To test the cooling fan and relay, turn the key on, disconnect the thermo switch and ground the pin in the single wire connector (harness side) to a ground source, the cooling fan should come on.
To test the thermo switch, with engine temp at or above 177*F there should be a ground signal from the single pin connector inside the thremo switch plastic.

Temperature gauge uses the straight pin thermo sensor next to the distributor, this sensor delivers a resistance to the ground signal it gets from the rear housing it's screwed into. As the temperature increases the resistance drops.
B-spec US: 215 Ohms at C, 8 Ohms at H.
B-spec Canada: 215 Ohms C to 15 Ohms H.
A-spec analogue: 233 Ohms C to 16 Ohms H.
A-spec Digital: 177 Ohms 1 dash to 17 Ohms 10 dashes.

I don't really know what effect adding resistance to the ground signal in a direct circuit 12V system but I have read that DC voltage flows from ground to positive (exactly the opposite direction every diagram shows) and will assume it has the same effect as adding resistance to the positive wire.
So to test the temperature gauge with a first gen, disconnect the coolant gauge thermo sensor, take an 8 Ohm resistor (5 or 10 watt, 430 /860 Milliamps, not sure of the amperage requirements of the ground signal going to the gauge but the only thing at risk is the resistor), (15ohms for the other types of gauges should be fine),connect one end of the resistor to the wire with metal loop that was connected to the thermo sensor and ground the other end of the resistor to an engine ground point (nut, bolt, housing you get a ground signal on a multi meter close to the sensor). Turn on the key and the temp gauge needle should be at H.
There is a good chance that the temp gauge wire could simply be grounded to get the temp gauge to read H (max) but for the cost of a resistor I wouldn't bother risking anything.

Also a potentiometer/variable rate resistor within the gauges specified resistance range could be used for full sweep gauge testing.
The factory workshop manual recommends holding the resistance steady for a couple minutes and make sure the gauge readings stay constant.

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Thermostat.
So what could cause both the cooling fan and temp gauge not to function properly at the same time.
Or what if they are both functioning properly, what could cause the temp gauge to never go past half way or even move and keep the cooling fan from coming on. Too cold rating on the thermostat, weak or tired thermostat that opens to far could keep the engine temps from reaching 177*F and turning on the fan.
A single stage low temp thermostat could cause both these symptoms also.
A properly working 2stage T-stat would definitely cause the temp needle to move between stages.

The 1992 Mx6 GT I drove years ago had a problem getting up to proper operating temp. It had to idle a wile and as soon as I would start driving it the temp on the gauge would drop and the car would go back into warmup mode and the cooling fan never came on. I dropped some coolant and pulled the thermostat housing expecting not to find a T-stat in there but it was there covered in a pile of red gasket maker and jammed opened. The gasket maker must of jammed up the passage enough for the coolant to reach 100*C (212* F) and open the thermostat fully, then jammed it there.

 

[ugly mx6 guy]

The coolant gauge side of this seems to be acting at least somewhat normally as best as I can tell. It still sets at the halfway point for most of my driving time but after the car setting still for several minutes it slowly creeps to the H side of the gauge face because the damn fan will not come on when it's supposed to. The fan motor itself is fine and I have tried two different fan switch sensors (one brand new OEM) but with zero luck. The relays are good - but my question would be should the connector to the number one fan switch be showing power when the key is turned on because mine does. Spec B GD cars are in my opinion are simpler than Spec A cars, but they are still different to me.

Just thought of this: I was looking at the WSM diagram of the path the cooling fan circuitry follows, and it actually goes through the ECU at one point. I wonder if this could be an ECU issue...just the ramblings of a tired old man.

 

[Mazda Carnage]

The relays are good - but my question would be should the connector to the number one fan switch be showing power when the key is turned on because mine does. Spec B GD cars are in my opinion are simpler than Spec A cars, but they are still different to me.

Do you mean the T-stat thermo switch for the fan when you say number one fan switch?
No it should not have power, it should only carry a ground signal from the thermo switch to the relay coil.

In my opinion from reading all the A and B spec diagrams and circuitry paths they are very similar, I find the A specs components more durable and some of the Bspec systems slightly more complicated but once you can read the circuitry diagrams they are both pretty simple.

Just thought of this: I was looking at the WSM diagram of the path the cooling fan circuitry follows, and it actually goes through the ECU at one point. I wonder if this could be an ECU issue...just the ramblings of a tired old man.

Yes the ground signal from the cooling fan thermoswitch Tee's to the ECU but the ECU can't rob or prevent the signal from going to the fan relay. It can either monitor the signal to know when the fan is on and off or send it's own ground signal to the relay. This would be a cool failsafe if the ECU triggered the cooling fan above a certain temperature if the engine started overheating unfortunately it doesn't, so it must just monitor the on/off signal.

 

[ugly mx6 guy]

Do you mean the T-stat thermo switch for the fan when you say number one fan switch?
No it should not have power, it should only carry a ground signal from the thermo switch to the relay coil.

In my opinion from reading all the A and B spec diagrams and circuitry paths they are very similar, I find the A specs components more durable and some of the Bspec systems slightly more complicated but once you can read the circuitry diagrams they are both pretty simple.

Yes, I meant the grey topped (for B-spec cars), number one switch. I definitely have a power signal to it. I am using an Innova test lamp that simply shows a red color for power, a green color for ground and white when the tester hooked up to battery but doesn't pick up a ground/power situation or not testing is taking place.

 

[ugly mx6 guy]

Yes the ground signal from the cooling fan thermoswitch Tee's to the ECU but the ECU can't rob or prevent the signal from going to the fan relay. It can either monitor the signal to know when the fan is on and off or send it's own ground signal to the relay. This would be a cool failsafe if the ECU triggered the cooling fan above a certain temperature if the engine started overheating unfortunately it doesn't, so it must just monitor the on/off signal.

Well, aren't you a buzzkill?

 

[Mazda Carnage]

Yes, I meant the grey topped (for B-spec cars), number one switch. I definitely have a power signal to it. I am using an Innova test lamp that simply shows a red color for power, a green color for ground and white when the tester hooked up to battery but doesn't pick up a ground/power situation or not testing is taking place.

I really don't think there should be a 12V signal at that connector.

The ignition doesn't switch any ground signal therefore the water thermoswitch must deliver ground to the relay coil, also a single wire switch can't deliver a 12V + signal, it would have to be delivered to the thermoswitch from a 2nd wire.

The manual states to supply a ground signal to the test connector to test the cooling fan relay and motor. Aspec have no test connector for the cooling fan because the relay is Normally Closed.
I do not recommend doing this if your finding a 12V+ signal in that wire, you might end up electrically soldering the ground to the test connector, or melting the wire or starting a fire (they don't fuse ground wires).

Well, aren't you a buzzkill?

Yes indeed, I am a certified boost cut and seat belt buzz killer.

Speaking of annoying sounds, the 88-92 626/MX6 has the least annoying turn signal sound of any car I have heard to date, a nice clicka, clicka sound, most cars signals make a plooka, plooka sound that highly irritates me. What kind of asshole builds a car that makes sounds to irritate the person driving it? But I guess nothing tops the old Chrysler Le Baron's, imagine someone doing paper of food delivery's in a talking Lebaron, If you ever drive by one that's burning on the side of the road that could be why.

 

[ugly mx6 guy]

Two different things: first, I stripped about 8 inches of black vinyl coating behind the affected connectors and found that someone has worked on the wires to both fan switch #1 and fan switch #2 and must have been color blind or thought it wouldn't matter if they didn't match wire colors when they went to solder things back together. I will have to fix their boo-boo when I have a chance. Second, I never really thought about how noisy the blinker is until you mentioned it. Sarcastically, it's its own warning beacon to the local and maybe aliens too.

 

[ugly mx6 guy]

Quick update: all seems well now as the backwards wiring is fixed and the fan now cycles on and off as it should. Interestingly enough, the coolant gauge seems to be unaffected by the fan when it comes on as the needle generally sits at the halfway point once the thermostat is fully open; given that halfway is withing spec I no concerns there.

 

[mx626]

Quick update: all seems well now as the backwards wiring is fixed and the fan now cycles on and off as it should. Interestingly enough, the coolant gauge seems to be unaffected by the fan when it comes on as the needle generally sits at the halfway point once the thermostat is fully open; given that halfway is withing spec I no concerns there.

Thanks for the update. Been quietly following this thread because my fan never shuts off and the coolant gauge is goes up to about 1/4 and never moves higher.

All of the wires going to the sensors on the thermostat housing are white. So you know that's professionally done. (Probably is even though I'm being sarcastic)