Replacing the Coolant Temperature Sensor

[LoveX1/9s]

I wanted to replace the sensor but it’s complicated. I wanted 2 fans and I had them all along but the sensor that is supposed to do the trick will not.

All indications are that the sensor is shot so the car will not start when hot (not saying that there aren’t other things and I have already fixed one).

The car starts when the gas pedal is depressed to the floor without pumping and turning the key. So, my air fuel ratio is off – faulty sensor, right???

I am no mechanic but I am becoming one – sort of.

As you can see, the sensor on the car is not the same as the one that’s supposed to replace it. I don’t know if I have 2 fans because I have air conditioning or if the radiator is aftermarket but what do I need to do to complete a simple job (Yes, the fuse is good)?

That said am I confusing the part with the Radiator Cooling Fan Switch??? I'm not a mechanic remember??? One repair book say's the sensor is on the back of the reaiator on the drivers side. Will doubble check with the shop manual too. BTW, isn't the sensor on the block some where on most cars??? Yes, I'm confused ;-)


This post is related to another post – Car will not start when warm.

 

[JimD]

There is a fan switch on the radiator, a water temperature gauge sending unit on the block and a coolant temperature sensor for FI screwed into the thermostat housing. What you have there (with the blue plastic connector) is that last item for the FI system.

That first picture looks like it is the fan switch. You should test the fans, check the grounds for the fans and make sure the fan relays are in the fuse tray. All those items, in addition to the fan switch, will affect the operation of the radiator fans.

 

[kmead]

I wanted to replace the sensor but it’s complicated. I wanted 2 fans and I had them all along but the sensor that is supposed to do the trick will not.

All indications are that the sensor is shot so the car will not start when hot (not saying that there aren’t other things and I have already fixed one).

The car starts when the gas pedal is depressed to the floor without pumping and turning the key. So, my air fuel ratio is off – faulty sensor, right???

I am no mechanic but I am becoming one – sort of.

As you can see, the sensor on the car is not the same as the one that’s supposed to replace it. I don’t know if I have 2 fans because I have air conditioning or if the radiator is aftermarket but what do I need to do to complete a simple job (Yes, the fuse is good)?

That said am I confusing the part with the Radiator Cooling Fan Switch??? I'm not a mechanic remember??? One repair book say's the sensor is on the back of the reaiator on the drivers side. Will doubble check with the shop manual too. BTW, isn't the sensor on the block some where on most cars??? Yes, I'm confused ;-)


This post is related to another post – Car will not start when warm.

You are talking about two different things.

Most cars up until quite recently have always had a simple thermostatic switch installed into the radiator to control the electric fan on the radiator. Newer cars have a sensor in the radiator but also use other sensors in the coolant stream for the engine ECU to gain an overall picture of engine temperature and directly control the relay for a fan.

The radiator on an X has a thermostatic switch in the lower left corner on the back. This switch completes the ground for the relay at a particular temperature, the relay then closes and powers the one fan. AC cars had two fans with the second fan for the AC controlled by switching the AC on which then closed a relay to provide current to the second fan.

If you no longer have AC but still have the second fan, the one switch can ground both relays and cause both fans to turn on at the same time and temperature. This would require some very minor wiring to make happen as the two relays are near each other.

There is another way to do this using a VW thermostatic switch to take the place of the Fiat one which has two sets of contacts and is able to turn on one fan at a lower temperature and the second fan at a higher temperature so you don’t waste electricity and don’t end up short cycling the fans. This approach requires repurposing an existing AC system wire to be moved to the new thermostatic switch at the radiator back to the AC relay position in the fuse box.

This same approach can be done with a late car (1979 or later) which never had AC as it has a place for the relay and much of the wiring exists in the car.

The nifty little switch you bought does go in the engine bay and is related to the fuel injection sensor system.

 

[LoveX1/9s]

There is a fan switch on the radiator, a water temperature gauge sending unit on the block and a coolant temperature sensor for FI screwed into the thermostat housing. What you have there is that last item for the FI system.

Thanks Jim... and LOL. I was looking on the fan switch all along. So, we can rule that out.
Looking for the placement of my part in the mechanics manual but it is a mystery.


I will order the 2nd part from Bayless but not sure where that is either.

 

[LoveX1/9s]

You are talking about two different things.

Most cars up until quite recently have always had a simple thermostatic switch installed into the radiator to control the electric fan on the radiator. Newer cars have a sensor in the radiator but also use other sensors in the coolant stream for the engine ECU to gain an overall picture of engine temperature and directly control the relay for a fan.

The radiator on an X has a thermostatic switch in the lower left corner on the back. This switch completes the ground for the relay at a particular temperature, the relay then closes and powers the one fan. AC cars had two fans with the second fan for the AC controlled by switching the AC on which then closed a relay to provide current to the second fan.

If you no longer have AC but still have the second fan, the one switch can ground both relays and cause both fans to turn on at the same time and temperature. This would require some very minor wiring to make happen as the two relays are near each other.

There is another way to do this using a VW thermostatic switch to take the place of the Fiat one which has two sets of contacts and is able to turn on one fan at a lower temperature and the second fan at a higher temperature so you don’t waste electricity and don’t end up short cycling the fans. This approach requires adding a new wire to be added from the thermostatic switch at the radiator back to the AC relay.

This same approach can be done with a late car (1979 or later) which never had AC as it has a place for the relay and much of the wiring exists in the car.

The nifty little switch you bought does go in the engine bay and is related to the fuel injection sensor system.

Thanks so much. I have an 85 with air which explains the 2 fans. Of course, the air does not work... Is the part visible and even a guy like me cannot look at the wrong part a 2nd time?

 

[kmead]

Thanks so much. I have an 85 with air which explains the 2 fans. Of course, the air does not work... Is the part visible and even a guy like me cannot look at the wrong part a 2nd time?

The thermostatic switch is extremely visible on the radiator and is in the first pic you posted, it is not the part you bought. The part you bought goes on the thermostat housing back in the engine bay.

The thermostat housing is at the left end of the engine, hanging above the transmission

Circled in the pic.

 

[JimD]

Thanks Jim... and LOL. I was looking on the fan switch all along. So, we can rule that out.
Looking for the placement of my part in the mechanics manual but it is a mystery.


I will order the 2nd part from Bayless but not sure where that is either.

Here are a couple pictures I just put together. Not great, but working with what I had on hand.

 

[myronx19]

Car will not start when warm.

so my car has this same issue. My guess is that you'll change the sensor, and the problem will persist. My problem got so bad that the car would not start at all (which sounds like the same issue you're having).

What would be helpful is to have a copy of the fuel injection service manual. I tried to attach it to this comment, but the file is too large. I'll gladly send it to someone who could post it up in the wiki section.

I've changed everything in my case, and nothing really helped. Usually rebuilding the injectors helps for a while - but the problem returns. My issue is vapour lock due to one or more injectors leaking down while off. I've never verified this - it's not easy to do (visual check anyway) - but a fuel pressure gauge would be helpful to diagnose.

I have a work around - I have a bypass switch that disables the "combi-relay" that's in the spare tire well. A guy I met with an injected Fiat Brava had the same issue, and he told me that he would disconnect the combi-relay, crank it to relieve the pressure, reconnect and crank and it would start. Sure enough, he's right. I installed a bypass button under the left of the dash so that I can trip it in order to get it started. Kinda a hokey solution, but it works.

I have a relay on the pink and black wire to the combination relay. I've been trying to find a solution for years - but really, I probably need a new set of injectors to solve my issue

Let me know if I can help.

 

[ng_randolph]

Thanks so much. I have an 85 with air which explains the 2 fans. Of course, the air does not work... Is the part visible and even a guy like me cannot look at the wrong part a 2nd time?

Have a look at this photo from the Midwest-Bayless site:

The big hole above the coolant port on the left is for the fan thermo-switch. The switch came in two varieties; early cars had a switch with two connector tabs on the back. Late cars (like yours) got a switch with wires coming out the back, and a connector at the end of the wires. The size / shape / thread size is the same between the two, the only difference is the electrical connections.

 

[Dan Sarandrea (Phila)]

Do you have a simple volt/ohm meter? Sometimes called a multimeter or VOM? If you don't then you should pick one up or borrow one. Even a cheapo from Harbor Freight is good enough for what we are doing.


Here is the FI troubleshooting guide on this site: https://xwebforums.com/wiki/index.php?title=FuelInjectionTroubleshooting

You really, really REALLY should start at step one. But, since you are asking about the coolant temperature sensor......

This is the test for the CTS (coolant temperature sensor):

Note that the drawing encircled in yellow is NOT for the X1/9. Karl and Jim have already posted excellent pix of where to find the CTS on the X1/9.

You may be wondering why the manual has you doing a sensor test from a point in the system that is nowhere near the sensor! That's a fair question with a very logical answer---testing the sensor's resistance NOT at the sensor, but at the place where the sensor's wiring feeds into the fuel injection control box, tests both the sensor and its wiring at the same time.

Also, take note of the fact that in the FI system on the X1/9 (it's called Bosch L-Jetronic), the Coolant Temperature Sensor is the sensor that has the most influence over the level of control that the FI control box can apply to this system. If the CTS is not right, your FI system will NEVER be right.

 

[LoveX1/9s]

Do you have a simple volt/ohm meter? Sometimes called a multimeter or VOM? If you don't then you should pick one up or borrow one. Even a cheapo from Harbor Freight is good enough for what we are doing.


Here is the FI troubleshooting guide on this site: https://xwebforums.com/wiki/index.php?title=FuelInjectionTroubleshooting

You really, really REALLY should start at step one. But, since you are asking about the coolant temperature sensor......

This is the test for the CTS (coolant temperature sensor):
View attachment 71640

Note that the drawing encircled in yellow is NOT for the X1/9. Karl and Jim have already posted excellent pix of where to find the CTS on the X1/9.

You may be wondering why the manual has you doing a sensor test from a point in the system that is nowhere near the sensor! That's a fair question with a very logical answer---testing the sensor's resistance NOT at the sensor, but at the place where the sensor's wiring feeds into the fuel injection control box, tests both the sensor and its wiring at the same time.

Also, take note of the fact that in the FI system on the X1/9 (it's called Bosch L-Jetronic), the Coolant Temperature Sensor is the sensor that has the most influence over the level of control that the FI control box can apply to this system. If the CTS is not right, your FI system will NEVER be right.

Hi Dan,

Working my way down the list but also taking side steps. In the middle of fixing my Thermostat issue now.

 

[dcioccarelli]

I have a work around - I have a bypass switch that disables the "combi-relay" that's in the spare tire well. A guy I met with an injected Fiat Brava had the same issue, and he told me that he would disconnect the combi-relay, crank it to relieve the pressure, reconnect and crank and it would start. Sure enough, he's right. I installed a bypass button under the left of the dash so that I can trip it in order to get it started. Kinda a hokey solution, but it works.

Well, not really relieve pressure but get rid of the unburnt fuel that has flooded the engine. You can achieve the same thing by disconnecting the fuel pump (or the fuel pump fuse) while cranking. You could even wire a normally closed push button in series with the pump so that you could hold it down when cranking under hot start conditions so no more fuel is added.

If this starts the car than it points to something flooding the engine. Could be leaky injectors, a leaky cold start injector or a faulty thermo-time switch.

Cheers,
Dom.

 

[Ics19]

a water temperature gauge sending unit on the block

Late to the party with a typo, but just for future reading reading reference for anyone else, the gauge sensor is in the head as per your excellent pictures

Replacing the Coolant Temperature Sensor

I wanted to replace the sensor but it’s complicated. I wanted 2 fans and I had them all along but the sensor that is supposed to do the trick will not. All indications are that the sensor is shot so the car will not start when hot (not saying that there aren’t other things and I have already...

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