Coolant Temp Sensor Identification

[mtbtim]

Hey Guys,

I've searched for a definitive answer but haven't found it yet so I'm asking for help. On my 85 1st Gen, the motor was swapped and I'm not sure what year it came from. As far as I know, I have 3 coolant temp sensors.

The one that sends information to the gauge I replaced already and that's the gauge on the intake manifold between the #2 and #3 cylinders.

There's one that screws into the water neck, pictured below. It has a single black wire running to it and it's a single prong connector. People are saying this is possibly for the AC system. Is this true?



The 3rd sensor is under the water neck next to the cold start solenoid (Green Connector). As I understand it, this is the one that communicates with the ECU. Is this correct?



The reason why I'm asking is I've been have some cold start issues and have read it could be a problem with the cold start solenoid sending a signal to the cold start injector or it could be a problem with the temp sending unit that communicates with the ECU.

Help clarifying this would be much appreciated. Thanks!

 

[CCDave]

For troubleshooting a cold start issue, start by checking the ohms on the cold start time switch (brown one under the water neck in your last photo). Also check the ohms on the Cold Start Injector. More info on both of these tests are here: LC ENGINEERING Tech Notes

 

[mtbtim]

For troubleshooting a cold start issue, start by checking the ohms on the cold start time switch (brown one under the water neck in your last photo). Also check the ohms on the Cold Start Injector. More info on both of these tests are here: LC ENGINEERING Tech Notes

I actually checked the cold start injector, cold start time switch and the water temp sensor yesterday.

The injector ohms is within spec and when I put 12v of power to it, the injector fires like it should.

For the Cold Start Switch, I followed a video showing how to test it. I checked the ohms with the probe submerged in room temperature water and also checked it submerged in 180 degree water. The ohms were within spec for both tests.

I did the same for the water temp sensor that's next to the cold start switch and it also showed within spec submerged in room temp and 180 degree temp water.

I took out the cold start injector and hooked it up backwards onto the fuel line facing the driver side of the engine compartment. I didn't want to tweak the metal fuel line enough to see if the injector was firing in the regular orientation. The engine was cold and I had my girlfriend turn the engine over. No spray of fuel came out. I next disconnected the connector to the injector and hooked my multimeter up to it while my girlfriend turned the motor over. I didn't get any power to the plug.

Since the cold start switch checked out fine and there's no power being sent to the injector on a cold start, that leaves me with the possibility of a wiring problem. But, before I start ripping the engine apart to expose the wiring harness, I'm just going to swap out the cold start switch and see if it fixes the issue. I did read somewhere that a guy tested his cold start switch like I did, found it tested fine based off the ohms specs in the factory service manual and assumed it had to be something else. When he ran out of possibilities and was super frustrated that he couldn't find the fix, he decided to replace the cold start switch anyway and it fixed his problem. So, it sounds like this component can possibly test fine but still be bad. It's a pricey part, but I'd rather drop $150 at this point for a chance it will fix the issue rather than spend the time to tear apart the engine to investigate a wiring problem.

 

[Cali 4runner]

Currently rebuilding a 22re and dealing with these sensors.

Coolant temp sensor on the top of the intake is for the instrument cluster.

Sensor in the coolant neck is for the fuel pressure up system, not AC. Also communicates with ecu.

Sensor on the front of the engine communicates with the ecu.

There is a factory service bulletin about a hot start issue, not a cold start issue.

 

[mtbtim]

Currently rebuilding a 22re and dealing with these sensors.

Coolant temp sensor on the top of the intake is for the instrument cluster.

Sensor in the coolant neck is for the fuel pressure up system, not AC. Also communicates with ecu.

Sensor on the front of the engine communicates with the ecu.

There is a factory service bulletin about a hot start issue, not a cold start issue.

Thanks for this response. For the coolant neck sensor, what exactly is it's purpose? I've never heard of a Fuel Pressure Up System. I've heard of a Fuel Pressure Regulator but this has me a bit confused. I wonder if a failure of this sensor could cause a cold start issue.

 

[Cali 4runner]

Sorry for the slow response. Here is some more info.

AC Shutoff Coolant Temp Sensor - confirm please

 

[mtbtim]

Sorry for the slow response. Here is some more info.

AC Shutoff Coolant Temp Sensor - confirm please

It seems the water neck temp sensor might only be on 4wd models. It doesn't have anything to do with the AC system but does something else nobody seems quite sure of. Where did you read it's for the Fuel Pressure Up System?

Here's a link: 86 22re 4wd thermal temp switch........