Cylinder head temp sensor test

[Decolliber]

Checking cylinder head temp sensor in 88 Carrera as per Bentley 240-18. Test is resistance across male terminals at white harness plug on left side of engine. My fixed harness connector (in the vertical locking plate) has only has one male connector, and a single wire (I think) line coming out the back, going to the sensor. I thought maybe one connector had been broken off but there is no sign of that. The detachable plug, however, has two female terminals in it. So female plug has two terminals; male connector has only one prong. Is there something wrong here? how do you do the resistance test?
Is it possible that a two wire temp sensor has been replaced by a single wire temp sensor, and the single wire is positive, so now the sensor is just grounded at the cylinder head? Has there been a design change not shown in Bentley? If so, I can test the sensor by measuring resistance between the single male terminal and a suitable ground?

[Early_S_Man]

Well, since the single-wire sensor is a known source of problems, why not just replaxe it?

http://www.pelicanparts.com/techarticles/911_cyl_head_temp_sensor/911_cyl_head_temp_sensor.htm


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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[Clark Griswald]

The original design was a single wire sensor. It used the threads of the sensor to make the ground connection.

This proved to be an unreliable way to make ground and a two wire sensor was developed and used in later cars. Some if not all single wire cars have two connections in the connector, like yours. That makes updating to the two wire sensor a lot easier.

This is a recommended upgrade since it is usually just a question of when your single wire sensor will fail. The part isn't that expensive so just bite the bullet and get it done.

To test the resistance of your single wire sensor, connect one probe of the ohm meter to the single contact in the connector and the other probe to ground.

The tech article is good, but no photos. I recently did the twin wire upgrade and took a bunch of pictures. You can see it here:

http://www.pelicanparts.com/ultimate/Forum3/HTML/011548.html

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Chuck Moreland '86 Cab - "Sparky", '77 Targa - "Sweet Pea"

[This message has been edited by Clark Griswald (edited 10-07-2001).]

[JDaniel]

Clark- Great write up. I'm going to change my head temp sensor as soon as it gets here on Tuesday (Damn holidays). I've had some "running" problems and decided to do the DME relay and Temp Sensor at the same time. One question. You mentioned a quick fix to get you home using an alligator clip. I'm assuming you'd connect it to the barrel of the sensor to properly ground it, and the shortcut someone else mentioned would not work for those of us with the 1 wire sensor?

Decolliber- what did you get for readings when you tested your sensor (and what temp)?





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Dan Tolley
1987 911 Coupe
http://www.cheaterswayside.com/911/gallery.asp?sort=0&userid=294

[Clark Griswald]

Actually, the easy way to get home with a bad sensor is to clip a resistor between the contacts at the connector. I think 1k ohm is about right, but Bentley should list the proper resistance. You are effectively simulating the sensor with the resistor.

Or course, if you have the foresite to carry the resistor, then probably also have the foresite to update to the two wire sensor.

Or maybe you get lucky and your car breaks down in front of Radio Shack.

[Decolliber]

I got 3.57 Kilo Ohms at about 50 degrees F., which is just about right, so the thing is working OK. But I guess I should get the new one.

[JDaniel]

I Got 3.2 (Hmm, that's funny) at about 50 degrees. I'm still replacing it.

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Dan Tolley
1987 911 Coupe
http://www.cheaterswayside.com/911/gallery.asp?sort=0&userid=294

[88911coupe]

Trying to track down a random "crank but no start" on my '88. I have the new 2 wire temp sensor installed since the rebuild about 2 years ago but wondering if I need to add this to my list of things still left to check. I have not seen the temp sensor listed as a cause of random crank but no start conditions but maybe I missed it...do I need to check and/or replace this? The RPM and reference sensors both tested perfectly per the data I've seen so they appear to be fine (they were replaced in the rebuild as well). Thanks

[kidrock]

Probably not the cause of your no-start issue, but you can do a cheap easy test on the CHT sensor by disconnecting it from the 3 plug harness and inserting a U-shaped paper clip into the female connector. This serves as a jumper or bypass of sorts and wii quickly tell you if the sensor is shot.

[Jcslocum]

This sounds more like a DME relay failure or failing as they can be intermittent.

[88911coupe]

Thanks for the reply. The DME Relay is new but I have heard of them failing out of the box so that is on my list. So far I've checked the speed and reference sensors and they both test perfect (both about 18 months old, replaced along with temp sensor when engine rebuilt). Very frustrating. Not sure if it matters but in both cases the crank/no start happened when I tried to restart the car after about a 30 minute drive coming home from work. When running it ran perfect but when I went back about 10-15 minutes later it would crank fine but just not fire up, then after I grabbed the relay and cable it started.

[Jcslocum]

DME Relay Information and Testing

The 944 uses the same relay. You can build the jumper to make it work.

[scarceller]

With key in 'RUN' does the idle control valve hum and vibrate?
Start here, report back.

Random no starts are often failing DME solder joints, I'm repairing more and more of these DMEs these days.

CHT sensors even if bad won't cause a total no start, the engine would still catch and die.