Head Temperature Sensor Replacement

[Clark Griswald]

The '80s Carrera head temperature sensor is a well known problem area. The factory shipped it with a single wire unit that relied on the threads of the sensor itself to provide ground. Unfortunatly, this has proven to be unreliable and these things fail routinely.

There is an upgraded part available that has two wires. The second wire provides a ground and proves to be much more reliable.

This is a smart preventive maintence upgrade to avoid trouble before it happens.

Pelican has a tech article describing the replacement. See http://www.pelicanparts.com/techarticles/911_cyl_head_temp_sensor/911_cyl_head_t emp_sensor.htm

But no pictures! So here I have documented my own replacement procedure.

To gain access to the sensor, jack up the driver's side of the car and remove the rear wheel. Here is a picture of the sensor hiding behind the shock absorber, just ahead of the lower valve cover. The white wire is connected to the sensor through a grommet in the sheet metal.


I opted to remove the shock to get better access. I have SSIs which made access from the front impossible. If you have regular heat exchangers, the removal of the shock may not be required.

The sensor screws into the head of the no. 3 cylinder and has a wire that passes through the sheet metal and up to the top of the engine. The sensor is accessible from the front of the engine. It is behind a grommet that the temperature sensor wire passes through. Remove the grommet to get access, just pry it out with a screw driver.

Here is a picture of the grommet after removal. That part that appears white is the head, and the round thing is the sensor which screws into the head in the same fashion as a spark plug.


The sensor wire then passes back through the sheet metal to route to the top of the engine. Again, passing through a grommet. Here is the grommet at the top of the white wire.

Pry out this upper grommet as well. Two other wires share that grommet, just set them aside.

Now move to the topside, inside the engine compartment. The wire routes up near the intake manifold to a connector. In the following picture you see the white wire from the sensor attached to an off-white connector. The connector mounts in a bracket that holds two other connectors. You need to remove the head temperature sensor connector from the bracket.

To remove the head temperature sensor from the bracket, use a pliers to pull a "key" out of the bracket. I don't know what else to call it but a key, look at this picture.


Here is a shot of the "key" after removal.


Once you have the key out, the off-white connector pulls right out of the bracket. You are ready to install the new sensor and wire.

Move back to the underside of the car.

Cut the old wire off the old sensor. Then use a socket wrench with extension to remove the sensor.

Here is the new sensor and wire. Note that it includes new grommets.


Since the sensor wire is hard to reach behind the manifold, I used the old wire to pull the new one through the sheet metal and back to the topside. I taped the new one to the old as follows:


Thread the new sensor into the head. Use a set of long-nose pliers to tighten it down. Apparently there is a special socket for this that accomodates the wire. Get it if you can, else just use long nose pliers. It doesn't have to be very tight.

Note the sensor has some compound on it. Probably an anti-seize and heat conducting grease. Try not to wipe it off.

Return to the top side of the engine and pull the wire up using the old wire that you attached to the new wire.

My car had a ground already available on the connector that the new sensor wire will attach to. This may be true of all Carreras but look to be sure that your car has it. Here is a picture of the connector with two wires:


Put the sensor wire connector into the bracket, then slide the "key" back in to hold it in place. Now snap the connector back on.

You are done. Fire it up and make sure it is running right.

Now take it out and drive it hard, the way it was meant to driven, with confidence that you won't have any head temperature sensor problems.
----------------
Chuck Moreland '86 Cab - "Sparky", '77 Targa - "Sweet Pea"


[This message has been edited by Clark Griswald (edited 07-28-2001).]

[Jorgeman]

Clark--nifty write-up on replacing the temp. sensor. A suggestion: If you are caught out on the road with the bad sensor and a lot of backfiring and run problems--grab a alligator clipped lead from that multi-meter you ALWAYS carry and clip it to the metal barrel of the sensor and ground the other end somewhere in the engine room. It'll get you home in a pinch.

George 86T

[Clark Griswald]

Jorgeman, good tip. If you need to do that roadside emergency, attach the jumper clip up at the connector. It is much easier to reach and doesn't require pulling the wheel going head first under the car to reach the sensor.

[Kurt S]

Maybe I haven't looked hard enough but I can't seem to find a cylinder head temp sensor that looks like the one in the above pictures on any vendor's web site or catalog. Anyone able to point me in the right direction of where to get it and perhaps even a part number?

Thanks, as usual, for the assistance!

Kurt S

[marcesq]

Kurt:

http://www.pelicanparts.com/catalog/shopcart/911M/por_911M_FUELMO_main.htm

You up for joining a bunch of other NoVA Pelican heads for a fun run this month?? Let me know.

[Kurt S]

Maybe When?

Kurt

[jwill911]

Just did this job my self recently. Not terribly difficult (turned out not to be the source of my problem, but that's another story).
If you need to change one and don't have the special porsche tool, you can make one from a 14mm deep socket and ~15 minutes of work with a die grinder. I think Sears sells a 14mm for ~$4. Just grind ~3/8" slot from top to almost the bottom and it works like a champ. Then add it to your drawer of special tools you'll never need again.

John
`87 Carrera with new:
head temperature sensor
and both flywheel sensors <- they fixed it

[Kurt S]

Well I took the plunge today and purchased both a new 2-wire CHT sensor and a new DME relay. Changing the sensor was not too difficult, for me the worst part was removing the grommet with the three wires in it, it was just hard to get a good grasp in there to pry it out. Once out however I found that the sheetmetal around it interferred with getting a straight shot in to the sensor with a socket. The angle was wrong and I had to bend the sheetmetal a bit in order to fit the deep socket over the sensor. On my car the sensor turned out however not to be a 14mm but a 13mm.

Regardless, it took about two hours to to it all from jacking it up to rolling it back out the garage. Didn't have a chance to test drive it long enough to really tell if the hesitation problem was gone but it didn't do it for the few miles of my test drive. Time will tell.

Interestingly enough the local shop I purchased the parts from thought that my symptoms just might be cured by the new CHT sensor but gave me a puzzled look when I said that I'd heard a new DME relay might also cure my ills. The shop said a DME relay will often cure a car that doesn't start but that it only sends power to the computer and shouldn't affect the way the car ran.

But, they said try it, you never know.

Anyway, thanks for the tech tips and the replies to my question, as always they were very helpful and informative.

Regards

Kurt S

[rbcsaver]

I am in the process of this replacement on my 84 3.2 Carrera as I write, so I can only add more info to this point of the project. It seems that someone previosly replaced the sensor and crossthreaded the threads on the cylinder. I'll have to sort that out tomorrow (8/12/03). The old sensor in my car is the single wire type, and is 13 mm. The new sensor from Porsche carries the 930.606.915.00 designation and is 14 mm.

The metal pan around the back of the motor seems impossible to remove and makes visualizatiion/access very difficult.
I was able to bend a portion out of the way.



(spelling ed. 4/18/06)

[dwightp]

rbcsaver:

You are correct. There are 2 different thread sizes. I ordered a recommended part number only to find out that it was the wrong thread size!!!! I ended up buying another that was correct size. There are 13 AND 14mm 2 lead units available.

Maybe someone out there who is a parts guru can give a detail of what size corresponds to what year AND the 2 different part numbers (sizes) for the replacements.

[rbcsaver]

I don't think that the "threads" are different sizes. Putting the side by side the threads appear identical. My guess is that the old single wire units are the 13 mm and the new 2 wire units are bigger to accommodate the ground wire changes. I ordered mine direct from the Porsche dealer.


(spelling/ typo edit 4/18/06)

[dwightp]

I can't honestly say that the tread pitch was different between the 2 but the major diameters were. Both of the new ones I ended up with were 2 wire. I wish I kept notes on the part numbers.

[ad7tcar]

Just replaced this sensor. Not bad, couple of hours total with a couple of breaks. It sure did the trick. The car runs 100% better at all speeds and no lurching and missing anymore. I just hope the sensor is tight enough, as I also used the needle nose pliers (vice grips) in lieu of the special socket.

I have a suggestion or two. When removing the grommet to the engine compartment, I took a small allen wrench and slid the short end into one of the wire slits, then turned it across the slit. Then I latched onto the long end with the vice grips, straight into the jaws. After a few tugs and some arm bruises, (it's mighty tight in there) the grommet popped out.

On replacing the grommet, after the wires are all put where they should be, I started the grommet in the hole, then sprayed a bit of white lithium grease on the area. I took a 12" long, 3/8" socket extension, and pushed the rest of the grommet home. I would imagine there are a lot of tools that would work for that.
Hope this helps a bit
Tim

[Kraut Wagon]

Great post. Thanks for the 'heads up' (no pun intended)

Ouch $61 bucks for that little thing...but most likely worth it so Im gonna do it.

Looks like the price of DME relay has dropped recently here at our host too...

[panel]

Is this for CIS 911 cars?

[Quicksilver]

This is for the 84+ Motronic cars.

Wayne

[autofil]

Hia!
Thanks to Clark Griswald for a brilliant Thread!
Have done the replacement today in a few hours! The car is running like it should, and hopefully it will pass the Norwegian CO test, witch was why I did the replacement.

[rbcsaver]

Originally posted by ad7tcar

On replacing the grommet, after the wires are all put where they should be, I started the grommet in the hole, then sprayed a bit of white lithium grease on the area....SNIP
Hope this helps a bit
Tim

Wow, interesting to see my words (mispelled....LOL) and issues of the day resurface years later. Isn't the internet
and all it's resources wonderful?

I have always been leery of using petroleum products on rubber (or unknown similar compositions) products. If the O ring doesn't deteriorate, the grease will always be "there" and possibly inadvertently assist in the O ring being dislodged. I personally prefer a soapy water solution on the rubber O ring in firewalls, wheel wells etc.
regards,
Hope to see y'all at the Hershey Swap this weekend (4/22/06) and 2008

regards
Steve

[cicsprog]

I must be having a bad day or its the beer I just drank. Read this post and checked CHT plug. Mine looks like the above in the last pic in the thread. How can I confirm it's the two wire?

Jason
88 Cab

[Wil Ferch]

See the pic of the greasy finger holding up the plug?...as you view inside...if you find two metallic thingies...it's a two wire system.

If..OTOH...you only see one metallic wire "end" and the other end inside the plug is plastic, or lost-in-action...it's a single wire plug.

- Wil