cylinder head temp sensor

tomk · 01-31-2013, 04:19 PM

hello, just finishing up a rebuild of a 1.7 and noticed something that is a bit concerning. i mounted the heads on the wrong sides, that is 1/2 head got mounted on the 3/4 side. i wasnt too concerned except that the temp sensor is now on the opposite head. i retained the d-jetronic and the engine is almost ready to start and I dont really want to take it apart and swap the heads around. the question is, does it matter? ( been following the thread on the trigger points, very helpful, great site and really find the technical correspondence first rate)

thanks for any comments in advance

TheCabinetmaker · 01-31-2013, 05:04 PM

No problem, you will need to extend the wire that goes to the sendor to reach, and you will need to cut a hole in the cylinder tin (sheetmetal cover) to allow access to the sendor. be sure to make the hole no smaller than the original one on the other side. The hole must be large enough to not only pass the wire, but also big enough to pass a 13 mm socket so that the sendor can be replaced when it fails. You dont want to be forced to remove the the engine tin to replace the sendor in the future.

an electricians chassis or hole punch is the best tool for cutting this hole. it will leave a nice round hole with no jagged edges. a hole saw would be a poor choice to cut this hole, and a big drill would likley tear up the metal pretty bad too. Use a hole punch (also known as a chassis punch.) These can be purchased at well stocked electrician supply store, or a ham radio hobby store, or mail order.

also be sure to block off the un-used hole in the tin on the original side. you dont want any loss of cooling air thru an un-used hole.

In short no problem !!!

tomk · 01-31-2013, 05:11 PM

many thanks for the excellent advice, i got me a greenlee punch for making holes, does a very, very clean job. once again, thanks

TheCabinetmaker · 01-31-2013, 05:22 PM

PS be sure to liberally coat the sendor's threads with antiseize compound, they can be a real pain to remove later after years on that cylinder head, I have had stuck ones shear off when i tried to remove them, and it is no easy task to remove them once the head is gone without removing the tin, even worse is if the soft aluminum head's threads get stripped, thus use liberal amounts of antiseize, you will be delighted you did if you ever have to replace that sendor, and they all will eventually fail.

They are also supposed to have a soft copper crush washer under them, This should help improve thermal and electrical conductivity by giving a larger surface area for transfer of heat and current

tomk · 02-01-2013, 02:36 AM

once again, thanks for all the great tips

SirAndy · 02-01-2013, 07:16 PM

Quote:

Originally Posted by TheCabinetmaker

In short no problem !!!

I disagree. The sensor is in the hottest spot on cylinder 3 for a reason.

If you're running it on the other side, i may very well not read correctly under load.

tomk · 02-01-2013, 08:20 PM

you could be right, however each cylinder is getting the same fuel, and doing the same work so the heat load should be the same, the oil cooler might have some effect in that it uses some of the air intended for cooling, i am going to seehow it runs and then decide, thanks for the thought though

TheCabinetmaker · 02-02-2013, 10:37 AM

Not enough differance to worry about.

The head temperature does go up under load, but any one of the four head locations could be hotter than the others. There is variation in the temperature based on the mixture which will vary with different injectors. The average D-jet system is not so finely tuned that you can assume cylinder #3 is always the hottest.

Secondly after some where around 300F, the temperature sendor resistance verses temperature graph levels off, there is very little resistance change at higher temperatures. The head temperature sendor does little mixture change at higher temps. The MPS is what is taking over as the main control sensor aftert the moto is warmed up..

Thirdly the ecu that senses the resistance of the head temperature sendor makes no more changes to the mixture at resistances below 300 ohms. This resistance is at about the 210F temperature, which is a pretty cool head. Heating the head more than that wont effect the mixture. Once you hit 210F there is no more variation in mixture based on the head temperature.

The head temp sendor is really there to give enrichment for warm up, it does not come into play at the higher operating temperatures.

Once the engine is warmed up, spikes in temperature because of the motor being put under load dont effect the mixture.

Granted there may be a slower response to warm up when the head temp sendor is placed on a cooler cylinder. But once warmed up there is no longer any effect.

Thus it is a non issue, good thought, but the hardware does not operate that way.