Engine Temp Sensor Location

noincome · Post by **noincome** » Sat Jun 23, 2001 9:44 am

I'm in the process of getting everything set up for fuel injection. My ECU should be here any day now. Where have you guys mounted your engine temp sensor? I've seen a few mounted in the oil sump, the instructions for the ECU just say mount it in the block of an air cooled engine.

Post by **Tom Notch** » Sat Jun 23, 2001 10:12 am

Do a search on this forum and you will find this topic quite well covered. It's buried in the T4 section somewhere.

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Tom Notch
Tom's Old VW Home

Steve C · Post by **Steve C** » Sun Jun 24, 2001 4:09 am

Hi
My friend Liegh ran his in his sump and has great success with it there, he used the blank off plate for the Type 3 filler tube. Before he ran it an aircooled Baywindow sensor which proved to be very eratic. His usual oil operating temp is around 85 - 90 celcius which works perfectly with his ECU.
Regards Steve C

Chuck Schneider · Post by **Chuck Schneider** » Sun Jun 24, 2001 3:56 pm

What sensor are you talking about. The engine temp sensor usually reads the cylinder head temp, CHT sensor for short.
I wouldn't run it in the sump it isn't an oil temp sensor so why would you put it there.
The engine temp sensor is for the computer to adjust the mixture based on the engine temp if it is in the oil it will take alot longer to get to operating temperature than the rest of the engine,(heads). This could lead to poor cold starting and cold running performance. This could really be a problem if you have a huge ass oil cooler that is cooling the oil to 180 degrees and the heads are running at 300 degrees. The result an overly rich mixture and poor performance.

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12.70 @ 104

noincome · Post by **noincome** » Sun Jun 24, 2001 7:17 pm

The engine temp sensor is normally used to sense coolant temps. In the air cooled application it senses the oil temp, thats why some people run it in the sump.

Steve C · Post by **Steve C** » Mon Jun 25, 2001 6:14 am

Hi noincome
I assumed that you were asking about the sensor location for an after market ECU, was I correct? I have a Wasserboxer motor in my bug and the coolant temp runs about 95 - 100 degrees celcius, my friends aircooled bug is using the same ECU (Autronic SMC) and as I posted before he is now running a coolant temp sensor in his sump. Your ECU should have a provision to cut out cold start enrichment at a certain point. You could also fit it in your oil pressure relief gallery plug. As Chuck mentioned the head temps have a to great a variation for most ECUs as they mostly designed for use with a watercooled motor.
Regards Steve C

noincome · Post by **noincome** » Mon Jun 25, 2001 9:30 am

Steve,

You were correct in your assumption that I was asking about the sensor location for an after market ECU. For everyone's clarification I'm not trying to use something that was not intended for that purpose. The instructions for the ECU states, "The sensor measures the temp of water and or oil. Ideally mounted in the head of water cooled engines, and the block of an air cooled engine. The sensor supplied has a metric 6 X 1 thread. This sensor measures the engine temp and compensates for cold start, easier and better idling when hot or cold". I've called, and E-mailed several people, and they recommended that I place the sensor in the sump, and I've seen several run this way in a few of the mags that I have. However I feel that there may be a better location to mount the sensor, or to use an CHT sensor. Chuck's comment about it will take it a lot longer to get to operating temperature than the rest of the engine is duly noted. That's why I posted here to see if any of you guys that have real world experience with EFI had any insight on this.

Thanks

Post by **Tom Notch** » Mon Jun 25, 2001 10:14 am

Somewhere buried in the t4 forum is my experiences and a few others with the coolant temp location. I prefer monitoring the air temp. coming off the heads. Much quicker rate of change compared to the oil. and the head temps are what determines how it runs anyway.

just my experiences

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Tom Notch
Tom's Old VW Home

Sharkey · Post by **Sharkey** » Tue Jul 17, 2001 12:49 am

Monitoring oil temperature and water temperature is two different things, and any computer that is designed to monitor one will behave strangely when asked to monitor the other instead. Water temperature climbs quite quickly initially due to a lack of circulation within the water jacket. Oil provides a much more important duty and must circulate at all times. Water completely surrounds each cylinder and combustion chamber on a waterpumper, while the only heat your oil really sees is what it absorbs when in contact with the valve area and the piston undersides. Translation: it takes oil temperatures much longer to rise.

I use a VDO oil temperature gauge to monitor my sump temperature and use my Wolf 3D (via another sensor) to tell my computer what temperature my oil is. The water/oil temperature sensors I am using are also VDO and operate to a maximum of 150 Celsius (as recommended by my Wolf 3D dealer). Any VDO sensor designed with a 150C max reading will work, and thankfully VDO makes all sorts for us ACVW owners (including standard 1/8", 1/4", and 3/8" NPT sensors, drain plug sensors, oil relief valve sensors, and dipstick sensors).

At first I installed my computer's oil temp sensor in-line with my external oil cooler. Problem was, it took too long for my oil to get hot enough to get my computer out of "cold running" mode. Not only was my computer providing unnecessary additional enrichment for nearly all of my 10 minute drive to work, but it wouldn't enter closed loop mode until the sensor detected a sufficiently high temperature. My fuel economy suffered.

The obvious solution was to drill and tap a hole in the side of the head so that the tip of the sensor was exposed to the splashing oil within the driver's side valve cover. The hole was drilled into the *head* just below the spring bale dimple, and the sensor easily clears the #3 exhaust valve spring. Problem now is that the measured temperature might be climbing too fast.

That isn't the major issue with me, however. It seems that my engine likes to operate with a temperature within the valve area at approximately 118-120 Celsius. This is just beyond the range that my Wolf 3D will allow me to use regarding my external oil cooler's cooling fan relay. I've added a 10 ohm (or was it 20 ohm?) resistor in series with the sensor wire to trick my computer into thinking it is about 10C cooler than it really is, which allows my fan to turn on and off on its own now.

That hasn't completely cured my problem, however, since the engine often reaches its steady running temperature and simply sits there (according to this sensor location). If I get hard on the throttle my computer might report a temperature increase of 1-2C, but it really doesn't go much beyond that. I have two choices:

a) set the computer to turn on the fan at 109C (approximately 119C real temp) and have the fan remain on pretty much all the time once that has happened, or

b) set it at a higher temp so that only hard driving turns the fan on (unfortunately, I have to manually turn the fan on if I'm just cruising sometimes or the oil temp will climb too high).

In the months of driving that followed this decision, my fan either comes on and stays on for the entire trip (trigger temp set too low), or doesn't come on at all and I have to turn the fan on manually (trigger temp set too high). We're only talking about 1-2C difference here, so I haven't been able to find a setting that works best for me yet.

But I've got an idea. I've got a bunch of days off in a row coming up towards the end of this week and the beginning of the next, so I plan to run two oil temperature sensors in parallel. Since each sensor is nothing more than a temperature sensitive variable resistor, wiring them in parallel will effectively half their resistance-- assuming both sensors are exposed to the same temperature.

I've done some experimenting and done the math as well, and I feel that two temperature sensors wired in parallel, with a 100 ohm resistor wired in series with the sensor located in the cylinder head, should provide me with the necessary behaviour I need. It will still be pretty accurate in the mid- to upper-temperature range where it matters most. For example, if both sensors measure 5C, the computer will see about 19C (that's with the 100 ohm resistor wired in series with the second sensor). At 20C, the computer will see 35C.

Where this idea really starts to shine is when one sensor reports one temperature and the second one reports a much higher temperature. If the first sensor is installed either in the sump or near inline with an external oil filter/cooler, we can monitor a temperature much closer to the true operating range of the entire engine. If the second sensor is installed in the head as I described above, we get an accurate reading of what the temperature is within the valve area of the cylinder head only-- which is usually much higher due to the proximity of the exhaust port and combustion chamber. If the first sensor reports 25C and the second reports 70C, my math says the computer will instead read 63C. 30C/110C translates to 82C, 80C/120C equals 101C, and 100C/120C equalls 109C. It isn't exact, but it does get me out of "cold enrichment" mode a lot sooner and still provides me with a sufficient temperature span at the high end so that I can fine tune exactly when my cooling fan turns on and off.

I realize that the above scenario may work for me and only for me, but I thought I'd share. It should work for anyone using a Wolf 3D. It should work for other aftermarket EFI computers too, since most of them use the same type of sensor (and digest the relative data in the same manner).

[This message has been edited by Sharkey (edited 07-17-2001).]

Chuck Schneider · Post by **Chuck Schneider** » Tue Jul 17, 2001 9:52 pm

Sorry I am just used to SDS. SDS is very flexable and uses a Cylinder Head Temp sensor to tell the computer engine temp in aircooled applications.

Steve Arndt · Post by **Steve Arndt** » Wed Jul 18, 2001 10:18 am

In my Autronic SMC manual is states something like if you are using an aircooled engine then use oil temp instead of water temp, but raise the parameters before the overtemp limp home mode initiates. Their overtemp warning and limp home starts at 200 by default for water. Change it to 230 for oil.
It uses intake air temp and oil temp. My 103 cylinders and big heads w/ high compression will warm up the oil plenty quickly

Steve

Sharkey · Post by **Sharkey** » Sat Sep 01, 2001 2:00 pm

Just a quick post to let people know that my parallel sensor idea works like a charm. My Wolf is predominantly designed for watercooled engines (which isn't a bad thing, considering that less than 0.1% of all engines made are aircooled) so the temperature range that the sensor input is used to seeing max'es out around 120 Celsius (which would obviously be too hot for a watercooled engine).

I've installed one of my sensors in the side of the cylinder head and run a 100 ohm resistor in series with it. My second sensor is tee'd into the oil flow to my remote oil cooler (with NO resistor). The two sensors are then run in parallel, and work perfectly well. Currently, my electric cooling fan comes on at 105C and shuts off at 103C.

noincome · Post by **noincome** » Sat Sep 01, 2001 2:21 pm

That's good news Sharkey! Hey quick question for you. I was checking out your page and wanted to know if you machined your own distributor block off or did you have it done for you? What was the cost, if ya don't mind me asking?

Cheers