Heat sensing thermocouples

[chawk_man]

My Honda 225 outboard has an overheat alarm. If it trips, it shuts down the engine with no warning. I want to monitor the temperature on both cylinder banks to make sure I’m not approaching an overheat situation.

My plan for doing this was to purchase two Omega thermocouples that are set up to measure temperature at a bolt, using a washer type-thermocouple that fits under the bolt head. I plan to hook each up on either one of the thermostat bolts or a head bolt on each bank. I would then run the leads to a set of Teleflex (PN 67020P) temperature gauges that are already mounted in my console.

Here is what I cannot figure out – what TYPE of thermocouple (K, T, J, or E) will drive the Teleflex gauges. The spec sheets on the different types of thermocouples are in millivolts over the appropriate temperature ranges (e.g. a Type K will produce 1.977 millivolts at 120 degrees F.) The sender code for the Teleflex gauges are labeled Type D and the Teleflex data sheet says Code D will be 240 ohms at 120 degrees.

Seems like something is missing here – is there anyway to calculate a conversion between the thermocouple output (in millivolts) and the required ohms input to the Teleflex gauges? Or am I simply barking up the wrong tree because I don’t know enough about how these things work?

 

[makomark]

two different set ups; aka trying to mix apples and oranges.

If you wanna use your exisitng temp gauges, you need to use the temp sender which are usually thermistor based. If you wanna use the thermocouple, you'll need a millivolt meter calibrated to degrees. VDO makes one and it appears to use a very similar "washer thermocouple" as its sensor.

I'd look at your 'needs' before making the selection. Any idea what the 'normal' range for the cylinder head temp is? (drive min and max values that need to be displayed) is the overheat alarm sensor on the surface or down in a cavity (drives range of temperatures to be measured)? How much accuracy do you want (thermistor based gauges are normally spec'd at +/- 10 deg F)? Thermocouples are more accurate but the gauge's resolution may not be what you want.

 

[chawk_man]

Makomark - thanks for the reply. Yep, I sort of figured I was mixing apples and oranges later today.

Normal water temperature range for the BF 225 is 140 to 160 degrees. I'm speculating here, but I'm guessing that the shut down temperature is around 200 degrees F.

Not too interested in high accuracy, just temperature changes to let me know that the motor is starting to overheat before it gets to the point of an automatic shut down and leaving me stuck in the middle of the Chesapeake.

The Honda heat sensor is a thermistor type that ranges from 5,000 ohms to 100 ohms over the temperature range of 0 degrees to 120 degrees F. However, the wires from the sensor to the ECM carry a 4 to 5 volts, so I cannot intercept that signal to drive the Teleflex gauges. Again, I'm speculating, but apparently the way that works is that the ECM reads voltage changes caused by the resistance in the heat sensor. If that voltage goes out of range (i.e. resistance goes to near zero), then it shuts down the engine.

There is no easy way to directly measure the water temperature. I guess what I need is a thermistor type sensor that I can connect to one of the thermostat bolts or a head bolt and will give me the appropriate resistance to drive the Teleflex gauges. Any ideas would be highly appreciated.

BTW - I did try using a standard inboard water temp sensor, wrapped in a copper sheath, and attached to a head bolt. That did not work.

 

[makomark]

some outboards have bosses that have been tapped in a pipe thread size. The trick then becomes getting the thread size and finding the proper sensor to drive the gauge that 'fits'.

Back when I worked in the shop, EVERY v-6 we installed on the bass boats got a water pressure gauge installed with it. There was typically a pipe plug that was removed and a hose barb adapter installed to connect the hose driving the gauge. Your engine may have that fitting. if so, you have two choices. 1) is use the existing temp gauge and find the proper sensor. 2) is to install the pressure gauge to monitor the pressure in the cooling system.

The trade is that the pressure gauge will give you an very rapid indication of lack of cooling water. No pressure infers no flow which means the overheat is imminent. Most bassers also found the gauge was useful for trimming the engine out. The temp gauge, on the other hand, gives you the temp of whatever the sensor is connected to. Depending upon the 'thermal latency' of the installed sensor, the ECU may still shut things off before you see enough change on the temp gauge to warrant concern.

May wanna check your service manual or your servicing dealer to see if the magic plug exists on your engine.

 

[chawk_man]

Makomark - I think I have this all worked out now.

As you suggested, I plan to install a water pressure sensor. There apparently a tap point for doing that. I will also install two Teleflex heat sensors on the heads.

I had the right idea before of bolting the sensors to the head bolts with direct contact to the head surface. However, after many hours on the Internet comparing output ranges, I found out that I have the wrong sensors for the gauges. So, I have ordered new sensors.

Thanks for your insight.