88 XR2 Temp. Guage Sending Unit

[chadbenton]

I would like to know if my sending unit is correct or installed correctly. The sending unit for my temp. guage is threaded into an adaptor plate on the port head. From there it just bottoms out on the head itself. This looks like it would be a water type sending unit, with sealed threads. Is this correct or should there be a different type of sending unit. Can it affect actual temp. readings....

 

[Silvertip]

Re: 88 XR2 Temp. Guage Sending Unit

Yes -- where the temp sender is mounted will provide different readings. Contact with the water cover means it's measuring the temperature of the water cover. The closer the sensor is to the spark plug, the hotter it will read. Sensors that have the tip immersed in the cooling water read the actual water temperature. After a long run, it probably won't make any difference since the temperatures will equalize. Its the short blasts that can result in different temperatures in the engine. That said, sensors are temperature sensitive resistors so you can measure the resistance with the wire disconnected. Just measure from the terminal to the body of the sender. Resistance should be about 30 ohms at 250 degrees F, 99-100 ohms at 175 degrees F, and 450 ohms at 100 degrees F. Senders and gauges need to be matched so these numbers may not be correct for your gauge. You can use a digital or analog meat thermometer and a pan of water to check the sender. Do not allow the sender to touch the pan during heating as it will measure the pan temp and not the water.

 

[chadbenton]

Re: 88 XR2 Temp. Guage Sending Unit

Would I be better off finding a place to install the sender where it is in a water jacket, so that I can get an accurate water temp. reading. My guage seems to show high at WOT (195-205) but I don't believe that the engine is acutally that hot, shows no signs of overheating. I just want the peace of mind knowing that my engine is being cooled properly............

 

[Silvertip]

Re: 88 XR2 Temp. Guage Sending Unit

You are experiencing the difference in readings between a water temp gauge and a cylinder head temp gauge. You can do one of two things. 1) Using an infrared temp gun, or temp sticks, measure the cylinder head temperature and compare it with the gauge reading. If those numbers agree they will be your reference. The key though, is to determine what acceptable cylinder head temperature is -- and it is NOT the same as water temperature. As I mentioned earlier, the closer to the plug you mount the sensor, the hotter it will read as thats the hottest part of the head. 2) I'm not familiar with the heads on that motor but look for a small plug that can be removed that is open to the water jacket. It will likely be around the top of the head. If the current sensor is not the correct thread size, you can order a proper one or use a reducer bushing availbable at any hardware store. It is important that you measure the resistance at the various temperatures. It may be this is an incorrect sensor for this application.

 

[gss036]

Re: 88 XR2 Temp. Guage Sending Unit

My 89 200 Merc has an overheat sensor installed on the starboard head, between the 2 top spark plugs, that sensors sets off the overheat alarm. The port side is usually used for the temp sender for the temp guage. I went through that process last year and had to go back to the Mercury OEM sender to get the correct reading. I had tried to use a TeleFlex sender but it just read way to high. I think the Teleflex was sending different ohm reading and was reading metal temp next to the spark plug.
I am not familiar with your motor so can only speculte at this time.
Like SilverTip indicated, once you get to know what a satisfactory reading then, that will be indicator.

 

[chadbenton]

Re: 88 XR2 Temp. Guage Sending Unit

Mine has the same setup that yours has. You said the correct sender came from Mercury, would'nt happen to know the part number would you. Thanks for all the help.

 

[Silvertip]

Re: 88 XR2 Temp. Guage Sending Unit

Get the serial number off your engine. Then go to Mercury Parts Express. Enter your serial number, select any parts distributor, and then view the parts list for your engine.

 

[chadbenton]

Re: 88 XR2 Temp. Guage Sending Unit

Found the part number, thanks for everything..