1991 Merc 135 / Temp Gauge

dogsdad

Lieutenant
Joined
Aug 8, 2003
Messages
1,293
I think I posted about high temperature gauge readings a couple of years ago, but I can't find that thread...so here's a new one.<br /><br />A couple of years ago I learned a lesson about impellers and running without muffs. I was under the mistaken impression it was okay to idle the motor dry as long as the motor didn't get too hot.<br /><br />Boy, was that mistaken belief corrected in a hurry.<br /><br />Anyway, the afternoon I unknowingly destroyed the impeller, we launched the boat with my wife at the helm. By the time I parked and walked back down to the dock and out to the boat, smoke was pouring out from under the cowl and the warning horn was blaring. When I got a handle on what had happened, I thought we had toasted the powerhead.<br /><br />I was lucky! A new impeller fixed things up, but then we found out a switchbox had failed---I would assume it was related to the overheat. After that was taken care of, we had no real trouble, but the temp aguge always read real high. I ckecked the motor with an infrared thermometer, so I knew it was really okay all the time, but the gauge reading always bugged me.<br /><br />I finally found the part number for the temperature sensor and installed it, and now things are looking right again.<br /><br />So, the information I am trying to relay here is that an overheat apparently can screw up a temp sensor, as well as take out a switchbox.<br /><br />Just thought I'd pass that along!<br /><br />PS---another thing I found out was that a bad (presumably shorted) cell in your battery can cause low readings on the voltmeter. I saw this happen too during the troubles, and thought I had a bad regulator.
 

KCLOST

Commander
Joined
Jun 22, 2002
Messages
2,095
Re: 1991 Merc 135 / Temp Gauge

True,<br /><br />As long as the temp sensor is measuring the same thing as you measured with your lazer thermometer... <br />It will Depend however if you temp sensor is measuring metal temps on the head or water through the jackets....<br />Let us know...
 

dogsdad

Lieutenant
Joined
Aug 8, 2003
Messages
1,293
Re: 1991 Merc 135 / Temp Gauge

KCLOST, obviously, the sensor is reading temperatures conducted through metal. The sensor is located in a recess right in the middle of the head, just like the warning horn sensor in the other head and has no direct contact with the water.<br /><br />I did verify a long time ago with a laser thermometer that the cylinder head temps were normal, and I also directly measured the temp of the pee stream.<br /><br />The only point I am trying to make here is that the overheat incedent appears to have damaged the original sensor. That is a bit ironic, isn't it?! The new sensor made the world spin correctly on its axis again.
 

KCLOST

Commander
Joined
Jun 22, 2002
Messages
2,095
Re: 1991 Merc 135 / Temp Gauge

That is strange to see that happen but I guess it happens...<br /><br />Did you change just the sensor, or also the lead wire and the guage?<br /><br />The switchbox failure due to overheat is fairly common on the original mercury powerpacks....
 

dogsdad

Lieutenant
Joined
Aug 8, 2003
Messages
1,293
Re: 1991 Merc 135 / Temp Gauge

Just the sensor with the short wire lead that is a part of it.<br /><br />I had great difficulty in locating a part number for the sensor on Mercury's website...navigating it is not very intuitive! It wasn't where I thought it should be, so after being frustrated for months I decided to go looking where I thought it would NOT be. Lo and behold, here it is: Temp Sender: 13536A13. Just in case someone else has the same experience!<br /><br />I understand that there are numerous sensors for various Merc motors. This one has only two wires---the ground wire, which connects to ground underneath the capscrew that holds the securing tab down, and the main lead which is a tan wire that connects to the wiring harness.<br /><br />I hope this info is useful to someone!
 

gss036

Commander
Joined
Jan 18, 2003
Messages
2,914
Re: 1991 Merc 135 / Temp Gauge

I went through the same with mine. I bought a new sensor ($20) and still did not get a good reading, so ended up buying a complete TelFlex set and that fixed the problem. I think my whole problem was a bad gauge, so now I have 2 extra sensors to hang on the shelf. I get kind of warm when idling out of the barbor, but not enough to set off any alarms. My new project is the Poppit Valve Assy. I have the parts, just need to take the time to do it.<br /> Also added a water pressure gauge, I think that was a good thing, now you can see where the water pressure is at.
 
Top