The thermocouples on my engine don't appear to work. I tried to remove one by pulling out the rubber gromet only to find the actual sensor appears to need the engine head removed to replace the sensor. This seams to be a very poor design to me... can anyone confirm this is correct?
How do these thermocouples work exactly? Are they a milivolt resister that approachs 0ohms or short circuit as the engine overheats? if this is correct, can their opperation be tested by disconnecting the wire to them and shorting it to the engine earth? should this sound the alarm?
I am thinking of installing a external temperature sensor to one of the head bolts with a guage on the dash instead of relying on the origional sensors. Does anyone know what temperature these heads should opperate at? Does anyone else rely on a simular setup?
Temp switches close the horn circuit to ground via the tan wire upon an overheat condition. Check the horn circuit by grounding the tan wire at the engine which should sound the horn.
Test the switch[es] by removing them after removing the cylinder head[s] and heating in oil [with a flash point around 300] to around 203 degrees F +/- 6. It should reopen at 162 degrees F +/-15.
The surface mount OMC/BRP and Teleflex models work well and are easy to install. You will like the fact that you get the ability to see a problem coming rather than waiting for an actual overheat.
Don't know the exact device in your case for OMC, but if you are like Merc (probably are) it's not a resistor, rather a disc. At Texas Instruments, the company name is Klixon (brand) Thermostatic Snap Action Switch.
It's a disc, formed into a concave shape that's temperature sensitive. Upon reaching within 5 degrees of the design temp, it snaps from it's quiescent shape to the over temp shape whereupon it activates an internal switch contact and delivers the 0 or 1 logic failure signal (open or closed depending upon the type).
On the outboard engine overtemp, only way to test is to put it in a pot of water and boil it (212F at sea level). About 195F it will show a short circuit from the tan wire to the case of the device. If not, trash it and get a new one.
Temp switches close the horn circuit to ground via the tan wire upon an overheat condition. Check the horn circuit by grounding the tan wire at the engine which should sound the horn.
Test the switch[es] by removing them after removing the cylinder head[s] and heating in oil to around 211 degrees F +/- 6. It should reopen at 175 degrees F +/-7.
The surface mount OMC/BRP and Teleflex models work well and are easy to install. You will like the fact that you get the ability to see a problem coming rather than waiting for an actual overheat.
The perfact reply, answered all my questions clearly. Thank you very much
That is true. I based my answer on mine which is 195F and if it's sitting in a pot of boiling water (lot cleaner than heating oil to that temp) and hasn't shorted in 5 minutes, it's bad. 203 is getting pretty close and of course altitude makes a difference too. I'm at 700 ft.
The head does not have to be removed to replace the sensor, only the head water jacket cover. The horn switches are an "on-off" switch. The thermostats are 143 and the head may run warmer. Hot alarm will sound at 211 and will go off at 175. I prefer the separate temp sender and guage, however they typically come with one sender, which covers only one head, or the exhaust cover. You have two thermostats and could have an overhead problem on one head and not the other.;...still the guage is better than the horn only.
These surface mount temperature senders, are they the same design as what is used in most motor vehicle engines? Are you relying on the contect between the brass sensor and the block? This would not be as effective at having the sensor tapped into the water jacket would it? has anyone tapped into their water jacket? Would there be any dramas using a sensor and gauge from an vehicle? The marine units are very expensive in comparison to the automotive ones... Do the marine units normally come with a bracket to hold the sensor against the head? Does anyone have a pic of this setup?
If you check you local marine store for outboard temp guages..some come with the sender and the bracket.
The sender is also sold separately without the bracket. I use mine as a surface (contact) mount, first cleaned the paint from the contact area. I know some on the board have installed in the head-depends on engine model. Some OMC's have round flat spots cast in the head which can be used as a contact area. Unscrew one of the water cover bolts and use it to hold the bracket.
how's this for an idea, purchase two digital backlite multimeters with temperature thermocouples ($25ea), extend the leads, and hey presto, cheap digital temperature gauges...
You can even add a resistor to the 9V battery circuit and wire up to the 12V system. A bit uggly but practicle and cheap!! I'm going to try it
1986 Evinrude 90HP Thermocouples/head temps
Linear Mode
