Overheat Alarm Wiring Question: 1984 Mercury 200 HP

[wcsd106]

Good afternoon,


I just replaced a overheat alarm sensor on my 84 Mercury 200 outboard because the wiring had corroded so badly that the insulation was gone and the wires were breaking.

I've looked through my OEM service manual, but I'm either overlooking the wiring diagram or it's not in there. Here's an issue I've discovered that I'm hoping someone can assist me with.

If I'm understanding the way the overheat alarm sensor works, the spring in the sensor will expand and make contact with the block when the temperature exceeds a safe limit. That should ground the sensor which is connected via a terminal block to a tan wire in the control wiring.

The problem is, as I was checking everything with a multimeter, it appears that the tan wire is already showing continuity with ground. When I remove the tan control wire, the terminal block no longer shows continuity with ground.

If this is grounded out, shouldn't my alarm be going off continuously while the engine is running?

Any help would be appreciated. Thanks.

 

[Dukedog]

if tan in harness is showing grounded at term block you should get a buzzer tha instant you turn tha key on...

check screw holding tha wire to tha plastic block.. might be a tad to long.. SHOULD NOT be same screw mounting tha plastic term block to tha block...

 

[wcsd106]

My original post left some roof for confusion. Let me clarify. I verified that the tan wire shows continuity to ground by first removing it from the terminal block and checking from the ring terminal of the tan wire to engine ground.

When the tan wire is removed from the terminal block, the terminal screw that the wires connect to no longer shows a short to ground.

Going to dig back through my OEM manual and see if I can find the path for the tan wire.

 

[wcsd106]

On a side note, does the engine need to be running for this circuit to work correctly? I have tried it with the key turned off and turned on with the same results, but I'm wanting to cover all my bases.

Thanks.

 

[Dukedog]

see edit above.. tan from term block on motor goes to one side of warnin' buzzer.. buzzer should have a black, tan or tan/blue, purple/switched hot....

 

[Dukedog]

term block at motor.. mounted to motor with one screw on thin part of term block?.. two tan wires under another screw on thick part of term block?

 

[wcsd106]

Thanks, Duke. I've gotten it figured out.

I was using the audible continuity function of my meter to check everything out. When you mentioned the path of the tan wire it got me thinking.

I just ran back outside and checked the continuity again, this time with the meter set to 200 ohm scale. I get a reading of 37.1 ohms.. That is likely the impedance of the warning buzzer. So, held enough above ground to keep the buzzer from sounding.

Chalk this one up to me being overly cautious and/or having a brain fart.

 

[Dukedog]

:thumb:

 

[sam am I]

Thanks, Duke. I've gotten it figured out.

I was using the audible continuity function of my meter to check everything out. When you mentioned the path of the tan wire it got me thinking.

I just ran back outside and checked the continuity again, this time with the meter set to 200 ohm scale. I get a reading of 37.1 ohms.. That is likely the impedance of the warning buzzer. So, held enough above ground to keep the buzzer from sounding.

Chalk this one up to me being overly cautious and/or having a brain fart.

Hmmmm, that doesn't sound right. I'm pretty sure the OT alarm sensor (Tan/Blu Wire on block) ("Over Temperature Switch/Sensor" NOT the Temperature Gauge Sender) is a bi-metal switch........i.e., it's open (>>1Meg Ohms @ <192F) or closed ( <1 ohm @ >195F)..


See post #6 etc...

https://forums.iboats.com/forum/engi...-a-ohm-reading

 

[sam am I]

Correction from above 240F not 192/5F.....lol, not sure where those numbers came from, pretty much just made those up looks like, geeeesh :facepalm:



Merc calls it a "sender" but, more of a switch or a sensor IMO :juggle:

Also, if I recall(low batting average today but........), the OT Switch/Sensor is a one wire device w spring, the temp gauge sender, is the two wire device.

 

[Dukedog]

first remember ya dealin' with merc junk.. they're not known for precise "quality control" with they're suppliers, 'specially tha 'lectrics….