Lehman overtemp idiot buzzer help

[Marco Flamingo]

I'm trying to troubleshoot why my overtemp idiot buzzer isn't working. Best I can tell from my investigation to date is that the ignition circuit breaker energizes the oil pressure buzzer (normally closed and open with pressure to stop the buzzer) and the overtemp buzzer (normally open but closes at 205F to activate the buzzer). That is what I thought I would find. So the wire to the overtemp sensor (left side of engine under the coolant reservoir) would be 12V hot, closing the sensor would complete the circuit by grounding, and the buzzer would go off. Picture below.

What has me confused is that the wire to the temp sensor is simply soldered to a braided wire, covered (sort of) by a plastic wrapping. Picture below. I haven't tested it yet with a multimeter, but I can't believe that that poorly insulated braided wire draped over the engine is 12V hot and that's what provides 12V to the NO sensor.

Am I missing something on how the overtemp buzzer works (when it works)?

Mark

[Marco Flamingo]

Forgot the second picture. The wire has been disconnected from the sensor and some of the plastic wrapping peeled back.

[Barking Sands]

No...you have it right. At least on most set ups. Are you worried about the hot wire being semi exposed? Pull it off and put the exposed wire right to some bare metal and see what happens. The buzzer should go off and that's it. That's a good way to test the circuit for the temp switch. It will draw only the current required to operate the buzzer. Although it should be covered a bit better to prevent nuisance activation if it rubs against a grounded surface.

[C lectric]

That connection could be a P.O. adaption. Seen similar bodge jobs. They work but are open to problems eventually.

[Barking Sands]

Originally Posted by C lectric

That connection could be a P.O. adaption. Seen similar bodge jobs. They work but are open to problems eventually.

Usually the spade tab on the switch breaks off leaving only a nub. And the only way to attach a wire is to then solder it. Ask me how I know...lol. I have a new switch ready to go in though.

[Bmarler]

Originally Posted by Marco Flamingo

I'm trying to troubleshoot why my overtemp idiot buzzer isn't working. Best I can tell from my investigation to date is that the ignition circuit breaker energizes the oil pressure buzzer (normally closed and open with pressure to stop the buzzer) and the overtemp buzzer (normally open but closes at 205F to activate the buzzer). That is what I thought I would find. So the wire to the overtemp sensor (left side of engine under the coolant reservoir) would be 12V hot, closing the sensor would complete the circuit by grounding, and the buzzer would go off. Picture below.

What has me confused is that the wire to the temp sensor is simply soldered to a braided wire, covered (sort of) by a plastic wrapping. Picture below. I haven't tested it yet with a multimeter, but I can't believe that that poorly insulated braided wire draped over the engine is 12V hot and that's what provides 12V to the NO sensor.

Am I missing something on how the overtemp buzzer works (when it works)?

Mark

The sensor should be switching the ground leg of power, not hot. The hot leg goes direct to the gauge or light.
If you check the wiring to the sensor when it’s open, you will read the wire coming from the gauge as positive 12 volts, but it’s not really, that just how a switch will read till it’s closed.

[Marco Flamingo]

Originally Posted by Bmarler

If you check the wiring to the sensor when it’s open, you will read the wire coming from the gauge as positive 12 volts, but it’s not really, that just how a switch will read till it’s closed.

My multimeter didn't read the wire as 12V because my buzzer behind the upper helm was so corroded that there was no voltage going through. I cleaned it up and I now get 12V at the normally open coolant sensor. When it closes, the circuit is made and the buzzer (if working) goes off. What I thought was strange is that the 12V wire to the sensor is bonded to a braided loom that is only protected by a plastic wrapping. I don't know what purpose that serves. Why not just have an insulated wire run all the way back to the buzzer?

Anyway, problem diagnosed with cleaning the buzzer contacts. It now works, but a new, louder buzzer is on order. Still need to pull the thermal switch out to make sure it closes at 205 degrees as it is supposed to. I'd like to think that it was only a faulty buzzer, but now I'm paranoid and need to check everything.

[Hyperion]

These important sensors should be regularly tested. That said, what would be an easy, effective way to test this one short of poring boiling water into the coolant tank?

[Bmarler]

Originally Posted by Hyperion

These important sensors should be regularly tested. That said, what would be an easy, effective way to test this one short of poring boiling water into the coolant tank?

Only real way to test is to pull the sensor and suspend it in water. Heat the water and monitor the temp with a candy thermometer to see if and when the sender closes.

[Barking Sands]

The temp sensors are a bit harder to test accurately. Depending on the mdium the sensor sits in there will be some kind of lag. Usually the medium will be a few degrees hotter than the switch innards as temp climbs. Common ways to test are:
boiling water (if it is within range)
oil bath if a bit more temp is needed
sand bath with hot air from heat gun- put the sensor probe right next to the switch
And for a quick test I use a copper pipe with steel wool in one end and the temp switch and temp probe next to each other inside the other end of the pipe. Then use a heat gun to heat the outside of the pipe opposite the sensor to bring the internal air up to temp very slowly.

In all of these...the slower you raise the temp...the more accurate the test. Then test the falling temp to open the switch again. Then repeat several times to not only measure activation point, but to check for consistency and that you have a nice clean electrical opening with no milliohms remaining. You also get to see the hysteresis between rising and falling activation point.

Even if your switch says 205 but it closes the contacts at 210 or 201 I would consider that within limits since there is so much error in this type of test. That is as long as it is consistent regarding everything above.

Mine had the tab broken off and a wire soldered to the nub so I recently changed it out. I did this test on both the old one and the new one. Both checked good. Same with oil pressure switch.

[Nepidae]

Your question will elicit many answers but only one person can provide the right answers about a Lehman engine.

Call Brian at American Diesel for that answer. His father developed the engine. Google the phone number.

They also have all the parts you will need.

Good luck

[Hyperion]

Originally Posted by Nepidae

Your question will elicit many answers but only one person can provide the right answers about a Lehman engine.

Call Brian at American Diesel for that answer. His father developed the engine. Google the phone number.

They also have all the parts you will need.

Good luck

Furthermore, If you have an FL it is your best interest to buy anything you need for your boat from them, even if someone else sells it for a few bucks less. We want them nice and healthy making spares for us.

[catalinajack]

These sensors are important but some may want to consider the alarm made by Borel. It senses an increase in exhaust hose temp at a level far below that which the 205-degree sensor on the engine. You will get an alarm before the actual engine temp has risen to a critical point.

[hytedin]

Just replace them all. The cost of failed alarm is far and overheated engine is far greater. And redo wiring where necessary.

And without a doubt, an alarmed temp sensor on the exhaust hose—aqua alarm is another option.

All cheap security.

And, you can get a sensor that grounds out at 200 rather than 205.