Lehman SP135 eng heat sensor

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Nov 11, 2022
On my engine, it looks like 2 sensors.
One is in the head port side vertical behind tank. Other is horizontal center rear of tank. I see no wires that were ever connected to this one, it has 2 connections. Other has one. Anyone shed some light here ?
Here's a pic. 2 prong sensor at top rear of tank has no wires near it. Other 2 senders are vertical into head just behind tank. One on stbd side wire is bundled with shut down wire


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Any questions regarding Lehman’s…call Brian at American Diesel. There’s probably no one who knows that engine better than him except his dad, but he passed away a few years back.
Ok, but there are no wires near it. I have done no repairs in that area. The other 2 are wired.
My Am Diesel 136 has two sensors, one on either side of the head under the tank. Same locations as yours.

One is a temp gauge sensor; the other is an alarm sensor. Both are single-wire although I don't know which is which and (of course) the wiring color is the same...

I purchased a replacement temp gauge sensor because I thought that one didn't work. The one Brian at American Diesel sent me was a "floating ground" with two connectors. The existing one ended up working when I did the engine run test, so I never installed the new one or traced the wiring to see which was which. I still have it in the spares box.

I am guessing that the third one you have in the tank was something added by an owner along the way for some reason.

The only way I know of to figure out which does what would be to trace the wiring either manually or with a chirp sensor. If you have butt connectors, you could disco them one at a time to see which one takes out the temp gauge, leaving the other as the high-temp alarm - but mine have threaded terminals with nuts, so that's not really an option.

I will be replacing all that wiring while on the hard so I can label them.


Looking back at your pic, you have two different color wires - yellow and what looks like gray or white. I'd call Brian at Am Diesel and see if he has an educated guess on which is for what. My WAG is yellow for gauge and gray for alarm, but there's a 50% chance I'm wrong, LOL
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Now that makes sense. I'll disconnect each one. I'm going to assume they are "make" to start. May have to do a pan of water
Well, ALL wires half way down on engine are black.............
I believe Brian told me once that the horizontal center is for a cold start circuit of some sort that hardly any engines have.
Bottom one is the overtemp sender. Don't just check it with hot water. Also check the overtemp buzzer. Mine was mounted behind the upper wheel. It was corroded so I replaced it with a simple buzzer from NAPA. Buzzer $4.95. Engine rebuild $19,446.95.
Pls define bottom. As both sensors are level w each other

I think he means the bottom one in the pic - which would be the one on the starboard side of the engine.

And he's correct that checking the alarm one won't work with hot water because IIRC, the switch doesn't close until the temp is over 220 degrees. Conversely, you *can* check the gauge sender with water and an ohmmeter.

I have used an oven to check alarm sensors for old car engines - but that may not be the best plan.
I believe Brian told me once that the horizontal center is for a cold start circuit of some sort that hardly any engines have.
Second that. My motors have that extra sensor with the wires disconnected. Brian told they they are for cold start and to leave them disconnected because they just cause problems.
Port side (yellow wire) goes to the temperature gauge, the starboard/more center one (red/white wire) is the over temperature switch for the alarm. Ignore the one with no wires.
This is the replacement for the port sensor.

VDO 323 479 Temperature Sender https://www.amazon.com/dp/B002IV6VIY?ref=ppx_pop_mob_ap_share

It has two terminals, one to the gauge and the other to ground. I ran a wire to the nearest valve cover screw for a ground.
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Tks guys! When alz sets in, I'm gonna wonder every time I see it
I'm gonna take it out & PLUG it.
A guess would be that one is a variable thermal resistor used for gauge readout and the other might be for over temp alarm, so it will be normally open switched (NO) or normally closed switched(NC). An ohm meter will give some information. The variable will have some resistance value at varying temperatures and the over temp will either be near 0 ohms or O/L (infinite resistance).
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