Lehman 120 Overtemp Sender

catalinajack · Jan 6, 2016

Who among us knows at what temperature the sender delivers its alarm? I recently replaced the thermostats with units from American Diesel. Brian sent 195-degree thermostats, the temp recommended in the manual that came with our Defever 44. The reason I ask is that I am getting ready to install a Noland Engineering NMEA interface box which will read the voltages from the senders such that a virtual dashboard can be displayed on the MFD. One of the features within the software is the ability to user-set the high temp that sets off an alarm. Somewhere in my dim brain I seem to remember an opinion expressed that the sender on the engine goes off almost too late to prevent damage. I'm thinking to set a lower temp for the alarm resident in the MFD. The warmest temp I have seen on my new Intellitronix digital gauges is 198 degrees, not very much higher than the optimum operating temp specified in the manual. I am aware that in some boats the Lehmans operate at lower temps with lower temp thermostats although I can't imagine why would want to do that.

Capt.Bill11 · Jan 6, 2016

As I recall its 205 degs.

DavidM · Jan 6, 2016

Well, first there are two types of temp senders: those that provide a variable resistance that can be read as a temperature range and the other type which is a simple on/off switch that grounds the lead to the block when the temp reaches its fixed set point.

With that behind us, I am a little puzzled by the "standard" 195 degree thermostat. Such a thermostat will probably be open mid point and the temp of the cooling system will be controlled to about 200 deg F in normal operation. That is pretty close to the typical on/off switch type sender's setting of 200/205 degrees.

You should also be aware that even though your indicated engine temp may be 200 degrees, that there are places inside the cooling path where it gets up higher than that. The boiling point of 50% antifreeze with the effect of a pressure cap is about 250 degrees, but due to localized heating you can have boiling even if the indicated temp is in the low 200s, particularly at high power levels. That localized boiling will cause less flow due to the steam blocking liquid coolant, so the temperature then spirals upwards.

For all of those reasons, Tony Athens- the Cummins guru on boatdiesel, often recommends a 165 deg thermostat to provide a little more cushion. Cummins standard thermostat is 180 degrees as I recall.

But for the Lehman that typically is putting out a fraction of the hp that Tony is considering, it is probably ok to use a 195 deg thermostat, but if I had a choice I would use a 180 deg one. Yes it will be slightly less efficient at lower temps, but it would take laboratory quality instruments to detect any difference.

David

catalinajack · Jan 6, 2016

djmarchand said:
Well, first there are two types of temp senders: those that provide a variable resistance that can be read as a temperature range and the other type which is a simple on/off switch that grounds the lead to the block when the temp reaches its fixed set point.

With that behind us, I am a little puzzled by the "standard" 195 degree thermostat. Such a thermostat will probably be open mid point and the temp of the cooling system will be controlled to about 200 deg F in normal operation. That is pretty close to the typical on/off switch type sender's setting of 200/205 degrees.

You should also be aware that even though your indicated engine temp may be 200 degrees, that there are places inside the cooling path where it gets up higher than that. The boiling point of 50% antifreeze with the effect of a pressure cap is about 250 degrees, but due to localized heating you can have boiling even if the indicated temp is in the low 200s, particularly at high power levels. That localized boiling will cause less flow due to the steam blocking liquid coolant, so the temperature then spirals upwards.

For all of those reasons, Tony Athens- the Cummins guru on boatdiesel, often recommends a 165 deg thermostat to provide a little more cushion. Cummins standard thermostat is 180 degrees as I recall.

But for the Lehman that typically is putting out a fraction of the hp that Tony is considering, it is probably ok to use a 195 deg thermostat, but if I had a choice I would use a 180 deg one. Yes it will be slightly less efficient at lower temps, but it would take laboratory quality instruments to detect any difference.

David

By standard I meant the temp which is recommended in the Lehman operator's manual. It is also the temp of the thermostat that American Diesel recommends. My understanding of engine design is for a target operating temperature at which the engine operates most efficiently. Operating with a lower temp thermostat for a cushion? According to the Stant website, thermostats are fully open at 15-20 degrees above rated temperature so, yes, using a lower temp thermostat will provide a cushion albeit at the sacrifice of running the engine at its design temperature.

I am aware that any sensor measures only the point at which it is mounted. I must assume that the sensor on Lehman 120s is where it needs to be for us to properly monitor engine coolant temp.

"Such a thermostat will probably be open mid point and the temp of the cooling system will be controlled to about 200 deg F in normal operation." With the 195 degree stats I have, my temps have never gone above 198 and typically range from 189 to 193 so it would seem that they are functioning exactly as designed. Still, I do think that, perhaps 180 degree stats are a good compromise considering they would fully open at no more than 200 degrees. One hundred sixty degree stats make no sense to me.

catalinajack · Jan 12, 2016

To djmarchnd:

"But for the Lehman that typically is putting out a fraction of the hp that Tony is considering, it is probably ok to use a 195 deg thermostat, but if I had a choice I would use a 180 deg one. Yes it will be slightly less efficient at lower temps, but it would take laboratory quality instruments to detect any difference."

I've given your thoughts much consideration and I plan to install 180-degree thermostats. Truth to tell, I did get a bit uncomfortable when one of my engines operated at 198 degrees on occasion and in cooler waters to boot. Now, is there something special about these Lehman engines that a common, properly-sized automotive thermostat cannot be used? I'm guessing the answer is "yes" but, after searching variuos forums and all over the net I have been unable to locate a part number for a 180-degree thermostat. Does anyone have the answer?

chetvaldes · Apr 16, 2017

2.125" diameter 180F thermostat: Stant 45358 = Murray MCS 3838 (O'Reilly's store brand)
Gasket stant 35062 = Murray MCS 2138
Enjoy!

Ski in NC · Apr 16, 2017

If you have an adjustable alarm setpoint, just set it say 5F above normal op temp. If you get nuisance alarms, set it a touch higher.

I have an adjustable oil pressure switch, I set it where I get a "chirp" when shifting at hot idle. Lets me know it is working.

Bob Cofer · Apr 17, 2017

Jack,

I installed the Noland Unit on our Lehman and set it at 205. I also left the original overtemp alarm installed as a stand alone backup.

This unit is fantastic plus it is connected to our N2K backbone and displays on our chart plotter.

jleonard · Apr 17, 2017

MY Lehman manual calls for a 172 or 178 F thermostat.

Cigatoo · Apr 17, 2017

" if I had a choice I would use a 180 deg one. Yes it will be slightly less efficient at lower temps, but it would take laboratory quality instruments to detect any difference."

David I follow your reasoning for the 180 stat. I am wondering if the OP would see increased smoking. The Lehmans seem to smoke until "fully" warm.

Lehman 120 Overtemp Sender

catalinajack

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Capt.Bill11

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DavidM

Valued Technical Contributor

catalinajack

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catalinajack

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chetvaldes

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Ski in NC

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Bob Cofer

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jleonard

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Cigatoo

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