Testing an ECT sensor
 

This sensor, (Engine Coolant Temp) is sending a varible voltage of up to 5V to the ECM to adjust fuel mixture. Can this sender be bypassed by jumping across the 2 wires in the sensor plug, or unplugging the wires, or can't be bypassed, only tested? I'm thinking it will either send a full 5 volts thru, or zero volts. Not sure what voltage the ECM is looking for when cold.

Motor just cranks, no start when cold. After several minutes, it will finally catch, then all is well. A marine mechanic has suggested to check this sensor.

Thanks

ECT sender is "engine coolant temperature". Typical two wire that uses reference voltage if you unplug it the ECU would not receive any voltage. If you jump the terminals the ECU would receive full reference voltage. With regards to the ECT sensor cold stat would be affected if the ECU thinks it is hot, as well hot restarts would be affected with an over rich condition if it thinks it is cold. You can ohm out the sensor cold and then again hot and it would tell you if it is in working condition but using a scan tool would tell you if it is with in ECU specific range or not.

If you have Diacom or something similar, they also make a "sensor test lead kit" that is optional.

It will test your range either installed or not. Often saves you from throwing parts at it.

It's basically the same thing as what is said above but sort of eliminates the need to be smart enough to read meters and know in advance what the numbers are..

Yep, knew it was coolant, but brain fart typed control duhh.

This is just to see if the motor will start with the ECT out of the equation for a fast easy check. Have no scan tools. Does the ECM look for no voltage or 5 volts for a cold start?

Unplug it and see. 5 volts is "usually" the max temp obviously but you did not mention which engine, which efi system etc.

With no tool it's a guess.

You can make a simple scan tool for about $5.00, I did. I found out how to make it here, on one of the forums. I would tell you how, if I could remember.

In my foggy old brain, logic would say that it starts at zero for cold and goes up to 5 V, but logic and boats rarely play together. :) That is probably some type of rheostat on a bi-metal strip much like a temp gauge sender would have, and I'm pretty sure these vary from low to high.

CIRCUIT DESCRIPTION:
The Engine Coolant Temperature (ECT) sensor uses
a thermistor to control the signal voltage to the ECM.
The ECM applies a voltage on CKT 410 to the sensor.
When the engine coolant is cold, the sensor (thermistor)
resistance is high; therefore, the ECM will see
high signal voltage.
As the engine coolant warms, the sensor resistance
becomes less, and the voltage drops.

Start on page 5C-86 for more info about how the sensors work.
http://www.boatfix.com/merc/Servmanl/16/16C5R2.PDF

the factory workshop manual will give a graph of resistance vs temp. you put a VO meter across the terminals cold... say on an ice cube and read the value and the put in in boiling water and read the value... that will tell you what you need to know...if you had a mechanical water temp gage you could plot your own resistance over temp curve and see if it matches what the factory says. generally tho if the boat won't start cold, thats where you look first as your buddy said.

Yeah I read the manuals and found the resistance reading @ temps chart. Not the brightest light when it comes to electrical terms other than volts. ;) The ice cube/hot water test I can relate to on the reading of the DVM and apply that to the sensor testing. Have an offer to borrow a scan tool, so if all else, we can read alot more chit and be more confused. :angry-smiley-038:

Thanks all