ECT Sensor Temperature vs. Temp Gauge Reading

[tpenfield]

I had a hunch that my temperature gauges were reading a bit high vs. what the MEFI module was seeing via the Engine Coolant Temperature (ECT) sensor. So, today I hooked up the computer to my engines to compare the ECT reading vs. the temp gauges at the helm.

Engines are per my signature below (7.4 L MPI w/ aftermarket closed cooling (full) )

The engines after warm-up and at no-wake speeds were ECT = 155-157 F Gauges = 167-170 F. At cruising speeds, the ECT's get up to about 165 F, gauges are about 180 F.

Because of the closed cooling system add-on, the ECT gets its temperature from a by-pass tube attached to the recirculating pump, rather than the thermostat housing as typical with open cooling systems.

I cannot recall if there is a temp sensor on the aftermarket t-stat housing or if the gauge gets a signal from the MEFI (ECM) module somehow :noidea:

Anyway. The helm gauges read 170-180 during normal running, while the ECT is seeing 157-165 F. I am thinking it would be better to have the gauges show what the MEFI is seeing, since it manages the fuel mixture, based on ECT and a few other things..

So, the helm gauges are reading 12-15 degrees high. I was thinking that I may be able to add a small resistor to the gauge sender circuit to make the helm gauges read closer to the ECT temperature.

Thoughts?

 

[tpenfield]

I should mention that the thermostats are 160 F and all seems to be working fine, So, I'm thinking the ECT temperature is the more 'correct' and the helm gauges are reading a bit high.

 

[Bt Doctur]

Have made that mod on a few motors and cars , nothing wrong with it

 

[tpenfield]

Have made that mod on a few motors and cars , nothing wrong with it

Thanks BT.

I did a bit of searching back through some of my pics from a few years ago . . . I think I found the answer to my question about the ECT vs. the Helm Gauge



The ECT and the Helm gauge are separate senders located in close proximity, but slightly different parts, of the coolant flow. It looks like the gauge sender gets temperature at the top of the thermostat and the ECT gets its temperature from a mixture of coolant from both the T-stat area and the Re-circ pump


the only difference in the actual coolant temperature would be that the T-stat is measuring the coolant after it has gone through the cylinder heads, whereas the ECT is measuring a combination of cylinder head and engine block coolant temperatures.

I would use my IR Temp gun, but this whole area is well covered by the Heat Exchanger when everything is put together and running.

Sort of makes me wonder if I could/should make a fitting adapter to get BOTH ECT & Gauge senders in the same place. And . . . would the 'block' or the 'heads' be the better of the temperatures to use/reference? :noidea:

 

[tpenfield]

Quick update:

I found a chart for the VDO temperature sender to get an understanding of the sender's resistance over the temperature range.



As you can see the resistance change vs. temperature change is non-linear, but the 160 - 180 F degree range is of most interest. Since the resistance change of the sender is about 29 ohms between 160 F and 180 F, I'll put in a 15 or 20 Ohm resistor to lower the gauge reading to be more in-line with the ECT reading that I was seeing on the Diacom (computer).

Probably going to do a 15 Ohm first and see how that goes. I'll just have to remember from now on, if I see anything on the gauge over 180 F it is getting too hot. . . . I would imagine the MEFI will start to say something at that point as well. :noidea:

 

[JoLin]

I think I'd just say to myself... "Self, the gauge reads 15 degrees high." :joyous:

My .02

 

[scoflaw]

I think I'd just say to myself... "Self, the gauge reads 15 degrees high." :joyous:

My .02

Agree...KISS

 

[Fun Times]

I think you've read this before Ted but if not then below is just a copy and paste of what will show up... But for better images, etc. search online ( Troubleshooting Teleflex Water Temperature Gauges) under james town marine.
PS, The digital type sending unit the scan tool reads has always been very accurate vs what most analog gauges read on all models with either style cooling system.

Water Temperature System Accuracy?


Temperature systems used on boats and other vehicles give a general indication of the temperature in which the sender is operating. We use the term "general" as the system can vary to a maximum 15 degrees from actual temperature.

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The gauge itself is accurate to within about 5 degrees at the mid-scale position. The sender is accurate to within 10 degrees at 200 degrees (as are competitive units). The sender tolerances widen as temperatures become higher or lower than 200 degrees. (cont'd. on next page)

Although sender/gauge tolerances rarely run to the maximums (at the same time), the system can possibly indicate up to 15 degrees difference from actual temperature at 200 degrees. These specifications are developed at a system voltage of 14 volts. The sender is also susceptible to "self heating" when electrical current passes through the sender. The self heating causes the sender to become warmer than the actual temperature of the fluid (the gauge is compensated for this effect at 14 volts). If the system voltage is varied to 13 or 15 volts the gauge may read 5 to 6 degrees higher also.

In a few cases it is necessary to put a temperature sender and a temperature switch (to operate a light or buzzer) in the only engine opening. In this case a "tee" is used. However, when the sender (or switch) is moved away from the water flow, the water around the sender bulb tends to run cooler. The gauge will also show a cooler reading than the actual water temperature inside the engine. Use of "extenders" or "tees" is generally not recommended.

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Water & Oil Temperature Sender Identification Guide

Use this chart for field identification of Teleflex water temperature senders. (ID code stamped on wrench hex of sender. See illustration at left of chart.)

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Water Temperature System (Inboards & Stern Drives)

1. To test gauge, voltage from "I" to "G" terminals must be 10-16 VDC. No wire on "S" terminal. Gauge pointer should rest below 120. Next, connect the "S" terminal to the "G" terminal (leave "I" and "G" terminals connected). The gauge pointer should rest above 240.
2. The resistance of the sender can be measured to determine the sender's correct operation. Remove wire to gauge. Connect an ohmmeter to terminal of sender and to engine block. Approximate values are: 75 (room temperature) = 600 to 800 ohms; 212 degrees = 55 ohms. (These are single station values- twin station values would be 1/2 these numbers).
3. If sender is shorted (0 ohms) gauge will read above 240F.
4. If sender has infinite resistance (Open) gauge will read below 120F.
5. If the gauge reads lower than expected, was sealer used on the sender threads? (See illustration.)
6. The accuracy of the system (gauge, sender, voltage range) can vary as much as 16 degrees at 180F.
7. Use of pipe extenders to plumb both a sender and temperature switch (for horn or warning light) from one port is not recommended. The amount of weight extended on the fitting of a vibrating engine could cause fatigue-related breakage. Also, the sender is removed from water flow inside the engine and will probably read cooler.
8. Teleflex does not offer senders for metric threaded ports.
9. Dual Station installations:
   • When replacing one gauge in a two station installation, the second gauge may need to be replaced as well.
   • Mixing two gauges from different manufacturers to one sender may cause an error in both gauges.
   • If one gauge fails the second gauge will read much higher than expected.
   • If sender fails "open", both gauges will read below 120F.
   • If sender fails "short", both gauges will read above 240 degrees.
10. Twin engines: It may be helpful in troubleshooting to switch either the senders or the gauges from engine to engine.

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Water Temperature Monitoring- Outboards

1. The temperature monitoring kit includes all parts to adapt to any of the situations noted above. Yamaha engines will require an adapter P/N 61886P in addition to the kit.
2. Outboard engines use raw water for cooling and are mostly aluminum, so they operate cooler than stern drives. Consult dealer or engine manual to determine maximum operating temperature of your engine.
3. The sender in the kit has 1/8-27 NPT threads with I.D. Code "10" or "60" stamped on the hex. Senders stamped "230" won't work correctly.
4. Part number of the sender is 52320S010; the flat bracket is 54552; the Z bracket IA55009.
5. Some engines include a threaded hole directly into a water line. The temperature sender can be threaded into the hole, using no sealant on the threads. The threads create the electrical ground.
6. Some engines have a blind threaded hole for a temperature sender that does not enter the water jacket. After applying thermal compound to the hole, thread the sender in snugly, again using no sealer.
7. Some Mercury engines have a blind hole flanked by two bolts. These are for tightening the flat plate. First place thermal compound in the hole, then drop the aluminum spacer in the hole. Thread sender into bracket over hole until it hits spacer.
8. The Z bracket installation is used most often as it fits any engine. The Z bracket through hole is held by any one of the engine's cylinder head bolts. The sender is threaded into the bracket's remaining hole down against the cylinder head. Scrape the paint from the head under the sender to get good thermal transfer.
9. The kit is supplied with one sender. If a sender is desired on each cylinder bank on V type outboards, a double pole ON-ON switch must be used to switch sender input to the gauge. The gauge can not accurately handle two senders at one time.
10. Testing the gauge for operation: Connect a hot wire to "I" and ground wire to "G". Gauge should read below the C. Next, Short the "S" terminal to "G". The pointer should go above the H. If the pointer sweeps this way, the gauge is fine.
11. Testing the sender: At room temperature, with no wires on the sender, it should measure approximately. 600-800 ohms on test meter.
12. Replacement outboard sender IA96035 cannot be used with this gauge. The gauge will read very high. IA96035 is only for pre-1995 Johnson/Evinrude engines with a 50-250F gauge.

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[tpenfield]

Thanks all for the comments and technical info . I'm pretty sure the gauges (and senders) are the European standard (VDO Brand) .

In addition to system accuracy, part of the issue is that the ECT takes the temperature at the recirculating pump, and the gauge sender is getting its temperature at the top of the intake manifold (where the t-stat housing is). This also happens to be where the coolant would have just exited the cylinder heads, so probably hotter than the block temperature.

I got my resistors all made up to just plug into the instrument wiring, so I'll see how this band-aid works. I have my FLIR One camera to take some temperature readings as needed to verify the gauge & ECT readings.

I think the alarm goes off at 200 F, but it could be a bit higher.

 

[tpenfield]

Update:

I added the resistors and took the boat over to get some gas and give them some sewage

I also used my FLIR One to verify some of the temperatures.

Here are the gauges at cruise



Here is the starboard Engine temp where the ECT is located (engine idling) after cruise



Here is a section of hose in the flow AFTER the T-Stat housing where the gauge sender would pick up its temperature




Here is the port engine ECT location at idle



Here is a similar section of hose after the T-stat housing.



While idling the MEFI temperature readings (via the DIACOM) were around 155-157 F.

So, in summary . . .

It looks like the gauges/senders were reading a bit high (maybe 10 F) and the ECT sensors are reading a bit low (about 5 F)

So, it is all good

On the down side, I did have codes on the starboard engine for MAP and TPS sensors. The throttle on the starboard engine has been a bit flaky lately . . . surges up and down once in a while at idle.

 

[tpenfield]

Another Update, different stuff . . .

While I was checking out the engines, I did come across a few issues. As mentioned, I got codes on the the starboard engine for the MAP and TPS. I also discovered the the combo water pump/fuel pump has an oil leak . . . probably the front seal.

The starboard engine seems to go into high idle/surging more frequently. It had been doing it occasionally over last year.. So, I'll be buying new sensors. Mercruiser prices on these sensors are ridiculous

MAP (864856A1) $200 ish
TPS (805226A1) $90 ish

I know the MAP sensor is a GM part for about $15, but I am not sure if the TPS is a GM part :noidea: Any help of that ???

On the port engine, I pumped some grease into the oil cavity (instead of oil), figuring that will hold it off until the end of the season for either rebuild or replacement.