Engine Temp indicator woes

OK, this discussion is aimed at the real electricians in the group (no offense to anyone else, but this isn't everyone's cup of tea).


I have 2 indicators, they both test good with the ground to the sender, needle moves to full hot (gauge powered of course). I have tried 2 new sending units for dual gauges (Sierra matched sender and gauges) and have read out the wire from the sending unit to the gauges. ALL individually check good. Start the engine, hook up the gauges to the sending unit and.... NOTHING. How is this possible?

I did the science project with the senders in hot water, they perform as advertised. A good wire run is a good wire run. Even though the gauges check good by grounding the S terminal, do you think it's possible the gauge can still be bad? I've got 2 new gauges on order.

This is such an easy circuit.

Did they both stop at the same time or were you having issues on both prior? Can you run a jumper wire DIRECT from engine to gauge and circumvent existing wire?

Sounds like you have a high resistance connection somewhere in the circuit. I agree with running a wire directly from the sender to the gauge in order to eliminate that part of the circuit.

Check for 12 VDC to the gauge.
Check the black negative wire goes to the other negatives on the engine panel.
Disconnect the power and harness sensor wire at the gauge and read the ohms between harness sensor wire and panel negative. Should be more than zero. My memory is getting fuzzy on this stuff. 33 ohms I believe if the engine is cold for a Teleflex gauge. 300 ohms for top of the range.

Whatever the sensor reads without any wires connected should be within a couple ohms read at the disconnected gauge leads but with the sensor connected. Am I making sense?



I think VDO are the same for temperature. VDO Oil pressure senders (low ohms = high pressure) are opposite of Teleflex (low ohms = low pressure).

Problem Solved

Well the skinny of the whole thing is Sierra brand senders and equipment are not calibrated instruments. Recreational marine instrumentation just isn't dialed in and calibrated like aviation stuff is (that's what I'm used to). Both upper and lower helm gauges work fine, I've tried 2 brand new Sierra sending units (same results with both) they read high, like 200-210F when the engine is at 160F (using an IR gun). I'm running a 180F thermostat. Then I put the original sender back in and both gauges read 160F when engine is up to temp out on a run. I left the original sender in.

I assume/hope you contacted them about the problem? Was the reaction that that is the way it is? What was their reaction?

If so then that's not a sign of a good mfgr. at all. Guages can be off a bit but that is not a bit.

My experience with Sierra dual station senders is completely different. I just replaced the thermostats on my Lehman 120s from 180 degrees to 160 degrees. After the switch all four gauges (upper and lower) read exactly as before, off a few degrees, plus or minus in the same amount, as measured by an infrared thermometer. My temp gauges are VDOs. My take is that this poster's problem is in the wiring to the guages but, of course, that is just a theory. If it means anything I freshly wired the guages. I really don't believe the senders are the problem IF properly matched to the guages.

Agreed but that is why I asked about the contact.

Dave_E said:

Well the skinny of the whole thing is Sierra brand senders and equipment are not calibrated instruments. Recreational marine instrumentation just isn't dialed in and calibrated like aviation stuff is (that's what I'm used to). Both upper and lower helm gauges work fine, I've tried 2 brand new Sierra sending units (same results with both) they read high, like 200-210F when the engine is at 160F (using an IR gun). I'm running a 180F thermostat. Then I put the original sender back in and both gauges read 160F when engine is up to temp out on a run. I left the original sender in.

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The gauges and senders should be pretty accurate, people have been running these for years and years on everything from semi trucks, boats, cars, they aren't all that bad.

Swapping senders around won't tell you anything useful. A systematic approach, test each component individually, then put them together one step at a time will yield results and identify the fault (wiring, sender, gauge, or a combination of these). You had originally said that the gauges were reading 160 when the IR said 180, is that still the case?

sbman said:

The gauges and senders should be pretty accurate, people have been running these for years and years on everything from semi trucks, boats, cars, they aren't all that bad.

Swapping senders around won't tell you anything useful. A systematic approach, test each component individually, then put them together one step at a time will yield results and identify the fault (wiring, sender, gauge, or a combination of these). You had originally said that the gauges were reading 160 when the IR said 180, is that still the case?

Click to expand...

Yep

Dave_E said:

Yep

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In the other thread, I posted a chart with resistance values for the senders at various temperatures. If they line up with that chart, the sender is working as intended.

Grounding the S terminal will only tell you if the sender needle moves it's full range. If the sender had the proper resistances when tested in the cup of hot water, and the gauge does not read the proper values when that sender is wired up directly (known good wiring) in the same hot water then either it isn't compatible with the gauge or the gauge is faulty.

What is the make and model of the gauge? Make and model of the senders you have tried?

I also use a 'decade box' to troubleshoot gauges that are malfunctioning, disconnect the sender from the gauge's S terminal and then hook up one side of the box to the S terminal and other other side of the box to ground. A decade box is just a 'dial a resistor' box with switches for each resistance value so you can set it for any value you want.

With the decade box wired up, dial in the resistance on the box that a single station sender is supposed to be. For example set it for 35 ohms for 220F. Observe the gauge. It should read about 220. Now add in another 50 ohms (85 ohms total) and it should read about 180. Set it for 340 ohms and it should read about 100F. The resistance values can vary by manufacturer. You can test each individual gauge that way. One bad gauge in a dual station setup can mess up the readings of both stations.

If this passes, hook the sender wiring back up to the gauges and the decade box up at the engine in place of the sender, grounding one side of the box to the engine block. Now it gets a bit confusing for the dual station senders, they will use different ohm values for the same readings because two gauges are hooked up to it. Often you can just halve the values so 17.5 ohms for 220F and 42.5 Ohms for 180.

Hopefully that makes sense. It's easier to show how to do it than to explain it clearly.