Nonfunctioning water temp gauge

I have gone back to a project that I have asked you about previously, Don--installing a new S-W water temperature gauge in my 35-year old Pearson. (My first attempt got miswired and I got a big puff of smoke. Assuming that I had probably fried the new gauge, I bought another new from you.) I wired up the new gauge and it is getting power, the sending unit is producing current which I can read at the gauge--and nothing happens. The gauge remains pegged at 240 (it moved slightly the first time I used it from about 220 to 240 and remains pegged there). At your suggestion, when I could not get the new one to work, I tried to put the original gauge back in (which had seemed frequently to be giving false readings). Now it doesn't work at all.

I don't see how the positive and negative/ground leads could be reversed. The lead to the "I" screw on the gauge comes from the oil temperature gauge, like in your wiring diagram. The wire for the "G" screw on the gauge connects to a negative wire that leads back to negative buss. Do you have any suggestions? Are there checks I can make with a multimeter to find the problem? I'm stumped.

[lhbradley]

The temp sender is a resistor whose resistance varies inversely with temperature. It is about 500 ohms at room temp, and decreases as temp increases.

The fact that your gauge reads full scale suggests that it is seeing a low resistance, which suggests a short circuit somewhere in the wiring going to the sender.

Disconnect the wire from the gauge to the sender, and see what the meter reads. It should read a very low temperature. If not, it suggests that the meter itself is faulty. If so, try disconnecting the wire from the sender at the engine, and run a temporary wire from the sender to the gauge.

[Don Moyer]

Hello to you as well. 2005 seems to be the year of temperature gauge
problems, so I'm not entirely sure that I remember all the details of your
particular situation.

In any case, we consolidated the following list of troubleshooting
techniques for gauge problems from telephone conversations as well as other
postings in our Atomic 4 Community Forum. My best guess is that your
problem may be in the ground between the case of the gauge and the engine
ground.

If you discover some other problem that is not yet in our list of things to
check, please get back to us so we can add the information to our list.

"First, inspect all the connections to the gauge. This includes the
connections at both ends of the sensing wire (between the sensor and the
cockpit gauge), as well as the positive terminal on the gauge, to be sure
the gauge is receiving 12 volts.

Be sure to check for a good connection from the grounding terminal on the
gauge to "engine" ground. I used to believe that the grounding terminal was
only for the light in the gauge, but I learned rather recently that Stewart
Warner gauges need to be grounded for the gauge itself to work.

If your wiring bundle disappears (like under the cabin sole) on the way to
the cockpit and cannot be inspected, we recommend running a single cheap
temporary wire directly from the sending unit to the gauge for a quick check
of the circuit.

After insuring that all connections are sound, and the gauge is still not
responding in any way, you can remove the wire from the sensor and touch it
to the head (essentially grounding it). With the sensing wire grounded to
the head, a gauge will usually move to one side of the gauge or the other.
If the gauge still shows no movement, the gauge or the sending unit is
probably defective.

You can make a simple functional check of the sending unit by checking the
ohms (resistance) between the terminal of the sending unit and ground when
the engine is cold, and then look for a change in the resistance as the
engine warms. While each company uses a somewhat different recipe in terms
of resistance versus particular readings on the gauge; in most cases, if the
resistance in a sending unit is varying with temperature, it's probably
working OK. Sending units tend to either work reliably, or not at all.

If, after insuring that the sending unit is at least functional, and that
all circuits to the gauge are proper, the gauge still shows no reaction, or
an obviously improper reading, it's probably time to replace the gauge.

When replacing a gauge, it's usually necessary to replace the sending unit
as well as the gauge, since all manufactures use different specifications in
terms of resistance versus gauge reading. You can try replacing the gauge
with one by the same manufacturer in hopes that the sending unit will be
compatible, but if the gauge and sending unit are quite old, the sending
unit might still not be compatible with the new gauge."

Don

[Bob N]

Thanks, Don. I'll try these things this weekend. The gauge and sending unit are brand new, so I'm hoping that they do not turn out to be the problem.

[Bob N]

Thanks for your suggestions, Don. What I finally found was that the panel was not wired as shown in the Universal owner's manual (the boat is a 35-year old Pearson). I had the positive and negative leads reversed on the gauge. The wire leading from the oil temperature gauge (a white wire--all the wiring on the engine is black, white, or red) was the ground wire rather than the positive lead as indicated on the diagram. I was puzzling over why I had plenty of current to the gauge and it was still not working when I noticed the "minus" sign on the meter.

Now that the gauge is installed, I'm glad I put up with the frustration of doing the job. The new one is much easier to read and does not jump around like the old one did.