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Old 07-30-2018, 06:54 AM
Joe Miller Joe Miller is offline
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temp gauge

Hello All,

While running my engine I notice the temp. gauge is pegged at maximum.
Is there an easy way to test gauge with a multi meter or other method.
Could it be a ground issue?

I have a second gauge and sender I could remove from my parts engine,
but would like to check original before I do a switch.

one note, I have only run engine from marina launch area to the mooring field, a 5 minute motor and 20 minutes charging batteries.



Last edited by Administrator; 07-30-2018 at 09:54 AM.
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Old 07-30-2018, 09:03 AM
tac tac is offline
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Gauge Sender Testing

You haven’t said which manufacturer. Most (?) A4 OEM gauges were Stewart Warner. These references should help. A search of this forum should give you plenty of past discussions.
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Old 07-30-2018, 10:13 AM
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ndutton ndutton is offline
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In this order:
  1. Take temperature measurements with an infrared temp gun (cheap at Harbor Freight Tools) to be certain you're not actually overheating.
  2. Disconnect the wire from the temp sender to see if the gauge zero's. If it does, that indicates a possible sender fault. If it doesn't, move to test #3.
  3. Remove the sender wire from the gauge S terminal to see if the gauge zero's. If it does, a sender wire short to ground is indicated. If it doesn't, the gauge is faulty.

Here's a Tech Tip from Don Moyer:

How can I determine if my sending unit or gauge is causing a faulty (or no) reading in the cockpit?

1) Check 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. Stewart Warner gauges (perhaps others as well) need to be grounded directly to the engine for the gauge 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.

2) After insuring that all connections are sound, you can remove the wire from the sensor and touch it to the head (essentially grounding it to “engine ground”). 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 is probably defective.

3) 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.

4) After insuring that the sending unit is 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.
1977 Catalina 30
San Pedro, California
prior boats 1987 Westsail 32, 1970 Catalina 22
Had my hands in a few others

Last edited by ndutton; 07-30-2018 at 11:32 AM.
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