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Old 12-09-2010, 10:56   #1
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Temp Gauge Trouble 4-107

Westerbeke 4-107, 1972 era, in Morgan Out Island 41. Temp gauge reads continuously pegged at max. Guessing in its simplest form, the system is a sender, a wire and a gauge, but unsure of specifics that will let me troubleshoot.

Engine has several units that look like sending units, a couple have two wires. Not sure which are oil and which are temp.

Gauge looks new and has a logo on the back of it with a 'T' 'cupped' in a 'U'. Donít know if an open connection in wiring would leave gauge reading max by default (boat is new to me and gauge has never worked).

Donít know what voltage or resistance on wire should read if sender is working working.

Any pointers or info appreciated.
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Old 12-09-2010, 11:19   #2
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Testing All Gauges:

1. Remove Sender Wire from “S” Terminal on Gauge, and Turn ON Instruments Circuit.

2. Test Voltage between “I” Terminal (External Power Supply) and Ground Terminal “G”.
Tested voltage must match nominal voltage of the Gauge (ie: 12V Gauge must test between 10 - 16VDC), and the Battery Voltage.
If no voltage - check Ignition or Instrument Switch(s) and/or Fuses.
If tests less than battery voltage, repair or replace (Violet) wire from Ignition or Instrument Switch.

If a Sender fails “Open” (Infinite Resistance), one or both (if Dual Station) Gauges will read Zero Scale.
If a Sender Fails “Closed” (shorted - Zero Resistance), one or both Gauges will read Full Scale.

Other confusing Ohmmeter results, such as readings lower than expected, could indicate a poor Ground Connection at the Sender Body (Sender to Engine Block). Check for pipe dope or tape on Sender Threads.

Temperature Gauges:

1. Disconnect Tan Sender Wire from Gauge (Terminal “S”) and turn Instrument Power ON.
The Temperature Gauge should indicate ZERO Scale (< Low Temperature).

2. Short the Gauge Sender Terminal “S” to Ground Terminal “G”.
The Gauge should deflect to FULL Scale (> High Temperature).

3. Measure Resistance from Tan Sender Wire (disconnected from Gauge) to Ground.
American Temp. Senders will read: 450 Ohms (Engine Cold @ 1000F)
or 29.6 Ohms (Engine Hot @ 2500F )
European Temp. Senders will read: 281 Ohms (Cold @ 400C) and 22 Ohms (Hot @ 1200C)

4. Disconnect Tan Wire from Sender on Engine Block.

5. Test continuity of Tan Wire.
Zero Ohms from Block End to Gauge End - If not, repair or replace Wire.

6. Measure Resistance from Sender Terminal (on Block) to Ground on Block.
Should read as per (3) above - If not, replace Sender.

See Basic Engine Gauge Theory & Testing
Basic Engine Gauge Theory and Testing

See also the graphics posted at:

Testing Engine Gauges (2) - See also Wiring Diagram &amp; Testing Instructions Cruisers & Sailing Photo Gallery
Testing Engine Gauges - See also Wiring Diagram &amp; Sender Resistances Cruisers & Sailing Photo Gallery

Engine Gauge Wiring Diagram - See also Gauge Testing Diagrams Cruisers & Sailing Photo Gallery
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 12-09-2010, 19:34   #3
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Originally Posted by GordMay View Post
Testing All Gauges...
Wow! Thanks much, GordMay! Exactly what I was looking for.

Just drove back home from the boat this evening, but lookng foward to getting up there again later this week to apply this info and fix this gauge.

Then start on the next things needed to get it splashed and deliver it to my home marina - gotta learn to change the diesel fuel filters and bleed the system. Oh, and start glassing a hundred or so blisters found while sanding off 15 coats of bottom paint... Guess its a good time for that as I believe this boat has been on the hard for three years.
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