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Author Topic: Console Temp guage  (Read 3325 times)
68Zproject
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« on: May 29, 2009, 11:10:47 PM »

Anyone know how to test the internals of the temp guage?  I know something is wrong with mine as it won't go to cold when grounding the green wire and doesn't go to hot when disconnected.  When I have it hooked up and power the hot side, all 4 terminals show 12 volts.  I think a wire must be disconnected or shorted and I have it out and would like to fix it, if possible.  Anyone have any experience with this?
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rich69rs
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« Reply #1 on: May 30, 2009, 12:02:59 AM »

Similar discussion here at the following:  http://www.camaros.org/forum/index.php?topic=3963.0

Disconnected green wire at the sending unit, key on should read C; if you ground the green wire with key on should go to H. 

Problem with the 86 ohm calibration resistor on the back of the gage?
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Richard Thomas
1969 RS
68Zproject
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« Reply #2 on: May 30, 2009, 11:37:37 AM »

Well, it's a new ceramic resistor, checks at 86 ohms.  I've read 30 or so threads on this and have had problems since I bought the car.  The problems is that:  First the guage does not go to cold or hot by grounding and disconnecting the green wire.  Second, it has 12 volts coming out all 4 terminals when I power the pink side (with resistor in place).  Third, Mark said in that link that the resistor goes horizontal?  All the diagrams I've seen it goes vertical.  I guess there could be a problem with the resistor.  I mean it shouldn't have 12 full volts going through it should it?  That's why I wondered if there was some kind of "bench" test for the temp guage to check what, if anything, is wrong.  This is such a pita.  All the other guages work great.
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rich69rs
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« Reply #3 on: May 30, 2009, 09:44:31 PM »

I've attached a JPEG of the wiring diagram for the 68-69 console gages.  Not the best, but it does show a ground fromt the bottom terminal on the temp gage.  Sketch also shows resistor vertical.  12 volts on all four terminals doesn't seem right. 
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Richard Thomas
1969 RS
Mark
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« Reply #4 on: May 30, 2009, 10:25:13 PM »

All 4 terminals will have 12V on them when the guage has power.  Resistor goes verticle (horizontal on fuel gauge).  Pink wire connects to two coils inside the guage, one coil connects between pink and green wires and is grounded thru the temperature sender.  That coil moves the guage upscale towards hot.  Second coil connects between the pink wire and the top terminal, then it runs thru the 86 ohm resistor to ground at the bottom terminal.  This coil moves the gauge downscale towards cold.  Gauge that goes hot and does not return to cold indicates a bad ground on the back of the gauge, a guage that never moves off cold indicates an open in the temperature sender circuit.  could be anywhere from the pink wire terminal to the sender itself.  Measure the coil resistance from the power terminal to the green terminal and from the power terminal to the top terminal  make sure neither is open circuited, also make sure neither is shorted to the case (although if they were you should blow your instrument panel fuse).
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Mark C.
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68Zproject
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« Reply #5 on: May 31, 2009, 12:03:21 PM »

OK, I measured ohms from pink to green and got about 50, pink to top about 60, green to top about 110.  The bottom terminal shows infinity (no short) between any of the other terminals.  The bottom terminal shows it's grounded to the case, 0.  None of the other terminals show they are grounded to the case, infinity.  When I hook up power from the battery to the pink side , have the resistor vertical and tightened, ground the bottom terminal to the case and leave the green wire off, nothing happens.  When I ground the green terminal, it moves toward cold about 1/4 inch.  BTW with the resistor in place, I get about 150 ohms from the pink terminal to the bottom terminal.
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rich69rs
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« Reply #6 on: May 31, 2009, 11:31:14 PM »

All 4 terminals will have 12V on them when the guage has power.  Resistor goes verticle (horizontal on fuel gauge).  Pink wire connects to two coils inside the guage, one coil connects between pink and green wires and is grounded thru the temperature sender.  That coil moves the guage upscale towards hot.  Second coil connects between the pink wire and the top terminal, then it runs thru the 86 ohm resistor to ground at the bottom terminal.  This coil moves the gauge downscale towards cold.  Gauge that goes hot and does not return to cold indicates a bad ground on the back of the gauge, a guage that never moves off cold indicates an open in the temperature sender circuit.  could be anywhere from the pink wire terminal to the sender itself.  Measure the coil resistance from the power terminal to the green terminal and from the power terminal to the top terminal  make sure neither is open circuited, also make sure neither is shorted to the case (although if they were you should blow your instrument panel fuse).

Following up on Mark's information, refer to the following link:  http://www.camaros.org/forum/index.php?topic=4227.0

In this discussion at the above link, there are two post with PDF file attachments which are pages 3-241 and 3-242 from the National Service Data TUNE-UP MANUAL; 1961-1969.  These two pages (PDF attachments) provide trouble shooting procedures for Chevrolet (not just Camaro) gauges along with a good schematic representation of all the gauges including the temp gauge. 

The temp gauge schematic and troubleshooting guide is contained in the second attachment, page 3-242. 

The calibration resistor is referred to on page 3-242 as the "A" coil or the coil on the left side of the temp gague schematic.  Needle position on the gauge depends on the current flow through the "A" coil to ground (cal resistor)  vs. current flow through the "B" coil and the temp sending unit to ground.  Current flow through the "A" coil is essentiially constant.  Current flow through the "B" coil varies and is controlled by the change in the internal resistance of the temp sending unit which varies with engine temp.

As I understand it, and if everything is working as it should;

1)  If you disconnect the green wire at the temp sending unit, then there is no current flowing through the "B" coil.  In this case, the needle will stay pegged left on the gauge at the "C" position. 

I have previously checked this on my 69 RS with the engine at normal operating temperature, needle on temp gauge inidcating normal tem.  As soon as I disconnect the lead at the temp sending unit, the needle goes to "C".  Reconnect the lead at the temp sending unit and the needle moves back to indicate normal engine temp.  If the gauge is constantly reading "C" this would indiicate either no voltage to the gauge or an open somewhere in the green line between the gauge and the temp sending unit or the temp sending unit itself is open and defective.

2)  If as soon as you turn the key to the "ON" position; applying 12v to the gauge, the needle deflects all the way to the "H" position, then there is no current flowing through the "A" coil - which is either caused by an open cal resistor or bad ground at the bottom terminal.  

What you are experiencing may well be an internal issue in the armature of the gauge itself, in which case a new guage may be the solution.
« Last Edit: June 01, 2009, 12:02:10 AM by rich69rs » Logged

Richard Thomas
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« Reply #7 on: June 01, 2009, 07:29:29 PM »

Thank you all.    Rich, that diagram is very similar to a Chilton manual I have that shows GM guages in it. Between everything here, I would guess that something is disconnected between the two coils.  Getting a new one is not really an option as it's original.  I found a place to repair it, but I'm too cheap.  I'm going to take it apart and see what I can do as I usually get things fixed.  If not, oh well, it's only $.  I just wanted to make sure everything from the outside was covered.  Thanks again. 
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68Z28
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