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Hooked up console harness to factory 4 cluster gauge set up. The fuel gauge pins at full and the temp gauge pins at C, any clues. This is an existing system that did not work at all when the car was purchased. It is a 69 rs/ss.
Thanx
Eric
 

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Discussion Starter #3
Marc,
Went throught the console harness and wiring, can not find the problem. Any hints or short cuts. Help!
Thanx,
Eric
 

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Something not documented in any of the various wiring diagrams for the console gauges is that you need to add a ground jumper wire between the two metal gauge mounting brackets, since there is only one ground wire in the gauge wiring harness, and it normally lands on the rear bracket. The fuel and temperature gauges need this ground, as do the lights since they ground to the metal bracket.

The fuel gauge is pretty simple, it only has one power terminal, one ground terminal and one connection to the sender. Some of the repro gauges require a 46 ohm resistor to be installed across the power and sender connections (take the one off the back of your old fuel gauage), some have the resistor installed inside the gauge.

The bottom terminal of the gauge is the ground connection. It has a plastic insulator on the stud and sometimes needs an additional ground wire added to the stud to get a good ground.

Check for voltage at the sender connection in the trunk with the key in the run position. If you can measure voltage there, then the guage is getting power (won't necessarily be 12V but will be greater than 0V). Make sure there is no voltage with the key off.

If you have voltage there, then ground the connector to the car body, gauge should indicate empty. With the connector disconnected the gauge should read beyond full.

While the connector is disconnected stick an ohm meter on the connection going to the sender in the fuel tank. You should get a reading between 0 ohms (empty) and 90 ohms (full). If you have half a tank you should see around 45 ohms. If you don't get a reading (infinite resistance)then the problem is between the connector and the sender ground wire. The sender is grounded to a stiffener right in front of the gas tank, there is a black wire coming out from above the tankattached to the underbody frame stiffener with a sheetmetal screw right above the rear axle.

Key needs to be in the run position for the following:

If the gauge reads empty all time then either it is not getting power, has an short circuit (ground) to the sender or has a bad ground at the gauge.

If the gauge reads full all the time, either it has an open circuit to the guage, or it's missing the resistor on the back of the gauge.

If you can manually put the needle in the middle of the guage and then disconnect the sender in the trunk and the guage does not move to the empty position (assuming it has power to the gauge) you are missing the resistor, or gauge ground.


The temperature gauge isalmost the same as the fuel gauge, it only has one power terminal, one connection to the sender, an 86 ohm resistor (91 ohms works better) that connects from the top terminal of the gauge to the bottom (ground). The resistor controls the amount of feedback current which drives the needle downscale. without this reistor once the needle moved upscale to the normal operating temperature position it would stay there forever.

Make sure you didn't use teflon tape, or any other non conductive sealant on the threads of the temperature sender because it will insulate the sender and result in an open circuit.

Also make sure you have a temperature sender installed that works with a guage, not a temperature switch that works with the idiot light.

Check for voltage at the sender connection at the engine block with the key in the run position. If you can measure voltage there, then the guage is getting power (won't necessarily be 12V but will be greater than 0V). Make sure there is no voltage with the key off.

If your getting voltage at the sender end of the wire touch the sender wire to ground, this should make the gauge go to hot. Remove the sender wire and the gauge should return to the cold position.
 

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Mark:
You'r truning out to be the gauge expert on the forum here so I just wanted to ask you a quick question re; my temp gauge. My Gauge always reads high (right in the red at about 180 degrees) so I was thinking of adding a 91 ohm resistor in line bewteen the s/unit and the gauge. I have after market OER gauges. If I already do have a resistor on my gauge, but it's just not calibrated to the s/unit correctly, and I add ANOTHER (91 ohm resistor), would that throw things way off?? In other words, would it add to the resistance on the current resistor or just bring the gauge down where it needs to be? There's no visible resistor on the OER gauge - I believe it's built in - so it's not like I would even know how to remove it if I needed to...

Also, if I go to Radio Shack and simply ask for a 91 ohm resistor, will they know what I am talking about or do I need some other specs to get the correct one (watts, dimensions etc etc).


Thanks Very Much!!!

Dan

'68 Convertible
 

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The 91 ohm resistor goes across the terminals of the guage. I replaced the stock 86 ohm resistor on the back of my guage with a 0-500 ohm 10 turn precision potentiometer because the guage always read about 1 line above 1/4 scale at 180 degrees. Now it reads 1 line under 1/2 scale at the same temperature. If you want to bring the needle down scale you need a smaller resistor across the gauge. The Pot allows you to adjust the scale of the gauge accordingly. After mine was set, I just buried it in some electrical tape so the setting wouldn't change.

Remember when you buy resistors they are either plus or minus 1 percent (precision) or plus or minus 5 percent tolerance (standard) so a 90 ohm resistor could be 86 to 94 ohms. So you buy a package of 10 (thats how they come) and measure each one with an ohm meter till you find the closest one. Pots are much simpler to use for an application like this. 1/4 watt resistors are fine for this application.

Check that you have the right sender before fooling with the guage though. You could put a resistor in line with the sender but I have no idea what value it should be. When I haul my car out of the garage today, I see if I can get a resistance reading off the sender cold and at normal operating temperatures for you to compare to.
 

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Mark:
That would be great! And thanks very much for the quick reply! I have an AC/Delco sending unit. I was thinking of changing the sender but who knows if the next one will be any better!? Plus, I'd rather throw an inexpensive resistor in line and see what happens rather than buying a $25 sending unit that may or may not work....

Thanks again!!

Dan
'68 Convertible
 

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Well cold (75 F or so) my sensor reads 460 ohms. Haven't started it yet, so later today I'll find out what hot is.

Measured between the connector on the sensor and the block, wire disconnected.
 

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No a 91 ohm resistor across the upper and lower terminals of the gauge (instead of the 86 one that was on original gauges) would make the guage read higher. If the resistor is built in to the windings of the gauge then somehow you would need to install a resistor parallel to this built in resistor.

There are two sets of windings in the gauge, one coiled in one direction, one coiled in the other. The winding between the power connection and the sender wire connection is the coil that moves the needle upscale (towards hot) The coil between the power connection and the upper terminal is the downscale coil that drives the needle downscale (towards cold). This coil is balanced by the external resistor (or the internal one on the repro gauges) so that it provides a fixed amount of current passing thru the coil and thus a fixed amount of electro magnetic force. Measure between the power connection and the connection to the sender with an ohm meter, then measure between the power connector and the upper stud on the gauge. This one should be 86 ohms higher than the first one if the resistor is in there.

On an original gauge the resistor spans the connection from the coil to the ground terminal (the bottom one) on the back of the gauge. If you want to make the gauge read higher, you just put in a large value resistor, if you want it to read lower you put in a smaller value resistor. When the resistor is built into the gauge you can't do that, you have to put the resistor between the power connection on the gauge and the upper terminal of the gauge (parallel to the coil). Your only trying to get the resistance of this coil to equal the other coil plus 91 ohms.

Assuming (I've never neasured it) the upscale coil is 200 ohms, and your down scale one is 300 ohms then you need to find a resistor that will bring it down to 291 ohms (remember adding 5 ohms to my original resistance moved the needle almost 1/4 scale so it doesn't take much of a change).

291=1/((1/300 {downscale coil resistance)}+(1/X {desired resistor value}))
X=9671.12 ohms

291= (1/(.003336)+(.0001034))
291=(1/.0034394)
291=290.74 (close enough)

Use your own numbers.

Now if your sender is off, all you have to do is put a resistor inline with the sender wire by the difference in value between yours and mine. Getting data from other peoples senders would help make sure of the values.

[ 08-19-2004, 11:03 AM: Message edited by: Mark C ]
 

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Mark:
Ok, I'm getting it. It's sinking in slowly but surely...I appreciate all the info. I know my sender is off. Certainly, it's not "matched" to my gauge. I'd just like to splice a resistor in line with the sending unit if at all possible. I read some prior post re; using a potentiometer to find the correct resistor I need to make the gauge read more to scale. Just wondering if you think this would work...

-Take the ohm reading of the sending unit at normal operating temp
-Using a potentiometer to adjust the gauge (with the sender out of the loop), find the spot on the gauge where I want the needle to be at normal operating temp.
-Measure the ohm reading of the potentiometer
-Take the difference in ohm readings between the reading of the "hot" sending unit and reading of the potentiometer where the needle reads where I want it
-Use that number, and that would be the resistor (in ohms) that I need

Think that would about do it???

Dan
'68 Convertible
 

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I just measured my circuit resistance back into the guage from the temperature sender lead and it's 52.1 ohms.

Can't get to any of my guage connections to measure those without ripping my console apart.
 

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Hot the sender reads about 94 ohms to ground at operating temperature and still dropping with heat soak after I shut the engine off.

I don't recommend removing the sender wire on an A/C equipped car without some heat resistant gloves. Putting it back may have to wait for it to cool.

Yes what you describe above should work fine just make sure you account for the resistance of the sender in you resistor selection. you shouldn't have to add more than a 10 ohm resistor (depending on how your sender responds).

I don't know what it will do the the linearity of the gauge but these gauges are just for spoting trends anyway.

One other thing you can do is just pop the needle off the gauge face and put it back on where you want it to be. Do it with the car at normal temperature though. Doesn't cost anything. Pop the needle off with a fork and then just press it back on.
 

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Thanks for the responses Mark! I appreciate it. I'll try the resistor first. That way, I'll be able to sound impressive when I tell people how I calibrated my gauge - rather than just popping the needle off...now why didn't I think of that in the first place!!!

Thanks

I'll report back on how it goes...

Dan
'68 Convertible
 
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