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Old 05-01-2004, 12:33 PM
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VDO Temp guage quit

Looked for loose connection, none.
Got a new VDO sender from NAPA, still no work. Have the sender in intake manifold (350 sb) as elec fan sender is in left head and rh head plug is broken. I have a multimeter, where does one start testing?
Thanks. Charles

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Old 05-01-2004, 12:42 PM
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Did you put any sealer or teflon tape on the threads of the sender? If you did the sender is more than likely insulated from the intake and not completing the circuit. The threads are tapered pipe and should not require any sealant.
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Old 05-03-2004, 03:22 AM
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gauge testing

Doc here,

I have found no procedure for testing of electric D'arsonol type gauges either.

Digging into my Shadetree Doctor Bag 'O" Tricks I have stored up over the years....This seems to work well but is probably not the way a dealer would do it...and if you feel it more than you want to handle .. the best thing is to go to the dealer or Auto Electric Shop.

Seeing the need to test these meters over the years I have a procedure I use that I developed basically on the "How It Works" premise. This is not to say it is perfect, but will give you a pretty reasonable idea of if it works or not.

All D'arsonol Meters are based on The same premise, A low current meter, and a "Whetstone" Bridge (4 balanced resistors or a resistive load in a loop) The meter is tapped into the bridge, positive and negative as well as sensor and ground. At about Zero meter indication, Everything is "Balanced"

The missing component is the sensor. At zero degrees (ideal) the sensor probe is about equal to the resistance to the the bridge or well above ground potential. The warmer the sensor the lower the resistance to ground. To test this theory, If you add Positive power, and Ground to the Gauge with no sensor load hooked up, The Gauge Does nothing!
It will remain at or below zero or decline from it's last reported reading to zero, or below, since now the bridge is balanced (no load to the sensor) Everything is equal thus the gauge returns to zero and / or remains there

As Tempeture at the sensor climbs, the resistance to ground potential is lowered causing higher current draw, The Sensor side of the bridge is now "Unbalanced" causing the meter to draw from the unbalance positive or negative, in an attempt to equalize total load of the bridge. As the ground potential becomes higher the gauge climbs. As the potential is lowered, the meter declines.

Depending on the type of gauge you have, they are orientated in the following format:
3 pole (3 connections)
(1) Ground
(2) Sensor resistive load
(3) Ignition, or batterey,switched +12 volts.


2 Pole (2 Connections)
(1) +12 volts switched ignition,
(2) Sensor. resistive load
Look at your specific gauge, Identify if you can the Connections

(Be sure of this, miswire can destroy or severely damage the gauge)

Some are MARKED with the following, "G" "S" and "I" at the rear of the gauge, these are: G for Ground, S for Sensor, and I for ignition (12 volts) If you are not sure of the pole connections...


Refer to the Manuel or Call an auto electric shop for the information! BEFORE TESTING ANYTHING UNDER POWER!

Next, If you own a Digital Volt / ohm meter Drag it out of the tool box...If you Don't own one, Now is a good time to think about getting one if you wish to do a lot of repairs on your own. They aren't too expensive, about fifty bucks. Some High end timing lights and dwell meters have them built in to.

Set your Meter (DVM) to Ohms, then "Auto-range" or ohms X1 scale.

Measure from the "I" post to the "G" post...It should read about 400 ohms. may vary from 350 to 450 but 400 is about right. This measures across the total circuit, Bridge, and meter. Next measure from "I" to the case ground, (If it's metallic) It should also read about 400 ohms. If this is all Correct, it means the Meter and bridge are not burnt up (open circuit)

Next move your probes To "G" and "S" poles. readings here should be about 320 ohms. This indicates that The sensor to ground side of the bridge is not open and / or Short.

Next move your probes to the "S" and "I" poles, your reading here should be about 80 ohms, this will also vary depending upon type of gauge and where the meter pointer is actually sitting at the time of testing.

REMEMBER: These are generic tests to check for functionalibilty. (<---is that a real word ?) If your in the ball park, the gauge should be working provided the meter movement is not mechanically bound! (pointer stuck on the face plate, or metal chips within the movement magnet)

Function Testing outside the Car:

The word here is caution!
A miswire can ruin your Day...not to mention the gauge

Secure A 12 Volt "lantern" Battery, or if you have one, a 12 volt bench power supply.

NEVER use a battery Charger FOR any DC TESTING! (These are not well filtered DC~~ever hear your Car radio HUM loudly when charging a battery? ) Can Damage Semiconductors!

Also avoid using the Car Battery, Since it can deliver lots of current real quick, It's just not too safe.

If you don't have a "Lantern" Battery, You can pick one up at like "Rat-Shak" For about a buck a volt...
And while you are there, get a Pack of 100 ohm resistors 1/4 or 1/2 watt (4 pack) about half a buck. (You will Need These)

With All your stuff set up, Hook a red piece of wire to the + terminal on your lantern Battery. Next connect it to the "I" pole of your gauge. Make sure it's not shorting on anything and the connection is good.

Next, obtain a piece of black wire, Connect it to the - terminal on your Lantern battery. Run it to the "G" pole on the gauge. As above, be sure it's not shorting on anything and your connection is good.

At this point look at the meter, you should see it decline towards zero, if not already there. (Balancing the load) If not exactly, don't worry, at least not yet.

Next, get your 100 ohm resistor from the pack and hook one end of the resistor to the "S" pole Don't short it!

Turn the gauge towards you, with the open line of the resistor accessible to you, If the case is metallic, ground the open line to the case, if not ground it on the - lantern battery post, and watch the gauge...
BE sure it does not shoot over to the right and peg real fast.
(It shouldn't under 100 ohm load)
If it does, remove the resistor ground to keep from damaging the gauge.

The Gauge Should slowly rise to about 210 degrees and hold there, it can vary a little but 210 is about right.
BTW you will notice the resistor will become warm to downright hot depending upon it's wattage. Don't run it too long, more than a minute or two, or the resistor may burn up.
(won't hurt anything but the resistor)

If you get the above indications, The Gauge is working! Look elsewhere!
If It pegs, or fails to move at all, get a new gauge.

BTW, as I Type this I have a full set of gauges in front me (to be sure ol Doc's memory is not too foggy) and a 12 volt supply, and 100 ohm 1/8 watt resistor, and am relating the readings as I get them.(Live from Area 51...) As I said they can vary from Gauge to Gauge but your in the ballpark!

If you want to check the Calibration of your temp gauge, Either remove your sender from the engine, or (somewhat Easier) obtain a new one from the auto parts store. Get an accurate Kitchen Thermometer and boil a pan of water. Let The water settle to about 180 degrees, take the whole test setup to it and run a wire to the sensor from the gauge. Place (slowly and carefully) the Sender in the water and compare the readings. Check it at various temps to check linearity, If your Readings are within a few degrees, your good to go! Replace the gauge in the car and install the new sensor.(Since you know it's accurate!)

Well Guy's I've tortured this text enough, REMEMBER: These are not dyed in the wool procedures, but only Generic ones that have served me well over the past 35 or so, years. If your not comfortable in performing anything, Go to the Auto electric shop! Better Safe than Smoked....and you can't put the smoke back in the wire once you let it out....Doc said it...

Hope This Has helped some!
shadetree Medical bag "o" Tricks
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