Can anyone tell me what the voltage reading is supposed to be on the two positive wires going to the temperature gauge when the ignition key is on and the engine is cold. And also what the voltage is when the engine is at normal running temperature. This is a VDO gauge. I'm trying to figure out if my gauge is broken or it's the sending unit. Thanks

Darryl, I was hoping Steve VW or Mwrench would answer, because both members are better at Electrical than I.

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You could try this. I would try getting a reading both hot engine and cold engine directly on the sending unit. Also try the reading on the wires at the dash both hot and cold. My guess is if the sending unit is not changing current I would be to replace the sending unit.

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I hope this suggestion starts some thought for the more knowledgeable members here. There is also another member here that commented on my dead batteries a few years back that might be able to help.

What is it doing. Does it spike to Hot?

Is it this one? https://www.vdo-gauges.com/ins...with-vdo-sender.html
instructions any help?

Since I do not have direct experience with the setup in the Breakaway I did not comment. However, most analog temperature gauges operate the same way.

The sensor itself is a temperature sensitive resistor (thermister) which changes its resistance as the temperature changes. Depending on the material used for the thermistor it may increase or decrease resistance as temps go up. (Also, may be one or two wires depending if it is internally grounded or needs a separate ground wire.)

The gauge has a coil that moves the meter needle. It is actually using current to create a magnetic field to pull on the spring loaded needle.

A constant voltage (ie 12V in the Barth) feeds the meter coil. The other end of the coil is connected through the sender to ground. As the resistance in the sender changes, more or less current flows through the coil and it moves the needle accordingly.

Verify 12v at the gauge feed, then find the sender. Ground the sender wire. You should see the gauge change. If not, the gauge or wires are bad. If the gauge moves, the sender is prob bad.

You can also disconnect the sender wire from the gauge and measure the resistance to ground. Warm up the engine, see if the resistance changes. Open circuit: broken wire or bad sender. Short circuit: wire is grounded or sender bad. Varying resistance means both wire and sender are good.

Hope this helps. (I have been accused of giving essay answers to true-false questions but I try to be complete.)

good luck

Do not ground the sensor side of the gauge. (full 12 volts across it may cause it to burn up)

Now for the rest of my story. If the gauge shows a rise in temperature as the engine warms up it is likely ok, unless it is erratic ( usually by dropping in indication which points to a loose connection either- wiring, connectors,sensor internal, or even in the gauge. Attack the problem in this order) If gauge reads either high or low and is responding normally- thermisters are resisters, just with a controlled change in resistance when heated, and can change over time from that same heat. Some other things to look for would be any poor ground connection like corrosion around sensor threads, and any thing that insulates the sensor from the actual coolant. Symptoms can give a clue as to where to start, but if all else fails to work change the sensor. sorry to be so windy, john PS if there was a calibration chart for the sensor it would be much easier, it would be a resistance vs temperature chart. Sometimes "easter-egging" saves time and money. Knowledge isn't always power.

quote:

Originally posted by john canning:
Do not ground the sensor side of the gauge. (full 12 volts across it may cause it to burn up)

John has a good point. I should have said "momentarily ground" the sensor wire. Usually a quick test will not fry anything but a sustained connection may cause damage.

Okay thanks everyone for the response. What happened was a wiring harness came loose on the engine and got into the fan blades and tore some wires loose. I lost the tachometer ,temperature gauge and the alternator was no longer charging the batteries. The wiring harness was in a location that was nearly impossible to get to so I paid a mechanic to do he repair.When I got it back everything was working except the temp gauge. The temp gauge has a ground a hot and the wire coming from the sensor. The hot wire reads a constant 12 volts. The wire from the sensor reads about 7.4 when plugged into the Gage. When unplugged from the gauge it reads zero. Any ideas?

Sounds to me as if they didn't wire it properly. I'm guessing that you have the gauge out or cluster out and can get at it. If it is a solo gauge you can take it out and go down under and make a splice wire to attach to the gauge and the sendor then also a ground. The other thought I have is depends on the gauge is lit or not. I would test that wire and make sure that he didn't cross those and that you're getting 7 volts at the bulb.