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M20Curtis

Temp gauge grounds/issues

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Trying to wire in an auxiliary fan and having some issues on my temp gauge.

Gauge works fine when switched power to fan is unplugged and headlight switch is all the way off. As you turn parking lights on it jumps up, and with headlights on it jumps up to the red instantly. I have been reading and remembering about ground issues but not finding much.

Also, when all is well, I try my aux fan switched power to a known good source or even direct to the battery + and it goes right to the red as well.

I'm stumped, thinking there is a weak ground or my gauge is going out somehow. Found that a PO added a ground from back of cluster to metal of dash but it does no change with or without it. Ideas?

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Hmm. Show a pic of where your sensor is mounted.

From Autometer:

Checking the accuracy of a short sweep temperature gauge (one that uses a single terminal sender): This gauge typically has 3 terminals on the rear of the gauge (not including the light connections). It sends a signal out to the sender from the “S” terminal, and it is the senders job to vary this signals resistance to ground. With this said, for a sender and gauge to operate, then sender must be threaded into something that is grounded since the ground path for the sender is actually the sender mounting threads. The gauge itself really is nothing more than a fine tuned & calibrated ohm meter. To check the gauge, you will need a controlled and measurable resistance to ground source. Many shops that deal with automotive electrical repair have what is called a gauge tester, which is really a switchboard with different resistance values. The Auto Meter 100 to 250 degree, short sweep temperature gauge uses a specification of 1123 ohms of resistance to ground to = 100 degrees F, and 65 ohms of resistance to ground to = 250 degrees F. You may contact our Service Department for further specs.

If you do not have access to a gauge tester, you may always start with the sender. To check the sender, warm the engine up to a stable temperature. Have an ohm meter ready, as well as the tool(s) needed to remove the wire off of the sender.

With the engine running/idling, notice what the gauge is reading, and write this down the reading. With the engine still idling, remove the wire off of the sender, then hook the positive wire of your ohm meter to the sender where the sender wire was. Now hook the negative wire of your ohm meter to ground. What resistance do you measure?

I will give you some examples to help you determine what to do next.

Let’s say that the gauge read 210 degrees, and then you took a measurement of 123 ohms. This would mean that the gauge was reading exactly as it should. And if you thought the engine was running hotter than that, or colder than that, either the sender is the problem (remember the gauge was reading perfectly based on the information from the sender), or the engine really is running that temperature.

Another example for you: You suspect the gauge is reading hotter than actual, and you measure the resistance of the sender again. You measure 253 ohms, and the gauge reads 210 degrees. In this case, the sender is at 170 degrees (based on the resistance measurement), and the gauge is not responding the way it should. In this case, either the ground to the gauge itself is insufficient (which will cause a too high reading), or the gauge is out of calibration.

Never trust how a ground looks. If your gauge shows higher than normal and the sender tests where it should, then run a new dedicated ground from the threaded portion of the gauge ground terminal to an engine ground and see if the problem persists.

If this seems like too much, or is confusing, we will be happy to test your gauge for you if you send it into us. Please contact our Service Department for further assistance.

http://autometer.com/tech_faq_answer.aspx?sid=1&qid=57

GL,

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I still need to ohm meter the sensor but while working at it today I messed with some issues.

First I tried a brand new sensor so I can rule that out (in theory).

Next I made some new grounds from both the fuel gauge and the temp gauge small threaded nubs on the back of that portion, to a new hole I drilled, cleaned off metal portion, and nice metal screw so I'm sure the ground is functioning.

As soon as I try and plug it all in and pull the light switch it pins itself fully in the red. To try and rule out the gauge, I tried another gauge I have on the shelf and it does the exact same thing.

Just to appease me, is there anything that should plug in besides the main plug into the back of the cluster? Thanks in advance and I can't wait to find what the issue is on such a simple system!

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