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Engine Coolant temp gauge working funky


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Just had my 70 MC engine rebuilt (350 5.7). Car came from factory with the gauge package. Before I took it to the rebuild shop the temperature gauge and the fuel level gauge which sits right below the temp gauge on the dash worked fine. Temp gauge would always read zero until needle slowly moved to the right as engine heated up. Can't remember how my fuel gauge read but I think it either went to far left when car was off and moved to the correct reading when car started or always remained at correct gas level reading whether care was on or off.

Got the car back from the shop and noticed that the temp gauge was not reading correctly, as it laid to the far right regardless whether car was on or not. Also noticed that the fuel gauge was not reading correctly Funny thing is, they both start to work correctly after I have driven the car for some time (1 hr or so). Also, because I suspect the water temp sensor may be faulty (it was not replaced in rebuild), I went to buy one at NAPA and it seems to be a little different that the one currently affixed. The parts guy stated that the sensor for a car with gauges is slightly different than the sensor for a car with idiot lights.  Could this be the problem?

I think someone mentioned that the sensor wire from the engine block goes to a relay (?) near the firewall that regulates the signal (?) to the gauge on the dash?

Trying to figure out what to fix or replace and trying to determine why the gas gauge problem would be related to the temp gauge. I thought they were independent.

As usual, any advice is appreciated.

 

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Try following the wiring from the Source to their respective connections and verify.  I am assuming it is a Factory Tach Dash with the Temp & Fuel gauges over/under on Dash?   They worked before Engine work tells me they are not connected properly or is one heck of a coincidence.  That's my take and Good Luck.................:)

Doug

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The temp senders are different for gauges vs idiot lights. The sender for idiot lights is just a trigger that turns on the dash light when it reaches a certain preset temperature.. it acts like an on/off switch. The sender for a gauge is different; it behaves more like a dimmer switch or a volume control, and yields a variable signal that ramps up or down as temperatures change.

Both the fuel and temp senders control the ground side of the circuit... if you unhook the wire from the sender there should be a voltage in the wire when the key is on. Grounding that wire should get a reaction from the gauge in the dash. Because you have a temp gauge, the fuel and temp circuits work the same way... you have  a voltage source going the the gauge and the other side of the gauge is connected to a sending unit which is grounded.  The sending unit controls whether the gauge sees a "good" ground, no ground, or something in-between, and the gauge in the dash points accordingly.

I'm not sure how the sender might have been changed during your rebuild, you would expect the same part to be reinstalled if it was removed.

I am struggling to figure out why the fuel gauge is acting up. Doing engine work shouldn't require messing with the dashboard or the fuel tank, and the fuel gauge circuit isn't connected to the engine anywhere.

 

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Hey Mark,

I think I understand the way the circuit works but am trying to learn to troubleshoot the problem step by step. As I think I understand what you are saying, I was going to test the voltage (12-13 v) on the wire for the Temp sensor by setting my multimeter to 15 DC V and attaching to that wire one of the meter's clips and affix the other clip end to some ground. With the ignition turned to the on position, the temp gauge needle should move ? (in what direction?). If I do that same step but without the key in ignition, what should the meter read? Should I test the resistance of this same connection setup with the key off?

I was also going to test resistance on the temp sensor without taking it out of side of engine by taking off the wire, setting the multimeter to ohms, connecting a clip of the meter to top end of the sensor (where the wire connects) and touch the other clip of the meter to the engine block to get a sense of whether it is bad. It should register resistance if it is still operable, right?

I just find it funny that both gauges revert to working fine once I've driven car and engine is at usual operating temp. This is one of the last things on the car that don't work as they should but are kind of important to monitor. 

May have other questions after I perform each of those steps. I realize it is probably an easy fix but wan t to make sure my fix is not hinky. 

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If you unhook the wire from the sender, you can test the wire with the voltmeter to make sure it has a voltage there. I don't recall if it is a full 12 volts.  Key should be on for the test.

With the wire still unhooked, place it somewhere where it can't find a ground. (no meter for this test) Observe gauge reading on dashboard with key on.

Now connect the wire to a good ground with a jumper wire. Observe gauge reading on dashboard, key on.

Without a ground (open circuit) the gauge should read full in one direction, and with a jumper to ground it should read fully in the other direction. These tests so far only confirm that the dash gauge is functioning properly.

To test the sender, it is as you described: Unhook the wire from the sender, and hook one voltmeter lead to the now vacant terminal. The other voltmeter lead goes to a good ground. With the voltmeter set to read resistance (ohms) it should give a reading at this point. Start the engine and let it begin to warm up.; as it warms, the ohm reading on your voltmeter should change as the temps go up.

If somehow you have an idiot light sender in there, it's near impossible to test.  It would show infinite ohms (indicating no connection to ground) until the engine begins to overheat and reaches the senders' pre-set limit. This would be a temperature above boiling. At that point the sender trips and the meter would show zero oms. 

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