Minimum voltage for a test light ?

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I've got a simple 3-wire sensor circuit that I need to check. It's a Fuel Tank Pressure Sensor and all it has is a 5V Reference Wire, Ground Wire, and Signal Wire. The signal wire sends a variable voltage signal between ranging from 0.1 to 4.9 volts to the PCM depending on what the fuel tank pressure is. Can I back probe the signal wire at the sensor with an incandescent test light? The plan is to put a T-pin in back of the connector where the signal wire enters the sensor and check it for voltage with a test light. Obviously, I'm not going to get a voltage reading with a test light, but at least I can tell if the signal wire still has continuity from the sensor all the way up to the PCM if the test light illuminates. DTC P0452 is the stored code. (Fuel Tank Pressure Sensor Circuit Low Voltage).
 
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Yes, but I'd only use a clear bulb and expect it to barely glow. Strikes me as weird the min voltage isn't .5V, so there's always a little getting through, but what do I know?
 
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Voltmeters are pretty cheap. Usually can find one for around $10. You can use some solid copper 20 ga wire like found in fluorescent light fixtures to push into the back of connectors to use for voltage probes.
 
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You can find flashlight bulbs that have 4.8V nominal voltage - that should work pretty well, assuming your connector can source the current. Also, I second the recommendation to get a cheap DMM. They are incredibly handy. Something like this will do the trick for just 25 Washingtons.
 

JHZR2

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I think I'd want a dvm. You already know the voltage is low. Notionally that beans below the minimum value but not necessarily. Good opportunity to clean connections too, which I imagine is related. How is 5v made?
 
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Coincidentally I'm working on the same test for my car. Hard-to-fix P0456. Smoke test went OK, gas cap seal looks fine, so I'm testing the pressure sensor now. Harbor Freight multimeters are cheap. And if there's no HF you can get a Craftsman for under $30. I'd get one before I messed around with a test light, they're handy just to have. Plus the pressure sensor is a PITA to get to (at least on my car) so I'd want to get as much data as I could without having to go back in later for another test. Just an aside, the shop manual on my car has me take the sensor out and measure resistance across terminals as the test to find a bad sensor; YMMV
 
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Use a DVM, A test light may excessively load the circuit and possibly scrap the sensor you are trying to test. Another possibility is that the output impedence of the sensor may be hight enough that the bulb wouldn't glow at all. The 5V ref. comes from the ECM.
 
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Test lights are so risky, that many service manuals specify an electronic tester. The Power Probe is one type. There are too many things that can get damaged, and cost more than a Power Probe or similar device.
 
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Originally Posted By: JetStar
Use a DVM, A test light may excessively load the circuit and possibly scrap the sensor you are trying to test. Another possibility is that the output impedence of the sensor may be hight enough that the bulb wouldn't glow at all. The 5V ref. comes from the ECM.
Right answer. You should never load a sensor output with a light bulb. Most likely it's an amplified strain gauge and can drive a few milliamps. Use a meter which essentially gives you the voltage without drawing significant current.
 
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Yes the two responses are correct. Also could fry PCM from excessive current draw. It is probably best to know what the sensors voltage is at lead to PCM and voltage to ground, and the actual voltage output from PCM
 

JHZR2

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Originally Posted By: JetStar
U The 5V ref. comes from the ECM.
I asked that because I suppose that the DC/DC converter in the ECM that takes 12V and puts out 5V could also have issues.
 
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Originally Posted By: JHZR2
Originally Posted By: JetStar
U The 5V ref. comes from the ECM.
I asked that because I suppose that the DC/DC converter in the ECM that takes 12V and puts out 5V could also have issues.
Absolutely, and proper voltage range should be verified as part of the check-out.
 
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