are my gauges causing a short? - Team Camaro Tech
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post #1 of 13 (permalink) Old Oct 9th, 09, 07:21 PM Thread Starter
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Jeff
 
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are my gauges causing a short?

guys,

Im installing fuel injection and need 12V key switch to the ECU. battery disconnected. So i find a wire that ive got tapped to the ign fuse terminal at the fuse block. i wanted to make sure that it was in fact 12V, so i did a resistance measurement to ground..expecting an open circuit (not grounded at all). Well, come to find out, i got around 77 ohms.

At this point, im wondering what is 77 ohms. thinking its my door switch grounding through bulbs, i shut my door. resistance is still there. So i wonder.. are my gauges causing some resistance to ground?

sure enough, i unplug all my gauges in the console and the resistance goes to open...

these gauges are all just voltage dividers right? they should have resistances.

But, i dont have my temp sender hooked up right now. So that circuit should not go to both ground and power because the sender isnt there to complete the circuit... Right??

bad ground somewhere? or perfectly normal?

just for kicks, do you have a resistance measurement across the temp gauge terminals? both plugged in and unplugged to the connector?

thanks

1967 SS 396, M20.
now a 10.4:1 540, AFR 315 heads, UDHarold SR Cam with a Tremec TKO
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post #2 of 13 (permalink) Old Oct 10th, 09, 07:55 AM
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Re: are my gauges causing a short?

If measuring gauge power wire to ground, yes, there will be resistance.
Fuel gauge sending unit, coolant temp sensor, coolant gauge itself.
All are connected as a parallel circuit. The sum of them will equal a resistance lower than the singular smallest resistance.
Resistance in a parallel circuit is calculated, not added as in a series circuit.
Try here:
http://www.1728.com/resistrs.htm

The coolant gauge is a voltage divider. Gauge will be between power and the sending unit as the gauge taps off a set amount of voltage to the sending unit for gauge indication. The lower the s/unit resistance, read hotter coolant, the less resistance, more needle deflection. However, the gauge itself will read to ground because of the circuit, voltage divider inside.

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post #3 of 13 (permalink) Old Oct 10th, 09, 08:21 AM Thread Starter
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Jeff
 
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Re: are my gauges causing a short?

ok, so heres a little drawing to help explain what i mean...

V batt + ____________________ (off, set to 0V)
|
|_________________ ohmmeter probe A
|
|
|
gauge needle (more current, more deflection)
|
|
____________________ to sending unit (which in my case isnt connected)
|
|
____________________ from sending unit (again, not connected)
|
|
______________________ ohmmeter probe B
|
|
ground

if this is a correct representation on the temp gauge circuit, then the ohmmeter should read open (no resistance at all) because the sending unit is not there to ground the gauge.

Is this a correct representation of the temp circuit?

heres more :
http://www.secondchancegarage.com/public/653.cfm

if this is right, why would i get anything but open without the sensor installed?

thanks

1967 SS 396, M20.
now a 10.4:1 540, AFR 315 heads, UDHarold SR Cam with a Tremec TKO
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post #4 of 13 (permalink) Old Oct 10th, 09, 09:19 AM
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Re: are my gauges causing a short?

A gauge has internal resistance, the coil of wire and its resistors making up the voltage divider network, depending upon manufacture. Not all gauges are the same for the same function.

As you place the meter probes across the gauge terminals, gauge by itself, out of the circuit, the needle may move and resistance will be indicated. Needle movement is dependent upon polarity of the probes to the meter.

Wire to sending unit to ground will indicate resistance depending on the temperature of the coolant.

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post #5 of 13 (permalink) Old Oct 10th, 09, 09:41 AM Thread Starter
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Re: are my gauges causing a short?

i think my gauge works... what im trying to understand is how my temp gauge would ever see ground if my temp sensor is not hooked up.

i know the gauge has resistance, but it shouldnt see ground without the sensor...

1967 SS 396, M20.
now a 10.4:1 540, AFR 315 heads, UDHarold SR Cam with a Tremec TKO
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post #6 of 13 (permalink) Old Oct 10th, 09, 11:55 AM
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Re: are my gauges causing a short?

Through the gauge internally. Yes it will by its own design.

The gauge has three terminal studs on the back, yes?
One is for power.
Another is for the temp s/unit.
The last is for ground.

Think of a "T"
You apply power to the gauge on the top left, maybe a resistor is also part of this section, at the junction of the crossbar and staff, the upper right of the T goes through the meter and out the to temp s/unit. The staff of the T is a resistor and its tied to ground. Maybe, depends upon design manufacture.

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post #7 of 13 (permalink) Old Oct 10th, 09, 08:13 PM Thread Starter
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Re: are my gauges causing a short?

my 67 original console temp gauge has 2 wires, pink (voltage supply) and green that goes out to the temp sender. maybe it grounds through the gauge body? if it does ground through the gauge body, i can see a parallel resistance... one through the gauge body and one through the temp sender. But, i dont see a specific ground wire on the gauge. it appears to be wired in serially with the temp sender being the ground... and its not installed right now.

ill look at it more closely tomorrow, but im still leaning toward a bad ground somewhere...

1967 SS 396, M20.
now a 10.4:1 540, AFR 315 heads, UDHarold SR Cam with a Tremec TKO
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post #8 of 13 (permalink) Old Oct 11th, 09, 07:19 AM
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Re: are my gauges causing a short?

Let me know what you find. Gauge could be grounded internally, but since it is OE, I doubt and agree with your suggestion on current flow; gauge in series with power and s/unit.

Are other gauges unhooked at time of measurement?

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post #9 of 13 (permalink) Old Oct 12th, 09, 12:07 PM Thread Starter
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Re: are my gauges causing a short?

nope, no other gauges hooked up. is this some kind of weird parallel resistance summation of all the other light bulbs in my car? thats what im kind of leaning toward

1967 SS 396, M20.
now a 10.4:1 540, AFR 315 heads, UDHarold SR Cam with a Tremec TKO
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post #10 of 13 (permalink) Old Oct 12th, 09, 04:50 PM
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Re: are my gauges causing a short?

Let us back up for a moment. You have a ohmmeter probe on the IGN spade in the fusebox and the other ohmmeter probe on ground. Meter reads approx 77 ohms; unplug the console gauge connector and an open is read?

If the above is true, I would probe each console connector pin to ground to see if the ohm value can be reproduced. Once found, go to the next device/item following the wire color.

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post #11 of 13 (permalink) Old Oct 12th, 09, 07:24 PM Thread Starter
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Re: are my gauges causing a short?

electrically, yes, thats exactly what im doing. heres what i think is going on... all the light bulbs in the car are pretty much wired in parallel and therefore creating all kinds of weird combinations of resistances. i pulled the bulbs out of the console and the low resistance went away on the console connector. on the main harness, the resistance was still there. makes sense now that i think about it.

so, kind of a brain tease for a second, but i didnt feel like blowing up my efi ecu, so i figured id make sure

thanks for sticking through this for me man

1967 SS 396, M20.
now a 10.4:1 540, AFR 315 heads, UDHarold SR Cam with a Tremec TKO
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post #12 of 13 (permalink) Old Oct 13th, 09, 03:19 AM
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Re: are my gauges causing a short?

You're welcome.
Lights weren't on IGN circuit unless desired this way.
Only courtesy lights are wired to BAT, i.e., dome, kick panel, glovebox, engine & trunk lights, and rear console lights and brake lights.

Instrument lighting would be wired to headlight switch unless desired otherwise.

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post #13 of 13 (permalink) Old Oct 13th, 09, 05:53 AM Thread Starter
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Re: are my gauges causing a short?

well, its the gray wire coming down to the console harness connector from the main harness ... i dont have a good schematic of that particular connector under the dash like that. but gray on the console goes to all the lights in the console. wonder if its my door switch again... ill check that tonight.

1967 SS 396, M20.
now a 10.4:1 540, AFR 315 heads, UDHarold SR Cam with a Tremec TKO
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