Temp Gauge

[Larry Stewart]

The temp gauge on my 63 fuelie is jumping all over the place...flips back and forth and buries itself sometimes then starts moving again. If I tap on the glass it comes back to normal and remains steady. Anyone out here experienced this before...just started occuring recently. No problems before that. Appreciate any responses.

...Larry...

[Jack H.]

Re: Temp Gauge

Check connections on back of gauge AS WELL AS fasteners holding gauge into cluster. Flakey ground reference can and will cause such symptoms.

Also, check wire run to temp sender. Depending on year of Corvette, some senders are in intake manifold while others are mounted in LH cylinder head. ALL wires were dressed inside black insulating loom intended to thwart nicks/melting resulting in premature contact to ground (this will PEG the meter -- current runs to ground bypassing thermister resistance in temp sender).

Last, where gauge wire connects to temp sender, you'll note the sender has button head, pinhead, or wafer contact post (depends on year and after market work on car) that's insulated by plastic/bakelite from the temp sender body that IS electrical ground. If engine compartment contaminants (oil, coolant, Etc.) coat bakelite insulator, you've got and unintended path to ground around the sender's internal resistance (plateau's out around 50 ohms keeping gauge needle from 'pegging').

[Jack H.]

Re: Temp Gauge

Check connections on back of gauge AS WELL AS fasteners holding gauge into cluster. Flakey ground reference can and will cause such symptoms.

Also, check wire run to temp sender. Depending on year of Corvette, some senders are in intake manifold while others are mounted in LH cylinder head. ALL wires were dressed inside black insulating loom intended to thwart nicks/melting resulting in premature contact to ground (this will PEG the meter -- current runs to ground bypassing thermister resistance in temp sender).

Last, where gauge wire connects to temp sender, you'll note the sender has button head, pinhead, or wafer contact post (depends on year and after market work on car) that's insulated by plastic/bakelite from the temp sender body that IS electrical ground. If engine compartment contaminants (oil, coolant, Etc.) coat bakelite insulator, you've got and unintended path to ground around the sender's internal resistance (plateau's out around 50 ohms keeping gauge needle from 'pegging').

[Fred Oliva]

Re: Temp Gauge

Larry,

The temp gauge circuit for your 63 is fairly simple. It consists of 2 components, the temp sender & a temp gauge wired in series. The gauge is actually an ammeter with it's face calibrated in degrees F. The sender is a device that varies it's electrical resistance with temperature.

One side of the gauge is connected to 12V, the other side is connected to your sender via the sender wire Jack speaks of. The sender body is connected to the engine and thus, the system ground. This creates a path for electircal current to flow from the 12V connection on the gauge, thru the gauge, thru the sender, and into the electrical system ground.

When the sender gets hotter, it's resistance goes down, allowing more current to flow thru the circuit, thus making the gauge needle deflect further to the right.

Now, armed with this theory of operation, lets see what can go wrong that can cause your problem. The first thing I would do is to connect a voltmeter to the 12V side of the gauge, start the car & see if the voltage is steady. A varying voltage is probably NOT causing your problem, but it's a good idea to eliminate this up front or else you could be chasing your tail later on.

If your voltage at the gauge is steady, disconnect the sender wire at the gauge, place your voltmeter in the OHMS mode & connect one lead of the meter on the button head of the sender, and the other on a good ground. A carb mounting nut is a fairly good ground. Start the engine & observe the resistance. You should see a steady decrease in resistance from around 600 to 800 ohms to a minimum of about 80 to 100 ohms.

Don't get too hung up on exact numbers. Your looking for NO jumping around of the resistance -- just a steady decrease. If you get erratic resistance readings, it could be that the either the sender is defective or there is a contaminant on the top or the sender causing a path to ground.

BE EXTREMELY CAREFUL not to short the sender wire to ground when you disconnect it from the sender or you could damage your gauge.

If everything checks out, it's either the gauge or the wiring. Next, I would reconnect the sender wire to the sender, & disconnect the wire on the sender side of the gauge. Connect one wire of your ohmmeter to the semder wire that was disconnected from the gauge and the other wire to a good ground. Once again start the car & you should see a steady decrease in resistance & no jumping. Make sure you've actually disconnected the sender side of the gauge & not the 12 V side.

If you see erratic resistance readings, try to wiggle the bulkhead connector on the firewall. It's just under the drivers side hood pad next to the master cylinder. If this effects the resistance readings, then you've got a bad connection here. Also check all your other connections as Jack recommends. You could have a bad wire or connector at the sender.

If everything still checks our ok, I'd borrow another gauge & see if this fixes your problem.

Sorry for the long append. Hope this helps.

Fred O

[Fred Oliva]

Re: Temp Gauge

Larry,

The temp gauge circuit for your 63 is fairly simple. It consists of 2 components, the temp sender & a temp gauge wired in series. The gauge is actually an ammeter with it's face calibrated in degrees F. The sender is a device that varies it's electrical resistance with temperature.

One side of the gauge is connected to 12V, the other side is connected to your sender via the sender wire Jack speaks of. The sender body is connected to the engine and thus, the system ground. This creates a path for electircal current to flow from the 12V connection on the gauge, thru the gauge, thru the sender, and into the electrical system ground.

When the sender gets hotter, it's resistance goes down, allowing more current to flow thru the circuit, thus making the gauge needle deflect further to the right.

Now, armed with this theory of operation, lets see what can go wrong that can cause your problem. The first thing I would do is to connect a voltmeter to the 12V side of the gauge, start the car & see if the voltage is steady. A varying voltage is probably NOT causing your problem, but it's a good idea to eliminate this up front or else you could be chasing your tail later on.

If your voltage at the gauge is steady, disconnect the sender wire at the gauge, place your voltmeter in the OHMS mode & connect one lead of the meter on the button head of the sender, and the other on a good ground. A carb mounting nut is a fairly good ground. Start the engine & observe the resistance. You should see a steady decrease in resistance from around 600 to 800 ohms to a minimum of about 80 to 100 ohms.

Don't get too hung up on exact numbers. Your looking for NO jumping around of the resistance -- just a steady decrease. If you get erratic resistance readings, it could be that the either the sender is defective or there is a contaminant on the top or the sender causing a path to ground.

BE EXTREMELY CAREFUL not to short the sender wire to ground when you disconnect it from the sender or you could damage your gauge.

If everything checks out, it's either the gauge or the wiring. Next, I would reconnect the sender wire to the sender, & disconnect the wire on the sender side of the gauge. Connect one wire of your ohmmeter to the semder wire that was disconnected from the gauge and the other wire to a good ground. Once again start the car & you should see a steady decrease in resistance & no jumping. Make sure you've actually disconnected the sender side of the gauge & not the 12 V side.

If you see erratic resistance readings, try to wiggle the bulkhead connector on the firewall. It's just under the drivers side hood pad next to the master cylinder. If this effects the resistance readings, then you've got a bad connection here. Also check all your other connections as Jack recommends. You could have a bad wire or connector at the sender.

If everything still checks our ok, I'd borrow another gauge & see if this fixes your problem.

Sorry for the long append. Hope this helps.

Fred O

[Larry Stewart]

Re: Temp Gauge...thanks for the help!!!!

Thanks Fred and Jack for the help...traced the wire up and there was a lose wire on the plug that connects to the gauge...one of the wires was making intermittant connection in the plug (did the nomal wiggle of the wire, can't believe I didn't start there in the first place). Was able to replace the contact connector with a new one I fashioned out of some old connectors in the garage. Works great now...but I really appreciate the help.

...Larry...

[Larry Stewart]

Re: Temp Gauge...thanks for the help!!!!

Thanks Fred and Jack for the help...traced the wire up and there was a lose wire on the plug that connects to the gauge...one of the wires was making intermittant connection in the plug (did the nomal wiggle of the wire, can't believe I didn't start there in the first place). Was able to replace the contact connector with a new one I fashioned out of some old connectors in the garage. Works great now...but I really appreciate the help.

...Larry...