Temp guage problem or sender?
Thread Starter
Racer
Joined: Mar 2002
Posts: 256
Likes: 35
From: Soddy Daisy TN
Temp guage problem or sender?
I know that this has been beat to death but I can't really find my answer in the archives. I'm as lost a last year's easter egg on this.
Recently, my 62 (340 hp) began to show the operating temp cooler and cooler each time I drove it. Yesterday, took it for a drive and the temp guage never budged. It was reading at the 100 degree mark bottomed out. So, I replaced the sending unit with a TU-5 from Autozone. I cranked the car and let it warm up. The temp guage started to respond as the thermostat opened, but would never get above (approx) 125 unless I raced the engine and then it would jump up to about 180 and would die back down as the engine returned to idle. The stupid guage would go up and down drastically with the engine revs. I put an infared gun on the intake at the sender and it would read approx 160 degrees (steady) with the car idling.
I have normal level in the expansion tank, the wiring is new and intact, there is no teflon on the sender threads, eventhough I can't get the new on to seal off and the timing is normal.
Plus, I grounded the guage to see if it would respond and it pegged high so I believe the guage is OK. (?)
Any ideas? Recommendations? Guage or sender problem?
Thanks, Steve
Chattanooga
Recently, my 62 (340 hp) began to show the operating temp cooler and cooler each time I drove it. Yesterday, took it for a drive and the temp guage never budged. It was reading at the 100 degree mark bottomed out. So, I replaced the sending unit with a TU-5 from Autozone. I cranked the car and let it warm up. The temp guage started to respond as the thermostat opened, but would never get above (approx) 125 unless I raced the engine and then it would jump up to about 180 and would die back down as the engine returned to idle. The stupid guage would go up and down drastically with the engine revs. I put an infared gun on the intake at the sender and it would read approx 160 degrees (steady) with the car idling.
I have normal level in the expansion tank, the wiring is new and intact, there is no teflon on the sender threads, eventhough I can't get the new on to seal off and the timing is normal.
Plus, I grounded the guage to see if it would respond and it pegged high so I believe the guage is OK. (?)
Any ideas? Recommendations? Guage or sender problem?
Thanks, Steve
Chattanooga
Tech Contributor
Joined: May 2005
Posts: 15,576
Likes: 118
From: Conroe Texas
Have you compared the Resistance reading at the sender, to the chart in this document to see if your sender is telling lies?
http://www.lbfun.com/warehouse/tech_...g%20gauges.pdf
Sounds like you might have an air bubble in the cooling system, causing hot coolant to only hit the sender during high flow moments.
http://www.lbfun.com/warehouse/tech_...g%20gauges.pdf
Sounds like you might have an air bubble in the cooling system, causing hot coolant to only hit the sender during high flow moments.
Tech Contributor
Joined: May 2005
Posts: 15,576
Likes: 118
From: Conroe Texas
Thread Starter
Racer
Joined: Mar 2002
Posts: 256
Likes: 35
From: Soddy Daisy TN
The problem started before harness replacement and I have have a good ground from the cluster to the valve cover. I will retrace it to make sure it is still intact
You've got me thinking about the air bubble thing, but it looks like over time, it would vent itself.
Safety Car
Joined: Nov 2002
Posts: 4,462
Likes: 1,293
From: minneola fl
Ifyou used any teflon tape or sealer on the threads of the sender, it might not be getting a good ground. Take the sender wire off, and while someone is watching the gauge with the ignition on, tap the wire to ground. The needle on the gauge should peg. If it does then the problem is in the sender/sender wire.
Good luck.
Mike Coletta
Good luck.
Mike Coletta
Tech Contributor
Joined: May 2005
Posts: 15,576
Likes: 118
From: Conroe Texas
Ifyou used any teflon tape or sealer on the threads of the sender, it might not be getting a good ground. Take the sender wire off, and while someone is watching the gauge with the ignition on, tap the wire to ground. The needle on the gauge should peg. If it does then the problem is in the sender/sender wire.
Good luck.
Mike Coletta
Good luck.
Mike Coletta
Tech Contributor
Joined: May 2005
Posts: 15,576
Likes: 118
From: Conroe Texas
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Melting Slicks
Joined: Jul 2004
Posts: 2,869
Likes: 196
From: Maryland
One item not mentioned although not likely is the thermostat. It might be stuck open but being summer the gauge should read a least 140* . Also take a resistance reading between the sender connector & the back of the temp. gauge.
Pro
Joined: Mar 2009
Posts: 576
Likes: 2
From: The French Connection New Orleans
Thanks
Joe
Tech Contributor
Joined: May 2005
Posts: 15,576
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From: Conroe Texas
If so, your car does not have a ground wire to the valve cover.
Your car has a ground wire that connects to the top of the cluster above the tach. That ground wire is part of the grounding system that originates at the ground wire screwed to the "Z" bar to the left of the radio.
That "Z" bar gets it's ground from the small ground strap that connects the frame to the birdcage underneath the left front corner of the driver's compartment.
The frame gets it's ground from the ground strap that connects the frame to the motor (in the case of my 65, at the left front motor mount).
The engine gets it's ground from the negative battery cable running from the battery to the engine. No A/C, it goes to the starter. Factory A/C, it goes to the bell housing (for a small block 65 at least).
Pro
Joined: Mar 2009
Posts: 576
Likes: 2
From: The French Connection New Orleans
A 66. I appreciate the info. I'm sure everything is grounded. I'll have to look into the air pocket possibility. Thanks for your help on this and all the other issues I've had.
Joe
Joe
Tech Contributor
Joined: May 2005
Posts: 15,576
Likes: 118
From: Conroe Texas
You are welcome. Have you tried, or are you familiar with, measuring the Ohms at the temp sender to determine what story it is telling? You may have addressed this in your own thread but I don't remember.
Pro
Joined: Mar 2009
Posts: 576
Likes: 2
From: The French Connection New Orleans
I've checked the resistance more than once and the readings seem to fall within the parameters I've seen on the charts. Although, I have not checked it under this particular condition since it just showed up while bringing it to the shop.
My mechanic is working on the a/c right now and he doesn't see how the air pocket could cause this increase temp reading on acceleration or just increasing rpm.
If the sensing part of the sender is and insulated from the iron manifold, what would be hotter, the air or water?
I'm starting to think that the quickness in the rate of increase must be an electrical issue. I can't see how the sensor could react that fast.
Will keep searching.
Thanks again, Joe
My mechanic is working on the a/c right now and he doesn't see how the air pocket could cause this increase temp reading on acceleration or just increasing rpm.
If the sensing part of the sender is and insulated from the iron manifold, what would be hotter, the air or water?
I'm starting to think that the quickness in the rate of increase must be an electrical issue. I can't see how the sensor could react that fast.
Will keep searching.
Thanks again, Joe
Drifting
Joined: May 2006
Posts: 1,469
Likes: 329
From: Southold New York
Your gauge is reacting to the increased voltage put out by the charging system as the revolutions are increased. Put a voltmeter in the system and you will see a direct correlation between charging voltage and temperature reading. Isn't there a damper element built into the gauge to filter out these voltage changes? Perhaps its time for a new or rebuilt gauge?
Pro
Joined: Mar 2009
Posts: 576
Likes: 2
From: The French Connection New Orleans
Your gauge is reacting to the increased voltage put out by the charging system as the revolutions are increased. Put a voltmeter in the system and you will see a direct correlation between charging voltage and temperature reading. Isn't there a damper element built into the gauge to filter out these voltage changes? Perhaps its time for a new or rebuilt gauge?
Melting Slicks
Joined: Mar 1999
Posts: 3,029
Likes: 137
From: Nashville TN area
I agree, this is an electrical problem.. an air bubble or pocket in the fluid won't cause wild fluctuation or even smooth rapid movement of the temp gauge - the sender cannot react that fast since the brass must cool to affect the resistance.
If there was a ground problem, or wiring problem in general, the gauge would likely fluctuate wildly every time the connection was broken, unless it has a weak ground, then power (amps) increase above idle would be noticeable in the temp gauge movement..
I suspect one of two things:
1) weak ground (but solid connection - i.e. the ground connection isn't loose, but it isn't sufficient in size)
2) voltage problem - the temp gauge is really a volt meter of sorts.. if your alternator voltage swings with rpm, then you might notice it in your temp gauge, but most likely the problem is internal to the gauge itself in that the resistance wiring internally (bi-metalic spring) has deteriorated and changing resistance value when alternator power (amps) is higher with higher idle..
good luck!
If there was a ground problem, or wiring problem in general, the gauge would likely fluctuate wildly every time the connection was broken, unless it has a weak ground, then power (amps) increase above idle would be noticeable in the temp gauge movement..
I suspect one of two things:
1) weak ground (but solid connection - i.e. the ground connection isn't loose, but it isn't sufficient in size)
2) voltage problem - the temp gauge is really a volt meter of sorts.. if your alternator voltage swings with rpm, then you might notice it in your temp gauge, but most likely the problem is internal to the gauge itself in that the resistance wiring internally (bi-metalic spring) has deteriorated and changing resistance value when alternator power (amps) is higher with higher idle..
good luck!
Pro
Joined: Mar 2009
Posts: 576
Likes: 2
From: The French Connection New Orleans
The bummer is I bench checked the gauge with a 12 volt power supply and pot and it appeared to be working fine. 
So - anyone out there ever changed a temp gauge without removing the cluster panel - I JUST RE-INSTALLED IT!!
Maybe I'll try to work some voo doo on it
So - anyone out there ever changed a temp gauge without removing the cluster panel - I JUST RE-INSTALLED IT!!
Maybe I'll try to work some voo doo on it
Drifting
Joined: May 2006
Posts: 1,469
Likes: 329
From: Southold New York
IIRC, mid-year cars had a form of wheatstone bridge in the gauge to supress battery voltage changes, later cars had a three wire regulator which would deliver a relatively constant voltage to the gauges regardless of what the charging system was producing. Early cars, like all others of the period, would have their gauge readings affected by the actual voltages in the electrical system, some cars to a greater extent than others. Live with it or replace the gauge.