Problem with temp gauge
Thread Starter
Instructor
Joined: Aug 2001
Posts: 184
Likes: 2
From: FL
Problem with temp gauge
Thought I would give the forum a try. I have a 68 with a 350 engine. (origional engine is awol). I have done a frame off and now have a problem with the temp gauge reading anything past 100 degrees. (All other gauges work fine) I have om'ed out the wiring, checked power going to the gauge, insured the gauge ground is good, replaced the sending unit with a new one from a corvette source, replaced the temp gauge with a new one thinking the origional was bad and still no luck. I have checked to ensure the sender is grounded to the block through the treads and the block has several grounds. Grounding the wire connector will only move the gauge needle to about 3/4 of the way to hot. I am more used to the needle maxing out if this is done. I have even used jump wires and bypassed the wiring harness thinking I might have something going on there. Still will not read correctly. Now here is the weird part. If I remove the sending unit and ground it and then put my pencle torch on it (flame) the gauge will register temperature. Any thoughts?? 
Le Mans Master
Joined: Jan 2006
Posts: 5,921
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From: GA "When I grow up, I'm gonna get a Trans Am and run from the cops". Direct quote from my 4yo son.
Is the sensor in an aluminum intake or aluminum heads? I had the same problem when I had it in the alum intake. Wouldn't go past 100. Put the sensor in the iron head now works as advertised. The aluminum didn't ground it worth a damn.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
If your gauge doesnt go to full peg when grounding the sender wire then the problem is "at" the gauge not necessarily in the gauge.(taking into consideration you said you used a jumper to rule out a bad sender wire)When you used the jumper did you go back to the gauge for ground?Do you have the white ceramic resistor across the back of the gauge if not look there?
Burning Brakes
Joined: Oct 2004
Posts: 904
Likes: 0
From: Murphy NC
Maybe Im being too basic here but is the engine coolant getting over 100*? Id check with a infared gun and make sure the coolant doesnt have any bubbles in it. Possibly the thermostat is missing?
Le Mans Master
Joined: Jan 2006
Posts: 9,192
Likes: 27
From: Boca Raton Florida
I have used tape, RTV, Plumbers putty, all kind of stuff and have never seen it interfere with grounding a sensor, the stuff filles in the voids in the threads between the two parts and the threads themselvs cut thru the sealant and gets a fine ground.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
If your gauge doesnt go to full peg when grounding the sender wire then the problem is "at" the gauge not necessarily in the gauge.(taking into consideration you said you used a jumper to rule out a bad sender wire)When you used the jumper did you go back to the gauge for ground?Do you have the white ceramic resistor across the back of the gauge if not look there?
EDIT-I also agree with Sixfooter on the thread dope.I've tried it both ways and it doesnt seem to make a differance-but the book says not to use it.
Last edited by ...Roger...; Jun 10, 2007 at 05:49 PM.
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Thread Starter
Instructor
Joined: Aug 2001
Posts: 184
Likes: 2
From: FL
still testing
I have checked the sender ground and it is good. The sender is in the drivers side cylinder head (iron) like the factory location. I have checked engine temp and it registers 189 or so at the thermostat. Both upper and lower rad (new) hoses have the temp drop that I want while sitting idleing in the garage, approximately 25 degrees. No issues with cooling. It is electrical. The new gauge does not use the resister as mentioned. I have 12.5 volts in the pink wire; power to gauge. The green wire; the one to the sender shows about 4-5 volts, the black shows a good ground when omed. It is odd that both the new and old gauge do exactly the same thing. I have been assured by the vender that the new sender is correct for the gauge and so on. Because everything has been appart, and I mean everything, I used a jumper direct from the sender to the gauge with the same results. When oming the sender I get a good reading. Don't you just love electrical problems? My tail lights were delightful as well. Love my 68 but want to get this temp deal fixed before crusing. 
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Does the back of your gauge look like this?You said the new gauge doesnt use the resistor did the old one have one on it?You have done a good job checking everything your problem has to be at the gauge.
Melting Slicks
Joined: Jan 2003
Posts: 2,713
Likes: 31
From: Shreveport LA
LA Events Coordinator
I had a similar problem...never went over 120*...checked the gauge, continuity of the wire, replaced the sending unit...and everything was working ...and after all that...found out the thermostat had failed in the "open" position, so the engine never got a chance to "heat up"
Hope this helps...Good Luck!
Hope this helps...Good Luck!
Melting Slicks
Joined: Oct 2005
Posts: 2,114
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From: upstate N.Y.
I HAD A SIMILAR PROBLEM WITH MY 76.Doesen't go over 100 and did all the same tests and new gauge and does the same thing.So i am just going on to other problems. Good luck and keep us posted on the outcome.
Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
The resistor "must" be in place and a fiber washer under the resistor.If the resistor is not there the gauge will read about 215* when you ground the green sensor wire.It will remain around 100* at all other times even when connected to the sender in the head.
Thread Starter
Instructor
Joined: Aug 2001
Posts: 184
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From: FL
Will do some more testing
Thanks so much for all the input. I think you are correct that this must be a problem at the gauge. I will run some more tests and see if I can isolate the problem. If and when I find it I most certainly will post it. Dwncchs, my old gauge did have the resister on it. The new gauge instructions said not to use the resister. I agree that the problem is at the gauge, just not sure what to do, and I do not want to kill the new gauge, if thats possible. Your picture is great and the back of my gauge looks like the picture minus the resister.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Thanks so much for all the input. I think you are correct that this must be a problem at the gauge. I will run some more tests and see if I can isolate the problem. If and when I find it I most certainly will post it. Dwncchs, my old gauge did have the resister on it. The new gauge instructions said not to use the resister. I agree that the problem is at the gauge, just not sure what to do, and I do not want to kill the new gauge, if thats possible. Your picture is great and the back of my gauge looks like the picture minus the resister.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Thanks so much for all the input. I think you are correct that this must be a problem at the gauge. I will run some more tests and see if I can isolate the problem. If and when I find it I most certainly will post it. Dwncchs, my old gauge did have the resister on it. The new gauge instructions said not to use the resister. I agree that the problem is at the gauge, just not sure what to do, and I do not want to kill the new gauge, if thats possible. Your picture is great and the back of my gauge looks like the picture minus the resister.
Thread Starter
Instructor
Joined: Aug 2001
Posts: 184
Likes: 2
From: FL
Thanks Dwncchs for all the information. I will look at the gauge and see if I am missing the washers you mentioned. I agree the problem must be at the gauge. I will also put the resister back on the new gauge and see what happens. I will let you know how all turns out. Won't be doing it this weekend as I will be at Vettes at the Village just outside Jacksonville FL. This is a primere NE Florida event at the World Golf Village held every year on fathers day weekend. 
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
Just for $hits and giggles I went out and removed the resistor and the fiber washer and the gauge performed Exactly as you described.Exactly.Check with who sold you your gauge but I cant imagine it would hurt the gage.The resistor is insulating the gage "from" ground by around 90 ohms.Give us an update when you find out something.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
I do enjoy getting to the bottom of things even if it takes a little extra time.Plus I'm just a little anel if no one noticed.
Oops can I say that on here!!!