misreading temp gauge
- Thread starter Thumpower
- Start date
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TrailBeast
AKA Mopars4us on Youtube
What does it say with the sender disconnected?
A quarter?
A quarter?
67Dart273
Well-Known Member
Thumpower, you need to determine if it's (in no particular order)
The sender
the gauge unit
The IVR
or a wiring / bad connection problem
You need to take some test resistors and check BOTH gauges. They are the same unit.
Hook a test resistor from the sender wire connection to a good ground, turn on the key for about a minute, and see what it reads.
Read this:
http://www.forabodiesonly.com/mopar/showthread.php?t=179517
Basically you need three resistors:
L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full
They don't have to be "that close." 25 ohms for example is close enough for 1/2 scale. Go to RadioShack and buy a package of four -- 100 ohm, 1/2 watt resistors. Hook all 4 in parallel, which gives you one 25 ohm resistor.
Now if both gauges read high, the very first thing I'd suspect is the IVR is out of tolerance. Do yourself a favor and buy an RTE which is solid state.
There can be issues with the panel wiring, with connections at the gauges, and so on, but this would actually tend to make the gauges want to read low, not high.
Also, the fact that the fuel gauge "reads OK" may not be conclusive, as the sender may be wrong, and say, example, the IVR is too high, may be "luckily" ocmpensating.
Another thread
http://www.forabodiesonly.com/mopar/showthread.php?p=1970209465
The sender
the gauge unit
The IVR
or a wiring / bad connection problem
You need to take some test resistors and check BOTH gauges. They are the same unit.
Hook a test resistor from the sender wire connection to a good ground, turn on the key for about a minute, and see what it reads.
Read this:
http://www.forabodiesonly.com/mopar/showthread.php?t=179517
Basically you need three resistors:
L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full
They don't have to be "that close." 25 ohms for example is close enough for 1/2 scale. Go to RadioShack and buy a package of four -- 100 ohm, 1/2 watt resistors. Hook all 4 in parallel, which gives you one 25 ohm resistor.
Now if both gauges read high, the very first thing I'd suspect is the IVR is out of tolerance. Do yourself a favor and buy an RTE which is solid state.
There can be issues with the panel wiring, with connections at the gauges, and so on, but this would actually tend to make the gauges want to read low, not high.
Also, the fact that the fuel gauge "reads OK" may not be conclusive, as the sender may be wrong, and say, example, the IVR is too high, may be "luckily" ocmpensating.
Another thread
http://www.forabodiesonly.com/mopar/showthread.php?p=1970209465
What I would do first is get a ohms meter. Pull the sender wire off the sender. Check sender resistance to ground with one probe on the engine right beside the sender, then again with that probe at battery negative. General ambient temp, cold engine, 80 ohms or better with about the same resistance anywhere checked.
Next run the engine until that gauge gets to its wrong or 3/4 as you claim. Pull that wire from the sender again and repeat tests. Approx' 14 ohms resistance to anywhere ?
Next run the engine until that gauge gets to its wrong or 3/4 as you claim. Pull that wire from the sender again and repeat tests. Approx' 14 ohms resistance to anywhere ?
BillGrissom
Well-Known Member
Read my recent post www.forabodiesonly.com/mopar/showthread.php?t=269976.
I measured ~340 ohm at 61 F for the coolant temp sensors in both my A-bodies. A thermistor chart (NTC) extrapolates to 20 ohm @ 203 F and 10 ohm @ 257 F. Similar to RedFish' values in post #6. With a 20 ohm resistor as simulator, my gage read what most consider "normal", tic just below halfway.
I measured ~340 ohm at 61 F for the coolant temp sensors in both my A-bodies. A thermistor chart (NTC) extrapolates to 20 ohm @ 203 F and 10 ohm @ 257 F. Similar to RedFish' values in post #6. With a 20 ohm resistor as simulator, my gage read what most consider "normal", tic just below halfway.
I'll pick up some resistors and get out my multimeter. I just thought there might be a common fault with these as they age. Still don't get the connection with the fuel gauge. Of course, they are both powered by 12v, but are two separate systems - aren't they? What's an IVR? Voltage regulator?
I'm pretty sure that, given my infrared reading, the gauge is wrong; also I drove it 200 miles home with it showing 3/4 hot and it never boiled. Also noted while after taking my reading, after I put the cap back on, I have a pinhole in the tank. Ah, well, it's 50 years old, there WILL be issues.
Never had a slant let me down yet.
I'm pretty sure that, given my infrared reading, the gauge is wrong; also I drove it 200 miles home with it showing 3/4 hot and it never boiled. Also noted while after taking my reading, after I put the cap back on, I have a pinhole in the tank. Ah, well, it's 50 years old, there WILL be issues.
Never had a slant let me down yet.
1968FormulaS340
Well-Known Member
Not exactly.....
The gauges are not 12v. They are 5v in a 12v system. The IVR (Instrument Voltage Regulator) cycles the 12v to feed the fuel gauge an "average" of 5v.
The fuel gauge feeds the other gauges.
....Or al least that's how I understand it.
67Dart273
Well-Known Member
This might be a tiny bit misleading. The IVR is IN the fuel gauge HOUSING in some clusters. The output of the IVR feeds the other gauges FROM the fuel gauge HOUSING IN an early dash cluster OR IN a RALLY cluster
So, electrically, 12V feeds TO the IVR, which (orginially) "pulsed" the 12V into a "fake" 5-6V. This IVR output then branches off and electrically feeds the other gauges.
nm9stheham
Well-Known Member
Are you using the IR temp sensor on the top tank? Is the paint on the tank FLAT black? Unpainted will usually give inaccurate readings as may gloss black. I would paint a patch of flat black on the tank and re-test before being certain that the temp guage is off; that is SOP for accutate IR readings.
I assume the top tank is where the fluid flows into the radiator from the engine? And a 3/4 temp gauge would not typicaly boil over with a good pressure cap and tight system. I have raced with elevated readings due to a tiny head crack, and the radiator fluid never boiled, either running or stopped afterwards when still hot.
So, IMO, I am not sure you have elminated the possiblity of the engine actually being hot due to something like a sticking thermostat, a compression gas leak into the water jacket, etc. If the fuel guage is normal, then that would explain it. Did the temp guage read normal before, or is the a new car to you with unknowns?
I assume the top tank is where the fluid flows into the radiator from the engine? And a 3/4 temp gauge would not typicaly boil over with a good pressure cap and tight system. I have raced with elevated readings due to a tiny head crack, and the radiator fluid never boiled, either running or stopped afterwards when still hot.
So, IMO, I am not sure you have elminated the possiblity of the engine actually being hot due to something like a sticking thermostat, a compression gas leak into the water jacket, etc. If the fuel guage is normal, then that would explain it. Did the temp guage read normal before, or is the a new car to you with unknowns?
1968FormulaS340
Well-Known Member
I thought you would come along and correct me. I'm glad you did. I only have a rudimentary understanding of how the system works.
Why did they do it that way is my question.
67Dart273
Well-Known Member
What, use the IVR? Who knows. Many claim it's because they already had 6V gauges. I don't think so. I think it was to attempt to get them more accurate. That is, battery voltage varies all the way from below 12V (lights on, idling, cold winter night, heater running at stop light) to 14V (charging down the road with battery "up) so that's a pretty fair variance. If this was your home power this would be about the same as a change from 100 to 120V AC, and THAT is a pretty large change!!!
So the IVR I think was intended to try and proved a bit better regulation. These were designed before "solid state" (transistors) really got off the ground
Ford by the way used essentially the exact same system. I can still well remember the day I climbed into my 60 Ferd Falcoon and the two gauges both went "through the roof."
Didn't take me long to figure that out, and this was long before Al invented the internet, LOL, this was fall of 67 before I joined the Navy.
So the IVR I think was intended to try and proved a bit better regulation. These were designed before "solid state" (transistors) really got off the ground
Ford by the way used essentially the exact same system. I can still well remember the day I climbed into my 60 Ferd Falcoon and the two gauges both went "through the roof."
Didn't take me long to figure that out, and this was long before Al invented the internet, LOL, this was fall of 67 before I joined the Navy.
Car is new to me (2 weeks). Drove it home from San Diego with gauge showing 3/4 hot, but figured that it would escalate if there really was a problem - never did. I was planning on an overhaul/replacement anyway, just not sure what the deal was. The sensor is adjacent to the thermostat housing on the slant. I'm just getting over surgery; soon as I can get around not bent over, I'll try disconnecting the sender and see what the gauge shows then. Thanks for all the help, guys.
nm9stheham
Well-Known Member
Oooo, good luck with the recovery! Take it easy and let it heal guy!
Question... Do you get a ping or spark knock under load ? Idle seem to climb and cause a diesel or run on at switch off ?
Questions are based on a my similar experience with or first Barracuda many moon ago.
Those gauges worked fine. Engine running a little too warm wasn't really a problem unless I got caught in slow moving traffic.
I wasted a lot of time and money on all of those less expensive possible cures.
A new radiator was the cure.
Questions are based on a my similar experience with or first Barracuda many moon ago.
Those gauges worked fine. Engine running a little too warm wasn't really a problem unless I got caught in slow moving traffic.
I wasted a lot of time and money on all of those less expensive possible cures.
A new radiator was the cure.
67Dart273
Well-Known Member
I've posted this several times because in my case the change was so incredibly dramatic Oldmanmopar posted this I do not take credit
My old original rad was tested by a shop and they said it "should cool." It did not. I swapped in a used, unknown, /6 rad, and the change was dramatic. The temp immediated dropped. Oldmanmopar thinks that years of thermal cycling causes the fins to crack loose from the tubes, reducing head radiation and heat exchange. This makes some sense, because my old rad flows lots of water. Its simply does not cool very well.
Both rads are two core, similar size, one the old 67 round tank, 273, the other the newer 73 style newer style.
My old original rad was tested by a shop and they said it "should cool." It did not. I swapped in a used, unknown, /6 rad, and the change was dramatic. The temp immediated dropped. Oldmanmopar thinks that years of thermal cycling causes the fins to crack loose from the tubes, reducing head radiation and heat exchange. This makes some sense, because my old rad flows lots of water. Its simply does not cool very well.
Both rads are two core, similar size, one the old 67 round tank, 273, the other the newer 73 style newer style.
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