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Fuel and temp gauge conundrum.
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ABloch
Posted 2016-12-01 3:40 PM (#527662)
Subject: Fuel and temp gauge conundrum.



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Hey all, quick question.

Fired up the '59 Sierra this morning so as to keep it from gathering dust. One pump to set the choke and one more for an extra squirt of gas and VROOOM! As it sat there and waited for it to warm up I checked the gauges. Oil pressure, check. Amps, check.
Temperature was flat because I just started it and fuel gauge flat......wait a sec. I knew the car had almost half a tank so what gives?

Then both the gauges began to slowly rise simultaneously. And they kept rising till they both maxed out. I turned the car off and came inside to search the forum for any info. Perhaps the voltage limiter has gone kaput? Have I completely fried my gauges now? What a way to start the day, huh?

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ttotired
Posted 2016-12-01 5:01 PM (#527668 - in reply to #527662)
Subject: Re: Fuel and temp gauge conundrum.



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No guessing, just checking (pull it apart and look), but I would expect the regulator, gauges should be fine

Now you have something to do on the weekend

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Powerflite
Posted 2016-12-05 12:20 AM (#527891 - in reply to #527662)
Subject: RE: Fuel and temp gauge conundrum.



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The fuel gauge on my '58 Dodge is always on and doing its usual fluttering. Only problem is that it is draining my battery. Is that the way they were setup originally, or has someone swapped wires on it?
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56D500boy
Posted 2016-12-05 1:24 AM (#527894 - in reply to #527891)
Subject: RE: Fuel and temp gauge conundrum.



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Powerflite - 2016-12-05 12:20 AM The fuel gauge on my '58 Dodge is always on and doing its usual fluttering. Only problem is that it is draining my battery. Is that the way they were setup originally, or has someone swapped wires on it?


As it happens, I just fixed my fuel gauge sender (needed a new cork float and a new wire connected between the harness and the sender). Based on that limited but successful experience, the fuel gauge only comes on when the ignition switch is turned to RUN (not start). When the key is turned to OFF, the gauge is not powered. Somebody has connected yours to "hot" battery power for some reason.

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ABloch
Posted 2016-12-05 10:23 AM (#527932 - in reply to #527662)
Subject: Re: Fuel and temp gauge conundrum.



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Location: Pacific Northwest
Pulled the regulator and tested it.....11.5v. now do I make one, or order one?
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wizard
Posted 2016-12-05 11:18 AM (#527937 - in reply to #527662)
Subject: Re: Fuel and temp gauge conundrum.



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ronbo97
Posted 2016-12-05 12:09 PM (#527940 - in reply to #527937)
Subject: Re: Fuel and temp gauge conundrum.


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Yes, sounds like the voltage limiter. For those unfamiliar, it's a small silver device located behind the gauges. It's job is to reduce voltage to 5V for the two gauges mentioned. You have to pull the gauge cluster to access the limiter.

Ron

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ABloch
Posted 2016-12-05 6:36 PM (#527967 - in reply to #527662)
Subject: Re: Fuel and temp gauge conundrum.



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Thanks Wizard! You rule! On my '59 the limiter is attached to the back. I was able to remove it without pulling the cluster.
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ABloch
Posted 2016-12-10 1:20 PM (#528355 - in reply to #527662)
Subject: Re: Fuel and temp gauge conundrum.



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Just received an "NOS" voltage limiter in the mail. Went out to install it this morning but before I crawled under the dash I tested it. Crap! Full voltage. No limiting what so ever.

So now I am off to Radio Shack.
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Polara61_383
Posted 2016-12-10 2:25 PM (#528362 - in reply to #527662)
Subject: Re: Fuel and temp gauge conundrum.



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Please be aware that building your own voltage limiter is a little more difficult than what is described above. You should include to small condensers and a blocking diode to make that device fail safe. I think a have the instructions for this somewhere but you can also google them. There should also be a thread about this on this forum. and iirc the mopar muscle magazine once wrote about this
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5wndwcpe
Posted 2016-12-10 3:05 PM (#528365 - in reply to #527662)
Subject: Re: Fuel and temp gauge conundrum.



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Or you could buy one of these and adapt it if needed.

http://www.rt-eng.com/rte/index.php/RTE_limiter

Edited by 5wndwcpe 2016-12-10 3:08 PM
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5wndwcpe
Posted 2016-12-10 3:19 PM (#528367 - in reply to #527662)
Subject: Re: Fuel and temp gauge conundrum.



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Little more info on voltage limiters:



On the back of most mopar dashes (with some notable exceptions) there is a 1" X 2" metal can, with 3 terminals. This device is known as a limiter. Its function is to regulate the voltage that is being applied to the fuel/oil/temperature gauges.

Some mopar dashes don't have a visible limiter device. Instead they have the limiter built into the fuel gauge. These special dashes can be identified by looking at the fuel gauge, and if it has 3 terminals then it has the limiter built in. A-Body Rallye dashes and 66/67 Chargers are two examples of dashes that have the limiter built into the fuel gauge.

The 70-71 parts book does list 4 different types of limiters, of which we have seen only two types. The E-body limiter has part number 2209216, and the most common push in type has part number 2258413.

The push in type is intended to be pressed into some mounting clips on the backs of the circuit boards. This type of limiter has three male spade terminals protruding from the face of the limiter. One terminal is spot welded to the case for ground. The other two terminals are for +12V in, and limited voltage out. There is usually a capacitor connected to the +12V spade when it is pressed in palce. This type of limiter is always found on dashes that have circuit boards. This type of limiter was used on a lot more different types of mopars than the other type, and this type of limiter is still available at your auto parts store.

The E-body limiter has a mounting tab spot welded to the back of the limiter. A small 1/4" head bolt is used to fasten this type of limiter to the metal dash frame, and this is how this type gets it ground. The +12V input terminal on this limiter is a dual male spade terminal. It has two terminals so that one can be +12V input, and the other is intended to be connected to the capacitor (sometimes called a condensor). The capacitor does not have any effect on the operation of the limiter, but is intended to stop AM radio interference. The 2nd terminal on this type of limiter is a female spade terminal, and this is the limiter output to the gauges. This type of limiter was used on just a few different types of cars, includingthe 70-74 E-bodies. This type of limiter is no longer available in the aftermarket. Note that there is a limiter that appears to be the same that was used for some ford dashes, but this limiter does not put out the correct voltage and will make your gauges read low if you try to use it.

In order to understand the limiter and why it is necessary to have one on your dash, it is first necessary to understand how the fuel/oil/temp gauges work. The typical Fuel/Oil/Temp gauge (henceforth referrred to as FOT) has inside it a bi-metallic strip, similar to the bi-metallic strip inside your non-electronic wall thermostat. There is some insualed nichrome wire wrapped around the bi-metallic strip. When current is run through the nichrome wire, it will heat up the bi-metallic strip, causing it to bend. The bending is caused because one of the types of metal expands faster than the other. See this URL for more info on how bi-metallic strips work:
http://en.wikipedia.org/wiki/Bi-metallic_strip

One important thing to know about bi-metallic strips is that the amount they bend is proportional to the amount of heat that is put into the strip. Therefore, if you want to have an accurate gauge, you must have accurately regulated heat. The heat is proportional to the current through the wire, and the current through the wire is dependant on the voltage being applied to the wire. Therefore if the voltage being supplied to the gauges were to vary, then the gauges reading would vary. It turns out that the voltage on a cars battery is not very well regulated, and it can change as much as 2-3 volts under different conditions (for example, just turning on the headlights and sitting at idle will have a lower voltage condition). Therefore the limiters job is to regulate the voltage being applied to the gauges, so they will be accurate and not vary.

The dash limiter is not temperature sensitive. In other words, if the external temperature varies between its normal extremes of +150F and 0F, the limiter will still put out its voltage with little or no variation. This temperature compensation is accomplished by the way the bi-metallic strip is constructed inside the limiter. The FOT gauges also have this temperature compensation inside them and so are not sensitive to the external temperature.

The way the dash limiter works is as follows. When power is first turned on (i.e. you turn on the key), then the points are closed because the bi-metallic strip is not yet hot and the limiter is putting out +12V. The nichrome wire inside the limiter starts to heat up, which in turn starts to heat up the bi-metallics strip. After about 3-4 seconds, the strip will get hot enough to cause it to bend up and open the set of points inside the limiter. When the points open. When the points open, then the nichrome wire stops heating the strip, and the limiter stops putting out +12V. The amount of time that the limiter stays open is proportional to the amount of heat that was put into the strip, so when the voltage of your car battery is higher, then the limiter stays open longer, thus regulating the average RMS (Root-Mean-Square) voltage coming out of the limiter.

There are several different failure modes that a limiter can have:

•If the limiter loses its ground (from corrosion for example) then there will be no current flowing in the nichrome wire, and the limiter will output +12V all tht time. The FOT gauges cannot tolerate this condition for more than about 20 seconds, and will burn out after about 20 seconds.
•If the nichrome wire burns in half or breaks due to mechanical vibrations, then the points will no longer open and the gauges will fry after 20 seconds.
•If the points weld themselves shut, then the points will no longer open and the gauges will fry after 20 seconds.
•If the output of the limiter is shorted to ground, then either the fuse to the instrument panel will open, or we have seen some limiters that had the bi-metallic strip burned in half. In this case the gauges won't be fried, but they will stop working until the limiter is replaced.



Currently there are some people that are replacing the insides of the limiter bi-metallic strip with a linear regulator. This does work, but it is not a perfect solution. The following problems exist with this solution:
•The linear regulator will dissipate a lot of heat because it is constantly dropping a lot of voltage across it in order to create 5V all the time. In fact a linear regulator will generate a lot more heat then the original limiter. It is necessary to thermally bond the linear regulator to the limiter case to keep the regulator from getting too hot under normal operation. For reference, it takes about 300mA per gauge to make each gauge read full scale. Therefore the amount of heat generated by the linear regulator is P=IV=300mA*(12V-5V)*3=6.3Watts. This is a lot of heat.
•The linear regulator requires a good ground just like the original limiter. If the ground is lost, then the linear regulator will put out +12V all the time, frying the gauges.
•The linear regulator will not properly emulate the original limiter in the sense that the original limiter puts out +12V for 3-4 seconds at initial power up in order to get the gauges up to their correct reading quickly. It can take up to 1 minute for the gauges to reach their final reading when using a linear regulator after turning on the key.
•The original mechanical limiter pulses on and off, and this has the effect of overcoming friction inside the FOT gauges. This makes the gauges more responsive to changes. Since the linear regulator doesn't pulse, it doesn't have this effect.
•Rarely some linear regulators can fail when their output is shorted to ground. Usually when they fail, they will short the input to the output, causing +12V on the output of the regulator.
•On a positive note, most linear regulators have heat sensors inside them and will shutdown when they get too hot (caused by the output being shorted to ground).








Edited by 5wndwcpe 2016-12-10 3:21 PM
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ABloch
Posted 2016-12-10 7:29 PM (#528376 - in reply to #527662)
Subject: Re: Fuel and temp gauge conundrum.



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Location: Pacific Northwest
Retested the limiter and it's working after all, however I'm pretty sure the gauges are fried. Temp gauge is beyond cold. totally flat. Fuel gauge is suspect at this moment. I'll need to ad more fuel to be sure.
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