RTE Limiter Faq
Limiter trouble shooting
- ) If the new solid state limiter doesn't function (LED doesn't blink), remove the limiter to gage wire from one gage at a time checking to see if the limiter
begins operation. The limiter will detect a short circuit and turn itself off until the short is removed. Also, if the limiter has too much or not enough voltage, then it will turn itself off.
- ) Note that some mustangs had build in resistance wire feeding the limiter. This wire must be removed or shorted around in order to make our solid state limiter work properly. The reason is that our limiter will see the voltage drop in the resistance wire, and it will think that the voltage is below 9V, so it will turn itself off to protect the limiter and the gauges.
Fixing dash clusters that have internal limiters
PDF copy of this same info: media:InternalLimiter.pdf
Some of the cars made my Mopar did not use external limiters. Instead these cars built the limiter into one of the gauges, usually the fuel gauge. Cars in this category include the 66/67 charger, 68-74 ABody cars with Rallye dash, and some older imperials.
When these internal limiters go bad, you have two choices, either replace the whole gauge (expensive and hard to find!), or disable the limiter built into the gauge and wire in an external limiter. If you decide to wire in an external limiter, then you should verify that the gauge still functions as a gauge, i.e. that it hasn't been burned out when the limiter failed. Another thing to consider is that it might be good insurance to just disable the internal limiter and replace it with an external limiter before the internal limiter fails. Here are some pictures showing how RTE disabled the internal limiter in an older Imperial dash cluster, and wired in an external solid state limiter. Real Time Engineering can do this for you for $50 labor plus the cost of the solid state limiter. The red arrow shows how we bent the internal limiter bi-metallic strip to disable the internal limiter.
Imperial internal limiter fix
A-body rallye dash limiter fix
- Step 1: Take the fuel gauge out of the cluster, and gently take the face off.
- Step 2: Bend the limiter points out of the way, so that they no longer contact. Make sure that the points don't touch anything.
- Step 3: Put the Fuel gauge face back on.
- Step 4: Mount the IVR3 (Ebody) style limiter using one of the circuit board ground mounting points.
- Step 5: Run a wire from the 12V stud going into the fuel gauge to the IGN terminal on the new solid state limiter. Use a ring terminal on the fuel gauge side, and a male spade terminal on the limiter side.
- Step 6: Run a wire from the old fuel gauge limiter output stud to the solid state limiter output terminal. Use a ring terminal on the fuel gauge side, and a female spade terminal on the limiter side.
- Step 7: Test the setup by turning the key on in the car. The fuel gauge, oil gauge, and temperature gauge should operate properly, and the red LED on the new solid state limiter should blink after the 3-6 second warm up time.
More information about dash 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:
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).
Real Time Engineering has a new solid state limiter that will replace the original mechanical limiter on the back of your dash. This new limiter has many advantages over the original limiter, and also has advantages over the linear regulators that hobbyist have been using as well. The biggest advantage is that our limiter doesn't have a mechanical set of points and a heater wire that can break and fry your gauges (which is what the original mechanical limiter had).
- Our limiter exactly duplicates the original limiters function by slowly switching 12V on and off.
- Our limiter has a built in polyfuse which protects the limiter and your wiring from short circuits on the output of the limtier
- Our limiter constantly looks for short circuits on the output of the limiter and will switch itself off if a short circuit is detected. When the short is removed, then the limiter starts working again.
- Our limiter has a built in diagnostic LED that blinks when the limiter is on, helping you see that you have power to your dash and that the limiter is operating properly.
- Our limiter has a warm up time at turn on, same as the original mechanical limiter. This means that your gauges will come up to the proper reading very quickly.
- Our limiter is always outputting a constant 5V average, no matter what the input voltage is (Within the range of 9V to 18V). If the input voltage goes too high or too low, then the limiter shuts off.
- Our limiter output is constant regardless of temperature.
- Our limiter is not sensitive to mechanical vibration.
- If our limiter loses ground, it will shut itself off. Contrast this to the mechanical and linear limiters, which will put out 12V and fry your gauges.
- If the alternator and/or the firewall regulator go bad and the alternator puts out too much voltage (over 19V), then our limiter will cause the instrument panel fuse to open, protecting the limiter and the gauges. Note: It is important that you have the proper size fuse in the fuse box for the instrument panel for this reason.
Advantages of our limiter over mechanical limiters:
- Won't fry your gauges when ground is lost
- No mechanical moving parts to wear out
- No points to stick closed
- No nichrome wire to break and then fry your gauges
- Insensitive to heat and cold
- Solid state so should last a very long time
- Short circuit protected
- Diagnostic LED built in
- Uses less current than the original limiter (because there is no heater wire)
- Original mechanical limiters tend to vibrate badly and lose regulation temporarily when going over very rough roads.
Advantages of our limiter over linear home made limiters:
- Won't fry your gauges when ground is lost
- Doesn't generate any heat (linear regulators make a lot of heat)
- Has a warm up period when turned on to make the gauges come up fast
- Diagnostic LED built in
- Uses a lot less power (Linear regulators can waste up to 8W of power in this application)