North American Turbocoupe Organization

New Design Solid State IVR
andrewjs18 Offline
Improved Design Solid State Instrument Voltage Regulator
By Jeff Korn, NATO member


It is well known that the stock mechanical / thermal instrument voltage regulator (IVR) used on virtually all Ford products from the 1960s to the early 1990s is problematic.  They tend to stick shut, causing the gauges (oil pressure, water temperature, and fuel level in the Turbo Coupe) to max out for periods of time ranging from a few seconds to minutes.  Often, a quick hit to the dash will restore more or less proper gauge operation. Some IVRs also tend to have two stable states, causing the gauges to read lower or higher for long time periods, rendering the gauges almost worthless.  I have developed an improved design solid state IVR using a LM338T 5 Amp variable regulator that works very well.  I have tested the design under worst case current draw conditions and temperature extremes, and found it to be 100% reliable.

Another article on the tech articles page written by James Johnson describes a solid state IVR based around a 7805 5 Volt 1 Amp regulator that fits in the mechanical IVR case.  With all due respect to James, I have several problems with his design. His design supplies the regulator through the 8.5 ohm resistance wire originally used in conjunction with the mechanical IVR.  This is equivalent to driving the regulator using a relatively high impedance power supply.  The voltage supplied to the regulator varies significantly with the current drawn by the gauges.  I believe this voltage variation exceeds the line / load regulation limits of the regulator, causing the output of the regulator to change under different operating conditions.  The worst case current draw of the gauges approaches .7 Amps, which I believe is too close to the maximum current limit of the 7805 for best reliability.  Solid state regulators dissipate a significant amount of heat when operating at higher current.  This heat must be carried away from the regulator by a heat sink to avoid damage to the regulator and to avoid the regulator from reducing its output current and/or voltage to protect itself from overheating.  While the 7895 is attached to the mechanical IVR metal case, this case does not provide a large enough heat sink.

The only drawback to my design is that the solid state IVR is that it must be remotely mounted due to the size of the heat sink needed to effectively cool the regulator chip.

I made my IVRs adjustable from roughly 4.5 V to 6.0 V so I could get the gauges to “read the way I want them to”.  I found that a voltage of 5.2V seemed to make the gauges read most accurately, based on indicated versus actual fuel level readings and temperature gauge reading with the motor fully warmed up.



I got some of the parts for the IVR from Digikey (, some from work, and some from my stock of junk in my basement and garage.

LM338T 5 amp adjustable regulator (DIgikey # LM338T-ND)

200 ohm pot – there are lots of styles of these available. I got a small one from Digikey (Digikey # 3329H-201-ND) I would not be surprised if Radio Shack also sells something similar.

120 ohm resistor (1/4 or 1/8 Watt) – either from Digikey or Radio Shack (I got mine from work)

300 ohm resistor (1/4 or 1/8 Watt) – either from Digikey or Radio Shack (I got mine from work)

.01 uF capacitor – can be had from Digikey or Radio Shack.  I had one in my junk collection.

A small plastic box to mount the IVR in – Digikey and Radio Shack have a selection of small plastic project boxes.  I found one in my junk collection measuring about 3” x 2” x 1”

A heat sink for the LM338T – Digikey has a large selection of heat sinks.  I found one in my junk collection. I suggest a heat sink with a MINIMUM surface area of 10 square inches, and preferably closer to 15 or more square inches!!!

White thermally conductive heat sink grease.  Digikey carries several versions, and Radio Shack may also carry it.  I bought a 2 oz jar from Digikey years ago for about $10 or so.  This is the same stuff used to heat sink a TFI to the distributor on EEC IV Fords.  DO NOT try to get by without using this grease between the LM338T and the heat sink!!!

A two terminal connector – I used a two terminal trailer connector available at any auto parts store.

A fuse holder and 5 Amp fuse - available at any auto parts store.  

9 Volt female battery terminal and a ring terminal – gut an old 9 Volt battery for the terminal.  


The figure above shows the solid state IVR mounted under the dash and to the right of the steering column.  Note the heat sink sticking out of the black IVR box on the left. I cut down a large heat sink I had in my junk collection to fit the plastic box. Also note the white voltage adjustment pot on the side of the box. The red marks are for 5.0 V, 5.2 V, and 5.4 V output.  Note that this is actually the IVR I built for my 86 Tbird 5.0.  The IVR I built for my Turbo Coupe uses an old CPU heat sink / fan I had in my junk collection (gutted from an old PC), which is really overkill as far as cooling capacity.      


The relatively simple circuit is shown below, along with the pinouts for the LM338T.  The LM338T was attached to the heat sink with a sheet metal screw, with the thermal grease between the heat sink and LM338T.  The circuit, with the exception of the 200 ohm pot, was soldered directly to the terminals of the LM338T, and was “potted” using clear RTV to eliminate the chances of a connection coming loose due to vibration.  The heat sink / potted circuit assembly was put in the side of the plastic box (the side of the box was cut out first using a Dremmel), and glued in with a little RTV.  The two wires that go to the instrument cluster were run out the side of the plastic box.  The IVR was powered from a 12 V source under the dash this is hot in RUN.  Since the maximum current draw is around .7 Amps, and is usually much lower, any 12 V hot in RUN circuit you can find will work.

If you would like to make a nonadjustable 5.0 V IVR, a single 360 ohm resistor can be substituted for the series combination of the 300 ohm resistor and 200 ohm pot.

Be aware that the mounting terminal for the LM338T is also another output voltage terminal, and when the LM338T is attached to the heat sink, the heat sink will also be at the IVR output voltage!!! It is IMPORTANT that the heat sink not contact any chassis ground when the IVR is mounted, or a short circuit will result!!  That is the reason I mounted the IVR on the panel under the steering column where there is no way the heat sink can contact any grounding locations.



Remove the instrument cluster from the car (a real fun job!).  Put the cluster face down on a towel so you don’t scratch the plexiglass face plate. The figure below shows the back to the instrument cluster.  Locate the old mechanical IVR. Pop the two terminal printed circuit connector off the old IVR, and remove the IVR by removing the screw that holds it to the back of the cluster.

As noted in the circuit diagram, two wires come from the new solid state IVR and go to the cluster.  The regulated voltage output wire from the new IVR needs a female 9 Volt battery clip terminal so it can clip to the male terminal on the instrument cluster printed circuit. The terminal at the “end” of the printed circuit that went to the old IVR is the male terminal.  I ripped apart an old 9 Volt battery to get the female terminal needed, and soldered a wire to the terminal.  After the terminal is connected, I wrapped it with several layers of electrical tape.  The female terminal on the printed circuit is not used, so I just wrapped it with several layers of electrical tape to insulate it.

The second wire from the IVR is the ground wire, and should have a ring terminal attached to it.  The ring terminal goes under the old IVR mounting screw.  Don’t forget to put the black wire back under the screw before tightening it securely,

I wrapped the two wires with electrical tape. And used a small tie wrap to secure them to one of the gauge mounting studs to act as a strain relief.  I used a two terminal connector for a quick connect / disconnect when installing / removing the instrument cluster.

Before reinstalling the instrument cluster, hook a DMM set on DC Volts to the IVR side of the two terminal connector, and turn the key to RUN.  Adjust the 200 ohm pot to get an output of 5.0 to 5.2 Volts.  If the IVR checks out, connect the two terminal connector from the IVR to the cluster, and reinstall the instrument cluster in the car.


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