Understanding the Rev Limiter

by Jeff Finley

You're cruising down the road and all of a sudden your car stops running or it's misfiring. Or maybe you go out to your car and try to start it. It cranks but it won't fire or run. What is wrong? Do you have an original Rev Limiter in your car? It may be the cause of your troubles. Disconnect and bypass it. The car runs fine without it? Then you know for sure.

Ford installed a Rev Limiter (a.k.a. RPM Limiter, a.k.a. Engine Governor Assembly in Ford speak) in the 1969 and 1970 BOSS 302 Mustangs and Cougar Eliminators. The Rev Limiter's function was to limit the engine RPMs below 6,150. Go over 6,150 and the engine started to misfire. How does it work and why does it make cars die or not start?

Take a look at the drawing (schematic) below. It shows the basic ignition circuitry including the Rev Limiter (Engine Governor Assembly). The Rev Limiter monitors the "DIST" terminal on the coil. The discrete and transistorized circuitry inside the Rev Limiter picks up the ignition pulses and "calculates" (without a microprocessor!) the engine RPM. When the Rev Limiter determines the RPM is greater than the limit (6,150 for BOSS 302) it switches (using solid state technology) a load onto the "DIST" line. The power is shunted away from the "DIST" terminal wire to ground. This causes a misfire or, actually, a no fire. Grounding this terminal prevents the high voltage build up and release from the secondary coil (HV) to the center distributor cap lead for the spark plugs. Maybe you have seen or heard of the same effect caused by one or both points in the distributor stuck closed or a bad distributor capacitor (condenser) providing a path to ground.

Rev Limiter Schematic

As you know, our cars don't need the Rev Limiter to run. Most cars don't have them, right? Why? Electrically speaking, the Rev Limiter is wired in a "parallel" circuit with the coil and distributor. The Rev Limiter provides a parallel path for the electricity to flow. The electricity can flow through the Rev Limiter (when the RPM limit is reached) or the ignition system (normal operation). If you remove the Rev Limiter, no more parallel path. All electricity (power) goes through the ignition system.

What goes wrong with these things that make them go bad and kill a car? Well, honestly we don't know which component(s) in the box go bad. From its position in the ignition circuit, when the Rev Limiter breaks and kills a car it either goes into permanent shunt mode or possibly it has pulled down the voltage at the "BATT" terminal causing no spark.

You want to keep that original Rev Limiter and wiring under the hood but don't want to take a chance it will die while driving? Yours has already died? You're thinking of buying one or you have one on the shelf and you don't know if it will work? Or maybe you even want to use a "fake" Rev Limiter? What do you do? Simple. Open the box. Look at the photo below. Detach (cut or desolder) all three wires from the circuit board. Wrap the ends separately, individually, with electrical tape so the wires do not touch each other, the circuit board or the case, and close the box. By doing this you will have completely taken the Rev Limiter out of the ignition circuit and your Rev Limiter troubles are gone.

rev limiter photos courtesy of Pat Whalen

Rev Limiter internal

How do you identify a BOSS 302 Rev Limiter? As usual, Ford made more than one version of this part, too. The photo below shows a 1970 Rev Limiter. The 1970 has an ink stamp with the part information and date code (7039 here) on the cover. The 1969 does not have an ink stamp. That's right, the 1969 is blank.

Rev Limiter external

1970 REV Limiter (above) has part information ink stamp on the cover. 1969 has no ink stamp.

The ink stamp is not the only difference. There are also differences in the Rev Limiter wiring harnesses. Again, 1969 vs. 1970.

The 1969 wire harness has five individual wires coming out of the tape.

1969 Rev harness

The 1970 wire harness has three individual wires coming out of the tape. Two of the terminals are double crimped branching out to make the same total of five wires.

1970 Rev harness

We added a little color to the diagram below to show the wiring installation. This illustration shows the labels that came on the 1969 harness. The 1970 is wired the same.

Rev limiter install sketch

Our friends at the 428 Cobra Jet Registry have more information on Rev Limiters. https://www.428cobrajet.org/id-governor.