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 it. The car runs fine without it? Then you know for sure.
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 (pretty obvious by the name) 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
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.
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.
|photos courtesy of
|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.
|1970 REV Limiter
(above) has part information ink stamp on the cover. 1969 has no ink stamp.
stamp is not the only difference. There are also differences in the Rev Limiter wiring
harnesses. Again, 1969 vs. 1970.
|The 1969 wire harness
(right) has five individual wires coming out of the tape.
|The 1970 wire harness
(right) 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.
|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.
friends at the 428 Cobra Jet Registry have more information on Rev Limiters. http://www.428cobrajet.org/id-governor.html
Their page includes a link at the bottom to another website which has a diagram showing
the Rev Limiter installation for 428 and BOSS 302 (including the hardware).