Various ignition systems and accessories are often configured to do certain things at a particular RPM level. These devices may be used to set a rev limit, a stage limit, trigger a shift-light, or even activate a nitrous system. The popular systems made by MSD utilize external replaceable modules called "pills" to select that point based on a pre-determined resistance value. The following pictures detail how to disassemble and reassemble an MSD RPM pill, thus allowing that point to be modified as desired. This is especially useful if your application requires a non-standard set point or it's 10pm the night before the drags and you don't happen to have the required pill handy. Even a potentiometer works wonders in a jam!

Refer to the chart below to select the proper resistor value. For most applications it's not essential to have the exact resistor listed and you may choose to adjust the value up or down slightly to fit your needs. If you're familiar with resistors, you can easily combine them to create virtually any value you require.

Notes:

All resistors are 1/4W rated. To enable the switch at all times, 0 ohms/shorting wire may be used. *Values below 500 RPM are not consistent, nor recommended and this range would typically be impractical.

Here's a graph of the resistance versus RPM:

I highly recommend using precision metal film resistors (like the quality ones used by MSD) and not the typical carbon ones found at places like Radio Shack. In a pinch they'll probably do, but the carbon based type are far more sensitive to heat/humidity and as a result their resistance can change, sometimes by a huge percentage. If you're mounting the RPM switch inside the interior of the car, it's probably not as big of a deal, but just be sure to do some of your own testing to verify proper operation under normal conditions.

Here's a typical MSD RPM Pill:

Here's one with the label removed by simply lifting an edge and peeling it back. There may also be a small amount of hot melt type glue underneath that you can easily pull out with a pair of tweezers.

There are a couple methods of removing the internal resistor. You can use a de-soldering tool (i.e. a solder sucker) and remove all the solder from the end of each hollow post, or if you don't have one of those available, you can simply heat one post at a time and gently pull the resistor lead out a little bit, then switch to the other lead, and repeat as necessary (see photo series below). If you use the latter method, you'll have to do the same in reverse to install the new resistor. The de-soldering method is much better and far quicker/easier, but either will work just fine.

Here's all the guts…there's not much to it!

Select your new resistor value, or the closest one available. Gently bend the leads at 90 degree angles, trim both to approximately 3/4", and place back into the holder. Re-solder by heating the post and flowing solder into it.

Viola! Ready for testing.

WHILE EVERY EFFORT HAS BEEN TAKEN TO ENSURE THE ACCURACY OF THE INFORMATION CONTAINED WITHIN THIS DOCUMENT, IT IS YOUR RESPONSIBILITY TO VERIFY THE USEFULLNESS AND SUITABILITY FOR YOUR PARTICULAR APPLICATION PRIOR TO USE.