For all the folks who are having shifting issues with their transmissions, for whom drain/refill cycles doesn't improve, one of few non-invasive possible solutions are shift solenoids and clutch pressure switches. Many have found through trial and error that the clutch pressure switches are maintenance items, and recommend their replacement. It kept bugging me however, that nobody ever mentioned a means of testing said switches, at least as a means of being confident in finding the "smoking gun" to a problem.
I'm in a unique position in that I have a set of clutch pressure switches from the old trans I just removed and 2 new ones that I'm planning to install. So I figured, why not test the new and compare to the old?
Here's the test setup: It consists of a portable air tank, a 1/2" tee, and other fittings cobbled together along with a 0-60 psi pressure gauge (better resolution than the 0-160 psi gauge on the air tank). The fitting used for installing the clutch pressure switch was a 1/2" brass plug drilled and tapped to accept the switch.
Since this is a pressure switch, the idea is that the switch closes once the pressure goes above a certain upper threshold (i.e. zero resistance) and opens once the pressure drops below a certain lower threshold (infinite resistance). To perform the test, I attach the leads of a multimeter to the switch as shown, then air up the portable air tank just like you would a car tire. I then record the pressure required to close the switch as well as how far I have to drop the pressure before the switch opens. Here's my results from measuring a set of new switches:
P/N 28600-P7W-003 (Brown): Closes at ~23 psi, opens at ~17.5 psi
P/N 28600-P7Z-003 (Black) : Closes at ~30 psi, opens at ~24 psi
Since I only tested one set of new switches, I don't know what the tolerance ought to be on the closing/opening pressures. My old black switch closed and opened within about 2psi of the new switches. However, it registered 3 ohms when closed instead of 0.1 like the new switches, and there was a very small pressure window (about 1 psi or less) over which it varied between 3 and 6 ohms. My old brown switch was clearly defective and likely the culprit in some shifting problems, as it sometimes registered 200 ohms, sometimes 90, but was highly inconsistent.
I think what my testing confirms is that these clutch pressure switches are indeed maintenance items, but for diagnostic purposes, you CAN test the switches fairly easily to confirm or deny their likely culpability in a shifting problem. The main item to look for is that it behaves likes a switch (low resistance when closed). However, you DO need the ability to apply pressure to the switch in order to test, as they are of the "normally open" variety (i.e. infinite resistance at atmospheric pressure)