Resistance is measured in "ohms", not "volts".

The values in the following advice are from my Haynes manual for the '91 3.0L - I assume it's correct but I don't have a FSM for the '91 - only my '89:

Key switch off - pull the 2-wire connector plug to the coil (not the high voltage lead to the distributor). Set your test meter to "ohms" (lowest scale) and check the primary resistance across the 2 terminals - it should be between 0.72 and 0.88 ohms.

Now set the meter scale to the highest range. Remove the high voltage lead to the distributor and check the secondary coil resistance between the high voltage terminal and the lower (the vertical blade terminal) of the 2 previously tested terminals - it should read between 14.5K and 19.5K ohms (K=1,000). Let us know what readings you get. If they're much different the coil is bad and must be replaced.

If the coil passes those tests, the power transistor is suspect. The power transistor is what triggers the coil, much like points in an older vehicle. The Haynes manual has no information I can find on the power transistor, so I'm giving you the test procedure from the '89 FSM and I'm assuming the power transistors are the same. Check as follows:

The power transistor is the black, flat contraption mounted next to the coil on the coil bracket. Remove the 3-wire connector. As you look at the 3 electrical connections, mentally number them 1,2,3 from left to right.... they have letter designations on the topside of the unit, but my truck isn't here right now and I can't give you the designations that way - so -

With your test meter set on "ohms", check between terminal 2 (middle) and terminal 3 (right).... there should be no continuity (high reading in ohms).

Connect the negative (-) side of a 1.5v flashlight battery to terminal 2 (middle terminal) and the battery positive (+) to terminal 1 of the power transistor (you'll need some small alligator clips) - Now there should be continuity (zero ohms) between terminal 2 and terminal 3.

If you connect and remove the battery (+) lead, you should see the meter move back and forth between continuity and no continuity just like a switch - which is what the power transistor is acting as when voltage is applied.

This test is difficult to do because of the tight quarters, but be patient and get whatever clips you need to get good readings without the clips touching each other. If it's easier for you, remove the coil bracket so you can do the test on a table or workbench.

If the power transistor fails this test, you've likely found the culprit.

If the power transistor passes, remove and check the coil wire with your meter leads on each end - I don't have a handy reference on ohms/foot but guessing it should be somewhere around 2K ohms max for the short lead.

Let us know when you've tried these tests.

Oh, if either the coil or the power transistor fail the tests, I'd suggest getting a complete assembly from a junk yard as electrical components can last for 10 seconds or 100 years.

Frank