By pass Volt reg install old style?

A Ford regulator won't work. It's the wrong kind of circuit. You can use the '70s style Chrysler electronic voltage regulator but that's going to leave you with the Check Engine light on.

How did you determine the regulator is bad? That 's a very rare failure.

There's going to be other problems but an external voltage regulator can be installed. First, since it is such an uncommon problem, lets start with some proper tests. Removing the entire wiring harness is WAY overkill in this circuit.

The engine has to be running to take these voltage measurements. Measure the voltage on all three wires on the back of the alternator. The large output wire bolted on must have full battery voltage all the time, engine on or off. One of the smaller wires must have full battery voltage when the engine is running. If either of those are missing, the voltage regulator isn't the problem. Holler back with those three readings.

Dandy. Now, with the engine running, . . . woops, wait a minute. I don't have a '91 service manual handy so I'm doing this from memory. Do you have those blue and green wires plugged into the two terminals on the back of the alternator, or do they go into a little black plastic block, then there's two metal tabs bolted to the two terminals with little nuts?

If you have two separate wires, ground the green one while the engine is running, but please do not raise engine speed. If the alternator is indeed okay, which I suspect it is, doing this "full-field" test AND raising engine speed can do a lot of damage. We don't need to make more work.

What you should see with the green wire grounded is the head lights get brighter, voltage, if you're monitoring it, will go up to over 15 volts, and you'll hear the alternator sing and load the engine down. That will prove everything is good to that point.

If you have the little block that the two small wires go through, don't pierce the insulation as that will just be asking for more trouble in a few months. Instead, go here to see which terminal to ground to do the full-field test:

http://randysrepairshop.net/which-field-terminal-do-i-ground.html

If you ground the wrong one, you can burn out a connection in the Engine Computer. I know from my students doing that on a "bugged" Dakota that I built. Again, this only applies if you can't tell which terminal goes to the green wire.

You can also look at this page to see if that sad drawing looks like your two smaller terminals:

http://randysrepairshop.net/interpreting-the-test-results-chrysler-charging-system-1970---1989.html

That is the diagram that will be used if you still want to install the external regulator.

This next page for the newer style systems shows that plastic terminal block I'm talking about, and it explains why you can't tell which wire goes where. That's when you have to go back to that first page I sent you to.

The fact that one small wire has a slightly lower voltage than the other one surprises me because that proves some current is flowing in that circuit, it's just not enough. That green wire should be down to around 4 - 11 volts. That also means there can't be a break in the wire or connector pins, but something like that is adding too much resistance. This is where the full-field test will tell us a lot. If grounding the green wire gets the alternator working, follow it to the next connector and ground it there too. Work your way along that wire all the way to the computer and keep grounding it at every accessible test point. You're looking for the first point where grounding it doesn't make the alternator charge wide-open.

You haven't told us which setup you have, two separate terminals or that plastic block. We aren't going to guess and have you mess something up.

Did you even bother to do the full-field test? If you modify this circuit with the external regulator, what are you planning on doing about the Check Engine light that is going to be on all the time? How will you know when some other problem develops?

If you didn't look at how this circuit is wired on the web page, here's the diagram again. There's a blue and green wire at the alternator and a blue and green wire at the regulator. The regulator has to be grounded to work.

I don't doubt the alternator is fine. It's the wiring that gives more trouble. Here's two drawings of Engine Computer connectors. I can't find a '91 service manual. The closest I have is for a '93, and it shows the big 60-pin connector on the right. I tried to help someone else recently with a '91 Dakota but he said he had the three connectors like on the left drawing. That is for a '96 and '97 model.

I thought a '91 had the computer next to the battery behind the left head light and the big connector. If yours has that type, does it also have a separate 14-pin connector next to it?

If you have the three smaller connectors, the computer will be on the right inner fender halfway between the head light and firewall.

Please do these extremely simple things first so I know this is going to work. You are working with by FAR the world's simplest and most reliable charging system ever produced, but if there's a problem in the wiring, this modification still won't work and you're going to be frustrated. I'm going to know what works and what doesn't before you start cobbling this system so it works when you're done, otherwise you'll have to find someone else who knows how to do it.

I need to tell you which wire to cut, but I won't know that until you tell me which computer connector style you have and how many plugs there are. There's three different versions of computer and they all have the green wire in different places. There isn't enough time left in my lifetime to type descriptions for all the possibilities, plus that would just add unnecessary confusion to the story.

Assuming you have those three smaller connectors, look on the bottom row of pins in the middle connector and see if the green wire is the second one from the end. If it is, see if you can slide a stretched out paper clip in alongside that wire. I can't remember if there's a cover over the wires. Don't take those connectors apart. They're REAL frustrating to put back together.

If you can't slide a paper clip in there, (it has to touch the terminal), you can resort to pushing a razor blade through the wire's insulation until it touches the wire. Now, with the engine running and the head lights on, use a piece of wire or small jumper wire to ground that green wire. Tell me what happens to the head lights when you do that.

No, power doesn't go to that wire. I just told you what to do, but for some reason your replies aren't showing up until after I post mine. Maybe we're both typing at the same time.

You have to ground that green wire. I want to be sure you have the right wire before you cut it, and only IF you have to cut it. You do not have to disconnect it to ground it. Just use a piece of wire to touch that terminal with one end and the body or engine with the other end. This is a real fast simple test that should take all of five seconds but the information it provides is important.

That's the 60-pin plug I thought should have been on your truck. The red arrow is pointing to that green wire in pin 20. Poke a paper clip in next to it so it touches the terminal, then use a piece of wire to ground it as shown with the yellow double arrow. (Put the wires back on the alternator and have the engine running when you do that). That five-second test will tell us how to proceed. Have the head lights on too. I need to know what happens when you ground that wire.