Okay. I'm stumped. I've scoured through my '84 Service Manual (I think the wiring is pretty much the same for an '87) and poured over the wiring schematics. I've replaced the ballast resistor, any suspect wires from the battery that I could find, the alternator, and the voltage regulator.

Here's the problem:

Recently, my 87 RC has been giving me trouble. At first, it just wouldn't start. I'd turn the key and nothing. I'd fiddle around with some wires, and jiggle the plug for the neutral safety switch, and then it would start. So I suspected a bad neutral safety switch, and I replaced that along with the plug, whose wires basically snapped as I removed it from the NSS. That seemed to fix my problem. For a day or so. Then the truck would just stop running while I was going down the road. Sometimes it would restart, and sometimes it wouldn't. I did some checking into the cobbled together fusible links on the drivers inner fender, and decided to cut them out and put in a fuse box (actually it was a mechanic friend who did me a favor when I was at work). So I did that. I put in all 20amp fuses for the four circuits, but the one coming from the big black wire from the alternator kept blowing fuses. The mechanic said to use a 30amp fuse. That helped, but the fuses would still blow. So I started measuring, and found out that the alternator was putting out 34VDC at that wire (and from the post terminal on the Alternator) at idle, and would not go any lower. I changed out the Voltage regulator, and that seemed to fix the problem. 14.5 volts. For a minute. Then it went right back to the 34v. I replaced the alternator (AutoZone checked it and it showed bad). I bought a new VR with the new alternator and put it in. There was no change. Still 34v. I checked the green wire for shorts. None. I check the blue wire for voltage with the ignition on. It showed battery voltage (about 12v now that I've been cranking it a lot) as it should. With the key off, the blue wire shows grounded (using ohm meter to determine that). I have replaced the ignition switch on the steering column, thinking it was possible a bad switch. No change.

I'm really at a loss here. Is there a way to determine if the VR is good using a ohm meter? I have four of them now, and two show .54 ohms resistance between the center (blue wire) post and the case, and the other two show .29 and .34 respectively. I don't know what the reading should be. Between the left post (green wire) and the case ground, none of them show any connectivity at all.

So what am I missing here? When I pull the green wire off the alternator while the motor is running, the voltage drops down slowly to about 1/2 Volt. When I put it back on, there is a spark as it is connected, and then we are right back to 34v at idle. I've seen it go as high as 70VDC at very fast idle.

Could I have just gotten another bad alternator, or could it have 'blown' from using the older VR before swapping in the new one? Any suggestions are welcome! Thanks!

**SOLVED** SEE POST BELOW

several things here.
That big wire from the alt supplies all the power to your car, so you need more like a 50 amp fuse.
If it is one of the newer square backs then 80 amps will cover you.

Next, you need to make sure your vr is grounded. Then you need to measure sensing voltage at the vr.
That taps off the ignition switch, and that means it passes through the bulkhead connector.
That power passes from the ignition switch back up through the amp meter and then back out the bulkhead again.
At any point you could be suffering from a bad connection or part causing a voltage drop on the sensing voltage.
That is if a good ground doesn't fix your problem.

Good point with the fuse size. I haven't found any 50amp blade type fuses at the local AutoZone. Maybe I should try Napa. I've measured the sensing wire on the VR (at the alternator, it's the blue wire) and it shows the same as the battery voltage (now at 11.5 volts). I realize that should be enough to cause the alternator to put out max voltage to charge the battery, but shouldn't it be limited to 14.5 volts or so, and not the 34 to 70 volts I'm seeing?

my guess again, is that the ground to the vr is bad.
Either get a block ground wire to it, or scrap paint under the metal where it touches the firewall.
See if that helps your problem.

If someone can give me a line to a current diagram, I can help you better, but here's how these things work, and I doubt that it's your alternator.

From the alternator, one wire (usually green) goes to the regulator. On these '70 and later systems, this green wire CONTROLLS THE GROUND so to speak through the regulator. That is, the lower the battey voltage on the sense lead--the other lead going to the key on the regulator--the lower this ground is pulled, making the rotor (brushes) draw more current and causing heavier charging.

The rotor (brushes) gets it's voltge right from "ignition run" usually the blue lead going right from the ign switch to one brush. It does not matte which brush these two leads are connected to.

One way to quick check the alternator, is simply pull (green?) wire going to the regulator. The alternator should NOT charge at this time. Using a clip lead, temporarily GROUND the exposed alternator terminal. This should (at a fairly fast idle) cause increased voltage at the battery and "full tilt" charging.

The "sense" lead to the regulator is fed from the ignition switch. THIS POINT MUST BE very close to battery voltage AT ALL TIMES. Follow this lead back to the key, and ESPECIALLY check this during running to be close to battery voltage. This means the connection at the regualator, the firewall connector, the connector at the ign switch, and the switch itself. On the "hot" (battery) side of the ign switch, FOLLOW IT if it's lower than "battery." Same thing--ign switch connector, firewall connector, and on back to battery positive.

One quick way to find out, is take an accurate voltmeter, and put one probe on the regulator ign terminal, (while running and charging, 'cruise') and the other probe on battery POSITIVE post. THIS VOLTAGE should be VERY low--the lower the better, and CERTAINLY below .5 (one HALF of a volt)

Same deal with your regulator ground. Stab one proble on the regulator CASE, and the other directly on the battery NEGATIVE terminal, again, while simulating 'cruise' charging conditions. AGAIN this voltage should be VERY low, less than 1/2 volt.

Once you get these two conditions met, and about the first thing I'd check is the aforementioned firewall connector and ignition switch and connector, if it STILL overcharges, then change the regulator.

Okay, lots of good suggestions here. I have checked the VR ground, and it shows good (VR case to engine block using ohm meter). Blue wire (voltage sensing) shows exactly the same battery voltage as when I measure between the posts of the battery. Have pulled the green wire while motor is running. Alternator stops charging. Putting it back on and the alternator puts out 34+ volts again.

Have replaced the ignition switch with new one, but that didn't help. My battery is at 11.5 volts now, so I'm going to put it on a charger overnight to bring it back to full charge and try again.

Thanks for all the brain power you guys are putting into this for me. It's much appreciated.

If you have replaced the regulator and it does the same thing try this. Pull the plug off the regulator. If it quits charging replace the plug. I have seen many go bad. You most likely are losing the voltage sense to the regulator [blue wire]. You will have voltage in the wire but the connection in the moulded plug goes kaput.

Now that is something I haven't checked. I'll be trying that first thing in the morning. Thanks, NITROUSN

Finally found the solution to my problem with the alternator putting out a measured 34.5V at idle. I now believe it was due to the way my mechanic friend had hooked up the wiring after he cut off the triangle block with the fusible links coming out of it. I mentioned that he had put in a fender mounted fuse block to eliminate the factory splice and he had hooked the return wire from the alternator along with it's fusible link to a port in the fuse block and put a 30a fuse in it. When the battery was fully charged (like when I first put it all back together) the VR was calling for no charge, and the alternator was putting out no significant voltage. At that point the 30a fuse held just fine. But when the vehicle operated for a bit, and was started repeatedly, the alternator put out too much current for the 30a fuse to handle (thanks Andrewh for that tip) and the fuse would blow. As soon as the fuse blew, the circuit from the alternator output to the battery was interrupted and there was no 'load' on the alternator, thus (I think) the alternator was 'freewheeling' and putting out whatever voltage it could. Since the return wire from the alternator still had about 5" of the fusible link attached, I decided to try putting a 'load' on the circuit by attaching it directly to the battery via the + post of the new fuse block, eliminating the extra fuse (but still having the protection of the fusible link). So I cut off the female spade connector the mechanic had put on the fusible link and put on a round terminal connector and bolted it dirctly to the + circuit coming from the battery. I started the vehicle and measured the voltage at the alternator. At idle, it showed 13.5V, and at a cruise rpm, went up to about 14.2V. It appears then, that the alternator MUST have the battery connected to it's output circuit in order for it to put out the correct voltage. Prior to starting the vehicle for this test, the battery was measuring at 11.2V, due to the three days of multiple engine starts for testing without getting a charge.

So, I do believe the problem is solved. I will continue to monitor the battery charging condition to see if the voltage coming from the alternator drops when the battery if fully charged.

Thanks to EVERYONE who contributed to this thread, your posts helped me look at some things differently which eventually led to fixing the problem. You guys are the best.

Once again, MOPARTS to the rescue!

Good fer ya. Ya know, it's hard to 'sit here' and visuallize what some of you guys do to wiring harnesses!!! Yep, not having the charging lead hooked to the battery makes a difference.

I 'spose' there are fuses that would handle an alternator output, but they would be very big and expensive, compared to a fuse link.

For, say, a 60A alternator, you'd probably need at least an 80 or 100A fuse, otherwise you'd have 'nuisance' blowouts. Also, the heat coming from the engine de-rates fuses, so keep them out of the way of the hot engine. After all, a fuse is essentially a 'melting' device

Thank you for posting the solution; too often that doesn't happen!
As a reminder for folks that have electrical problems, this is yet another example of problems with aftermarket fixes. I'm NOT saying factory stuff is perfect, but if you have a problem with a circuit or system, look at any additions FIRST!

Man I didn't know one of our alt's could put out 34.5 volts! unregulated high 15's maybe but I guess I've never had one that was disconnected from the battery while running.

HAd one with a bad connection pump out over 100 volts.
Fried the big wire under the dash. Was over 1 inch of it missing, and about 2 inches of insulation on each side burned away.

Put in a 100 amp alt and converted to the newer style voltage reg, but the guy that put in the wiring, had a bad connector so it dropped contact.

Prior to that happening, it swelled and boiled out 3 batteries.

yeah I bet 100 volts WOULD boil out a battery!

I'd think 100V would blow one up. I remember I was charging one once & I crossed some wires & blew the (2) caps off.made me permanently carefull when working with batteries & their cables.

Alternators WILL PUT OUT well over 100 volts.


Years ago, one could buy a real POS--a little switchbox that you wired into your vehicle. THE ONLY thing that these were, was a box, an outlet, and a toggle switch. The toggle switch did two things--when you wanted "110 volts" --and this is DC, by the way, not AC, the toggle would switch the output of the alternator to the outlet mounted on the box, and disconnect the regulator and 'full field' the alternator. You could plug lights and (supposedly) other AC/DC appliances into the box, and rev the engine up and get 110V out of the thing. You could run things like brush motors--maybe a drill or side head grinder, etc.
I'm sure these were not only hard on alternators, but a big dissapointment to customers who discovered that they would not run AC motors.