For the past week my tach has been dancing around, coupled with lights dimming. I installed a new Advance Auto (I know, I know!) alternator yesterday.

Things seemed fine for about 6 hours of driving around town, 14.6 volts at full load, tach works great, battery @ 13 volts after parking the truck.

I start it up to go see a movie with the wife last night, and the tach is dead. Under the hood I got 12.3 volts while running. Nothing coming from the alternator.

Now, when the tach is dead, the temp gauge also stops working, it sticks at a point just under the halfway mark, but I'm also NOT getting the battery light.

What gives? Could the alternator not be getting a signal to go full field? Advance can't get another replacement for 2 days.......And I have to return to work tomorrow (We're visiting family in DC).

 

Well, hate to say it, but maybe now you will understand why a cheap rebuilt is not the way to go.
I would either find a local starter/alternator shop rebuild one for yo and get your money back from the part store or buy a good one from British Parts of Utah.

 

And the Advance guys can test your suspect new alternator, which will remove the question of the field wire. That extra third wire supplies a pulse signal for the tach to count, it reads about 270 hertz at idle on mine. Have been through a couple of rebuilts. I suspect that the reason is that rebuilders use the least expensive electroncis (diodes, regulator) that they can, and these don't keep up with the heat in a Disco alternator. The fan shroud shoves hot air front the radiator into the alternator, and mine can hit 240 degrees plus (IR thermometer aimed at the case). While this might be great to bake out the water from a wading session in Borneo, it is really tough in stop and go traffic. If rebuild shop can't squeeze you in, maybe a trip to local salvage yard (to salvage the vacation...).

 

I found out one of the best way to go is to get the original Magneti Marelli rebuilt. There is a very good and reliable shop near my home. It costs me 80$ parts and labor.

 

I had the same ISSUES....your belt is on backwards sir

 

I wish it was that easy, belt is on perfectly.

UPDATE:

Didn't have time to dick around waiting for Advance to get me a replacement. Bought a Yellow Top Optima and drove from DC to Roanoke, VA. The Alternator only worked for 60 of 225 miles.

Both the Rover and the Optima have lived up to their respective reputations.

Time to pull out the wiring diagrams.........

 

From the ETM (Electrical Troubleshooting Manual), the generator (we call it alternator) delivers DC power thru the 100 amp main fuse in the under hood fuse box to the battery and everything else. So that fuse has to be good, and connections clean and tight. The two wires near that big fuse should also be clean and tight in the fuse box.

The generator has two other leads. One is the "exciter" voltage that comes through the "charge" light - if the bulb is out, no charge. The charge light is fed from fuse F14 on the fascia fuse box. Usually if that fuse is out you will drop other guages.

The plug in lead is the tachometer pulses (will read somethng on the AC scale of your meter).

The alternator is a three phase generator, being converted to DC voltage. So in theory you could have two bad diodes, and drop 66% of the output. Might not be enough left to charge the battery or run very much, but would extend your reserve running time on a battery, and charge light would not be on. Of course, if charge light does not come on at crank up, the bad bulb will mean no charging.

You did pretty good to make it that distance with just a battery alone, so the alternator might be putting out, but very low amps. Did you do this with AC on?

Reference pages section E2 page 12 (warning light) and B1 page 9 (alternator wiring) of the Electrical Troubleshooting Guide.

 

That 100 amp fuse is also for the battery, since I was running off the battery I know it's good.

If it was just a couple bad diodes wouldn't the tach still work somewhat? It's completely dead.

Charge light comes on with all the other cluster lights at crank. So it's not that.

When I was running off the battery we had everything non-essential turned off, I attribute the range to that, and the power of the deep cycle yellow top optima.

 

Well, you have eliminated the fuse problem, because other gauges work, battery bulb is known to be good from start up. The tach has to get pulses from the alternator to count, and those come from the diodes at some point. Considering that you were running a fuel pump, ECM, several other modules, and ignition, you had a well charged battery. Will local stores of the same chain honor the warranty? They can bench test it, sometimes they can test in the car. Of course to do that you should charge up battery for a while before limping over to the store.

I went thru two, I had not found this forum at that time. If mine goes again I'm going to the local rebuilder.

 

I don't want to insult your intelligence but that's the same thing I said, months before I found out it wasnt. And by your response I doubt you even checked. Do yourself a favor an at least check, remember over the alt and under the fan then back over the ps pump.