Ok, SO I have a 1997 disco . This will be a bit over explained so that I dont miss any details. In late October early november I noticed that when I first started my rover the tach would hold at 500 rpm for a few minutes before finally registering acurately. A few weeks later and it took ten to twenty minutes to register... Thanksgiving day I was driving with all options on (wipers, headlights, both seat heaters, defroster, etc...) and the rover started sputtering going up a hill... turned the seat heaters and radio off and it made it up the hill. I bought a new Interstate heavy duty battery... ran ok for a week, then started to sputter again. I hooked a volt meter up and it said 13 volts at idle then I would rev it up and the voltage dropped to 11.2. Went to local wrecker and swapped the alternator for a bosch that was on a 97 in the junk yard. Worked great for a week (including RPM gauge) then started dying if I shut it off and left the positive bat cable hooked up. Died quick 4 hours from full charge to dead... traced it to the alternator as being the cause... the diodes were fried. Went back to wreckers and made sure to get a marelli magnets alt from a 96 disco. This one only worked for about 8 hours before it left me stranded on the side of the road at night making popping and sizzling sounds from the alternator while it was running. Heres where it gets weird, When I pulled over the vehicle was still able to run but not drive... in the dark when I popped the hood it looked like a disco light show under there. Every where the plug wires came anywhere near a source of ground they were arcing. I replaced the coil and the wires but am leary to spend money on another alternator before checking here. Could a bad coil and wires cause alternators to go bad so quick? and why did the first one take months to go bad and the next a week and the last a day? Ive searched the posts here and found no similar thread. Thanks in advance for the replies

 

IMHO you may also have a loose or corroded ground cable from engine block to frame or frame to battery. This may have allowed the spark discharge around old cracked insulation on cables to find a new path to thru the alternator to the battery ground, peak inverse voltage ratings of those diodes is not enough to hold off the ignition coil output.

 

I have checked the ground from bat to above wheel well and then to frame. both were tight and clean the one that runs from frame to block is difficult to get at but if Im correct it mounts to one of the through bults that mount the starter correct?

 

I believe so. You can also voltmeter or ohm meter things to confirm. Measure volts from negative battery to engine block. Should be extremely low. Can then connect a battery jumper cable from negative battery to engine block and see if any change.

 

Ive tested that... sed 2 points on block to frame with jumper cables and there was no change

 

Then I think you are good to go.

 

Would the bad coils and plug wires cause an alternator to go bad? Or was the coil and plug wires just another symptom of a bigger problem... is there a known problem that could cause plug wires, coils, and alternator to go bad?

 

See pix of the internal of the alternator. The brushes make contact with the rotating shaft and carry power to/from the wound coils. The brushes are made from a graphite compound, and wear down by constant contact. When they reach the limit of their travel, the power connection goes on/off multiple times. The electronics inside the package is a set of diodes to rectify the three phase alternating current to direct current. If on diode hgoes out, one phase is lost and the laternator becomes a 66% unit, or a 33% unit if more fail. The electonics package also includes a regulator, which can also fail.

 

Sounds like possibly a short exists somewhere that is damaging those alternators.

I would also use the ohm meter to see what kind of resistance readings you get between the frame and the engine block, and between each of those to the body. You could have some ground issues like Savannah said.

The grounds may appear to look good but could have a high resistance.

My starter to frame ground gave me some issues initially as did the battery ground connections.

 

Ok, well I will double check... and Im pretty sure the diodes are blowing... because in the oem alt they just stop charging, but in the bosch alt instead when the diodes went they just become a short inside the alt that drains the battery when parked, but it still charges and runs the vehicle just fine, as long as I remember to unhook the pos terminal when I shut it off. The 2 oem alts dont drain the battery they just wont generate a charge anymore