Battery's died and now the tach is way off?

[mallet2442]

So the other morning I go out and the batters where dead! Nothing I mean dead. I put the charger on them and found the problem (bad door switch) no problem. Now the tach is way off! Idling at 1800rpm and cruising on road at 60 the its over 4 k? Any ideas?

 

[NVW]

Tach signal comes from the alt. Bad alt. may be why your batteries died. Can you test the volt output of the alt?

BTW welcome to TTS

 

[Pruittx2]

X2 Dead Alt

 

[THEFERMANATOR]

Alternator is the likely culprit here since the tach is run off of it for the 6.5's.

 

[mallet2442]

Thanks for the response! Good point but the volt gauge is running just a little below 14 volts? Can just the part in the alt that runs the tach be bad and not the charging part?

 

[WarWagon]

No, bad alt means the entire unit needs service. Usually they just die. However you may have a bad battery that is pulling your voltage down and causing the alt to fry attempting to charge the bad battery. Have you charged the batteries with a charger since? Alternators are not designed to charge a dead battery after a jump start and this weak unit WILL fry after attempting to do so. Heat damage to the diodes adds up and couple days after the dead battery gets jumped you get the sudden red light and dead alternator.

I recommend a CS-144 to replace the weak CS-130 POS alternator. Couple changes, bigger charge wire, and new rear bracket you make and it can happen. Threads on here about doing this.

Disconnect and check batteries with load test, check grounds by disassembling and cleaning. Test alternator. Your problem should show up.

 

[THEFERMANATOR]

The tach is run off the voltage regulator leg that sends out stator output. Theres a chance that the regulator is burned and shorted slightly and causing a battery drain as well as cross shorting the stator. What year truck is it?

 

[mallet2442]

It's a 99 c3500. I know what drained the batters down and have fixed that. Its just that it sits for days and cranks rite up. No sine of bad battery sells! When I found them dead. I put the trickle charger on the for at least eight houres then removed the charger befour cranking it. I do have a carbon pill battery tester with good gauges I'll try tomarow. Thanks for the help

 

[THEFERMANATOR]

Sounds like your alternator is done, and for a 99 I would go with the AD244 which is pretty much superior to the CS144 in just about every way. You can buy one for a 01 chevy 3500 with the DURAMAX and 130 amp charging system. I'm about 40 minutes away, but I have a good AD230 you could put on to verify that your alternator is the problem.

 

[WarWagon]

Good catch - I was thinking dark ages 1996 and older...