Tach erratic / Alternator appears ok

[shrink]

Hello- I know an erratic Tach may mean a bad Alternator but I've had the issue for 2 months and my battery is ok although I NEED to test the alternator myself. So turning on headlights, honking horn, turn signals will cause the Tach to move up and down, and it never reads more than about 1000 RPM even when on the highway. It tends to be jerky and perfectly timed with electrical current changes when turning things on and off. It's a '95 Disco I, v8. (i need to fix up my signature I know!)

[Savannah Buzz]

The tach wire on the back of the alternator is a "flying lead" that pushes on to a metal tab. It does not bolt down (at least on my 97). It sends pulses there, and my frequency meter said about 270 hertz at idle. If alternator is doing the job, battery volts will be between 13.6 and 14.4 at idle, nothing else running. Will drop a little as amp drain increases, but should stay above 13.2 with everything on. Schematic attached, part of the RAVE package.

[Sam95disco]

Mine did that right before the alt went out, Its doing it again. I used my multimeter and batt is fine, disconected the batt with truck running and everything worked. I think mine does it cause of the code 44 I just got.

[Danny Lee 97 Disco]

From what I have been told by others who own a Discovery, you should NEVER DISCONNECT THE BATTERY WITH THE ENGINE RUNNING!.

I know this was a common practice years ago, if the alternator was bad, the engine would merely die. With todays sensitive electronics, you can create havoc. Bad idea.

My tach bit the dust quite a while back, it works when it wants to.

[Savannah Buzz]

If tachometer is indeed all that is dead, and battery is charging corectly -

Does vehicle have an OBDII port to plug in a scanner, I'm not sure when/if Gulf vehicles got that. If it does, you have a $69 fix from Ultra Gauge, that will give you lots of gauge displays, plus code reader scanner.

[Danny Lee 97 Disco]

My tach continues to get more eratic, behaves like a bad solder joint. Does not want to work in the cold, gets erratic and suddenly sprigs to life, drops suddenly if I let off the gas, jumps a little, then runs fine for however long it decides to. It works as little as I do>>>>

[shrink]

Originally Posted by Savannah Buzz

The tach wire on the back of the alternator is a "flying lead" that pushes on to a metal tab. It does not bolt down (at least on my 97). It sends pulses there, and my frequency meter said about 270 hertz at idle. If alternator is doing the job, battery volts will be between 13.6 and 14.4 at idle, nothing else running. Will drop a little as amp drain increases, but should stay above 13.2 with everything on. Schematic attached, part of the RAVE package.

Thanks for the help on this to you and everyone else. I will check out the schematic that you've attached. I also just got the Rave manual last night. The reason why I think the Tach is ok is that I've been running light this for 2 months and figured it would have caused the battery to die already. I am going to have a look at it tomorrow in the day light and If I find a resolution I'll post it. Will definitely do the alternator tests too.

[shrink]

Originally Posted by Savannah Buzz

If tachometer is indeed all that is dead, and battery is charging corectly -

Does vehicle have an OBDII port to plug in a scanner, I'm not sure when/if Gulf vehicles got that. If it does, you have a $69 fix from Ultra Gauge, that will give you lots of gauge displays, plus code reader scanner.

I don't believe it does as it's a '95, but I'll check when I get some daylight to take a peek.

[Savannah Buzz]

We have had people with a short in the wiring harness make tach drop, but that's hard to track down a skint place.

[Danny Lee 97 Disco]

Who knows, a skint wire could cause havoc. Getting to the back of the tach to put in a replacement is a bit of a pain.