New battery and now tach is jumping

[scott540]

Battery was totally dead yesterday. Replaced the battery, car started right up. I was out driving today and noticed the tach jumping occasionally at idle from 1 to all the ways to 6 and back. Is this a voltage regulator problem or alternator? Alt ligh has not come on while driving. Thanks. It's a 79SC. Where is the voltage regulator? Easy to replace? What's also crazy is two of my guages have fogged up or become cloudy since I put in the new battery

[lfot]

Originally Posted by scott540

Battery was totally dead yesterday. Replaced the battery, car started right up. I was out driving today and noticed the tach jumping occasionally at idle from 1 to all the ways to 6 and back. Is this a voltage regulator problem or alternator? Alt ligh has not come on while driving. Thanks. It's a 79SC. Where is the voltage regulator? Easy to replace? What's also crazy is two of my guages have fogged up or become cloudy since I put in the new battery

First, why was your battery totally dead? When the tach jumps around, does anything else happen? Does the engine stutter as well, or does everything remain fine except for the tach? At night, do your dash lights get brighter than usual? Headlights do an up and down surge?

The voltage regulator is attatched to the alternator. That's on the flip side of your engine fan inside the shroud. If the alternator has never been replaced, you'll probably be better off getting a Carrera style alternator with the built in voltage regulator... one of those "while you're in there" things.

Seems to be a pretty common problem. First, check all of your grounds, especially on the battery you just replaced. It could be something simple like that.

[scottb]

Originally Posted by scott540

What's also crazy is two of my guages have fogged up or become cloudy since I put in the new battery

Which gauges? Is it possible that your alternator is putting out too much juice, causing the gauges to heat up and throw off condensation on the inside of the glass?

Dead battery + jumpy tach = bad alternator or regulator.

Do a search here on the board for the exact numbers, but IIRC, you should be getting 13+ volts out of the alternator at normal (~2000) RPM.

[scott540]

Not sure why the battery was dead. I hadn't driven it for almost 2 weeks. I tried to jump it but it wouldn't start. Gas/oil gauge is fogged. Temp/pressure guage is a little fogged. Tach is the only thing acting strange. Time to buy a volt meter I think! Thanks

[dshepp806]

alt/reg, for sure.

[Mo_Gearhead]

Well the fogged gauges would scare me. Can you tell if its condensation?...or smoke?? If smoked glass, you have wiring troubles and I would be unhooking the battery till I could chase them down! Good chance of short/FIRE!!

[scott540]

Must have been condensation. It's been really humid and rainy here until today. Left it out in the sun and the gauges are clear

[JV911SYDNEY]

Originally Posted by scott540

I was out driving today and noticed the tach jumping occasionally at idle from 1 to all the ways to 6 and back.

i have the same prob...here's what i posted on rennlist

hi all,

since the 3.0 conversion my tach has been doing strange things

while cruising along the tach will swing wildly around to 6k and back to 3k and then hover at say 5k. Its done it a few times for just a few seconds but last night it did it for what seemed like 30 seconds.

note the car runs fine, doesnt splutter or jerk

my initial though was "alternator" but from reading other posts on the subject alternator probs are usually accompanied by spluttering or at worst the car will shut down.

suggestions?

many thanks

JV

it was suggested that i check the voltage and grounds which havent been able to do yet as someone stole my toolkit right out of my garage

[JV911SYDNEY]

tested the voltage last night...13.5 at idle and a little higher at 2000rpm

drove to my parents, 20 minutes each way and no bouncing tach

[lfot]

Originally Posted by JV911SYDNEY

tested the voltage last night...13.5 at idle and a little higher at 2000rpm

drove to my parents, 20 minutes each way and no bouncing tach

mine did it for a while last summer, then it went away.
came back a few weeks ago more consistently, so i replaced the alt.
would i suggest delaying like i did? no.

[scott540]

Seems like one of the battery cables was loose. Tightened every thing up and no more tach jump.

[JV911SYDNEY]

Originally Posted by scott540

Seems like one of the battery cables was loose. Tightened every thing up and no more tach jump.

great! - hope mine is the same easy fix