Electrical 85 CJ7 tach fluctuation

[hoods5]

Hey Guys,

The tach on my 85 CJ isn't working properly so I'm looking for a little advice.

When I turn the key to the run position before starting the Jeep it reads about 12.8 volts. When I start the Jeep the gauge just pegs out instantaneously. I've replaced the alternator figuring it was the diode but that wasn't it.

I've attached some pics so you can see what I have.

Let me know your thoughts. Thanks

Dave

 

[KRUPP]

I haven't touched the Tach since I got it running last year. BUT, sometimes my OEM Tach just won't wake up until I tap it a couple times. Ha!
CJ gauges are notoriously finicky. There's probably 1,000 posts on the interwebs for their Fuel/Temp Gauges alone.
I wouldn't continue to replace parts though. Look at wiring first.

 

[Patrick]

Voltmeter

 

[TSB8C]

I believe you are referring to the voltage gauge and not the tach. The volt gauge is actually one of the easier gauges to deal with in a CJ. It relies on a ground and a single voltage wire (yellow) from the ignition that is only hot when the key is turned on. And of course there's also an orange wire to the light bulb in the gauge as well as part of the dash light wiring.

First test with a multi meter is to disconnect the yellow wire from the gauge and measure the voltage between that wire (with the key on) and ground. Should read 12 volts with engine off (just battery voltage) and about 13.6 volts with engine running (alternator output). If that's good, then measure between the yellow wire and the black ground wire on the gauge. Should still get the same readings if the black ground wire is indeed properly grounded. If these tests pass, your gauge is bad.

If you get other readings besides what I described, you have a wiring issue.

 

[HowDidIgetStartedOnThis]

I suggest you put the voltage regulator on your list of things to investigate. Your reported 12.8 V is an acceptable voltage for a fully charged battery, engine off. But an off scale reading suggests something is amiss with the voltage regulator. Check the voltage at the battery with a voltmeter, engine off. If the voltage at the battery is also 12.8 v the gauge is good.
When you start the engine, the alternator begins to rotate and produce electricity. By design the alternator should produce more electricity than used by your Jeep, at full electrical load. I suggest your alternator is running at full bore and overpowering your electrical system. That is why the reading on the gauge is off-scale.
The job of the voltage regulator is to control the output of the alternator and match the electrical supply to the electrical load of your ride. The intent is to keep the state of charge (SOC) of the battery at the high end, ie always fully charged. When you turn on driving lights, or winch, or compressor, etc the electrical load goes up. The voltage regulator responds by allowing the alternator to deliver more electricity (voltage) to the battery. Right after starting the engine the voltage should be in the range of 14.5 +/-, depending on loads (headlights, seat heaters, blowers, etc). Once the battery has regained the energy consumed by the starter motor the voltage may reduce a bit and sit just above 14.0.
On my 85 CJ7 the regulator was easy to replace and relatively cheap ($45 purchase).

 

[hoods5]

Here are a couple things that I did and I think I'm ok now based on what HowDidIGetStartedOnThis said in his last post.

I bought a new battery. The ground on the old one was constantly getting corroded. Figuring I may have had a bad cell or two.

Took the new alternator back and had got a replacement alternator. Although they tested it and said it was ok but was nice enough to give me another.

Installed new battery cables.

Bought a new volt gauge for the dash.

My multimeter was old so I bought a new nice one.

So I tested the battery with the jeep not running and I get 12.96 volts.

When I tested the Jeep running, I get a reading of 14.56.

The new volt gauge also reads 14.5 volts when running.

So based on HowDidIGetStartedOnThis comments above I think I'm good. Any thoughts?

Thanks

Dave

 

[TSB8C]

Doubtful corroded battery cables was the issue, although good to replace them anyway. Corroded connections would not cause high voltage readings, but rather low due to poor conductivity. Your voltage readings are spot on with battery voltage when not running and then alternator output voltage with engine on. Did you take any of those electrical readings before replacing the alternator? If so, should know whether the replacement alternator fixed the issue or not. If readings were not taken before the replacement, then no way to know for sure if the alternator was it or the new volt gauge. But since the auto parts store tested the alternator and it showed good, I'd bet the volt gauge was faulty and the new one is now showing you the real reading.

 

[hoods5]

I don't remember the original readings but I think i'm good for now. I really appreciate everyones input. Thanks

David

 

[Lumpy Grits]

Suggest you run a hardwire lead from the dash panel to the battery NEG post

 

[hoods5]

All fixed guys. After all I did it ended up being a bad gauge. Thanks for all of your help.