low voltage at idle with 160A alt & optima

[MachineMan]

I have the same low voltage gauge issue. It must be a global problem and I've seen several threads about this. I have a 145amp alt with optima. Replaced positive relay wire with 4ga, added a battery to chassis ground 4ga, added a Alt to ground 4ga, replaced the Alt positive to relay with 4ga. When idleing the voltage across my battery is 14v+ volts strong while the inside guage is around 12v or so with no accessories running. So the question is, where does the gauge tap into? And can it be wired to a more representative source....

 

[langer1]

The XJ voltmeter is nothing more than a mechanical idiot light.
To do a good job of monitoring your charging system you need to install a good amp gauge.
To do that you need to run the 4 awg wire all the way to the amp gauge and back or use a 200 amp shunt at the battery and wire to that.

 

[Matthew Currie]

I have the same low voltage gauge issue. It must be a global problem and I've seen several threads about this. I have a 145amp alt with optima. Replaced positive relay wire with 4ga, added a battery to chassis ground 4ga, added a Alt to ground 4ga, replaced the Alt positive to relay with 4ga. When idleing the voltage across my battery is 14v+ volts strong while the inside guage is around 12v or so with no accessories running. So the question is, where does the gauge tap into? And can it be wired to a more representative source....

I've never bothered to see exactly where it taps into, except that it's pretty clear it shares some flimsy circuitry with the heater fan. It's a good question though. It ought to be possible to figure out which wire or connector it uses, and simply snip it and reroute it to a better spot. I've never bothered. I figure that once you're familiar with its normal state it doesn't really matter just what it reads, only that it's normal.

An ammeter would, of course also be a nice idea, because it would tell you just what the charging rate is as it is happening. I wouldn't lose the voltmeter, though, because an ammeter cannot tell you what state your battery is in, especially (obviously) when the engine is not running. Both are handy.

 

[5-90]

One of these days (especially if I can get my hands on a gage cup or two!) I plan to replace as many gages as possible with some variety of aeroplane gage - and wire them as directly as possible.

Why? The damn things work. Period. And, because I "used to was" a pilot, and I'm addicted to information (and OEMR gages, let's face it, largely SUCK.)

I've not considered wiring the in-dash voltmeter closer to the source - because its resolution is not fine enough to be acceptable to me anyhow, and it tends to vary (when the voltage stays rock-steady. I've checked this.)

I'd like to have a voltmeter and an ammeter - and eventually, I will.

5-90

 

[hippymill]

An ammeter would, of course also be a nice idea,

talk to some of the FSJ guys about this. The old chiefs and wags were notorious for burning(thats right, FIRE) because of ammeters. Check you voltage and amperage at the battery. Also-have you or someone else done a hack job on any of the wiring? It is possible to mess up the reference circuit, making the alternator run constantly. this burns out the alternator. With the optima batteries, you can run for months without realizing that the alternator is shot. I went through 3 alternators on my wag before figuring this out. It also wrecked the battery, but the truck ran fine on just the battery(for 7 months and 2 6 hour trips). If your voltage is off at the battery, have the alternator checked.

 

[ifixit8]

One of these days (especially if I can get my hands on a gage cup or two!) I plan to replace as many gages as possible with some variety of aeroplane gage - and wire them as directly as possible.

5-90

I thought that the aeroplane gauges run differently from cars? I have noticed that on my wifes xj the factory volt gauge drops drastically when the defrost is on. I think that happens because the electric fan kicks on and off. Like others have said your best bet is a new gauge that is hooked up directly to the source. But with that setup you shouldn't have any troubles running the typical stuff. I have been doing sound systems in cars for many years. with that size alt, optima bat, you could run a 1000 watt sound system with no problems along with the typical draw of factory accessores.

 

[5-90]

Nah - the basic gage mechanism is the same, and there are 12V aeroplane systems, 24V, 28V, and 48V systems as well.

Checking around yielded a couple sources for 12VDC aeroplane gages - check Wicks Aircraft Supply and Aircraft Spruce and Supply, for starters. Both of those outfits cater to kitbuilders, who usually work on a 12VDC system (since they're usually already familiar with it...)

5-90

 

[DIGITRUCK]

I'm in a similar boat. '88 XJ Laredo Reman Renix 4.0L, 170A Alt, Red Top Optima.

I solved mine by running a 4ga Black Wire from the Neg Bat Term to Firewall. Once I did this the "idiot" voltage meter didn't fluctuate quite nearly as much.

The only times it really takes a dive is when I run the A/C, Heater, Vents, Neons, Hellas, A/V/Nav/Computer High Pwr Multi-amp and crank it

Killed the stock alt and several bats b4 I upgraded the alt and bat.

But if I'm idling I won't run "everything". lol

btw - Anyone have the Driver's Side and Front Pass Side SWITCH PANELS with the Handle for Power Windows/Locks and colored Black and Silver? In good condition for sale? Would like to see some pix if you got 'em. Thx.

Thx, Phil

 

[ifixit8]

Nah - the basic gage mechanism is the same, and there are 12V aeroplane systems, 24V, 28V, and 48V systems as well.

Checking around yielded a couple sources for 12VDC aeroplane gages - check Wicks Aircraft Supply and Aircraft Spruce and Supply, for starters. Both of those outfits cater to kitbuilders, who usually work on a 12VDC system (since they're usually already familiar with it...)

5-90

You hit it right there, the kit planes that run on a 12v system. Good to see you know what you are looking at. I couldn't remember if they did have the 12v systems or not. When I was in school for aircraft maintenance, most of the stuff was 24v or higher.

 

[jeeperjohn]

Many aircraft gauges run on 28 volts but there are also many that run on 12 volts. The price of new aircraft gauges would quickly send you back to the automotive manufacturers. I would suggest a good set of autometer or VDO gauges. High quality and cheaper than aircraft gauges.

 

[JoBo]

well am I correct in thinking (usually not I find here).. That the battery will never see a "high charge rate" unless I use alot of power while the rig is off? I mean while running the alt is said to output 65A min and thats more then I will ever use. So the alt is running all the load Amperage and battery is just trying to keep the voltage up?

That brings up voltage though. Is it true that Alt no matter what amperage, will not help Voltage? And the only way to keep voltage up would be to add a 2nd battery?

I will check the voltage today when I get home at the battery when the gauge shows low. See what I get.

I don't agree with langer1. The regulator will try to maintain somewhere right around 14 volts. It's a voltage regulator. In a perfect world you should not see 9 volts. Your right, once started the alternator is running the load the battery is just "storing" for the next start or when your ideling. I would check your belt for the proper tension and replace every inch of old battery cable.

JoBo

 

[5-90]

You hit it right there, the kit planes that run on a 12v system. Good to see you know what you are looking at. I couldn't remember if they did have the 12v systems or not. When I was in school for aircraft maintenance, most of the stuff was 24v or higher.

Hey - I was a pilot once - Piper Cherokee, Piper Cub, and Stearman biplane(!) I may not have my A&P, but I'm at least familiar with how things work (and I've service ground equipment - fuel trucks and GPUs...)

5-90

 

[BillyBob]

In looking through to 89 FSM, the voltmeter(battery indicator as they call it) uses a 20gauge wire that is tan in color with a green tracer when it leaves the instrument panel connector. From there it goes behind the fuse block & into the bulkhead connector on cavity "G2". On the engine compartment side of the bulkhead connector the wire remains 20 gauge in size, BUT becomes TAN in color. It then goes to a 6 pin connector located near the rear of the battery on cavity "C". From there it goes to the plug on the alternator in cavity "L".
I didn't notice any splices along the way, so it evidently does not share the power to any other device. I will say though any place there is a connection(plug and/or connector) there is the chance of a loss of continuity and therefore the possibility of a loss in the proper voltage or current flow. The easiest places to check & clean of course would be the alternator connection & the 6 prong plug located near the battery. It is common on many of the older cars to have to remove the bulkhaed connector/s & clean the connections/terminals due to moisture contaminants in the engine compartment and just general aging of the connections due to some corrosion.
A "possible" simple way to test this would be to just run another piece of 20ga. wire (or bigger if you must) from the alternator plug to the instrument panel harness to see if it makes any difference.

 

[ifixit8]

Hey - I was a pilot once - Piper Cherokee, Piper Cub, and Stearman biplane(!) I may not have my A&P, but I'm at least familiar with how things work (and I've service ground equipment - fuel trucks and GPUs...)

5-90

I love stearmans. I use to be a fueler at burlington WI airport. and a stearman would come in there quite a bit, I even got the owner to let me take a pic of my 2000 Grand am GT with his!!! sorry to hear that you aren't flying anymore, I can't find a job in that field right now. so I'm doing the next best thing, my hobby of car audio/video/security installation.

 

[5-90]

I don't mind - I let the license lapse, and my eyes are probably just bad enough to not pass the physical anymore. At least I can say "I vas dere, Cholly."

Hell, I couldn't afford an aeroplane anymore anyhow - and you don't need a license for an ultralight, if I get the bug again... I'd feel a little strange flying an airplane that weighs less than I do, but I've done odd things before anyhow.

I've done short-hop, acro, and I even jumped out of airplanes for a while - so "I vas dere, Cholly!"

5-90

 

[BillyBob]

Even though most of the older XJs have a "lazy" voltage guage, doing a upgrade to a larger amperage alternator, replacing with a larger capacity battery & doing the wiring upgrade is not going to cure the low output at idle speed. I have the same problem and am having a friend make me a smaller diameter pulley for the alternator so the output comes in sooner. This will be on a "trial" basis til we see which pulley works.

 

[lawsoncl]

Adding to 5-90's above, it should be noted that one of the main reasons that alternators superseded DC generators in cars is the ability of an alternator to be overdriven without damage. A DC generator will overheat and burn out if it is run too fast, and as the speed range of engines increased, it became increasingly necessary to underdrive the generator, with the result that most generator equipped vehicles charge poorly at idle. Because an alternator does not suffer from this problem, it is possible to design it so that it charges well at idle and the voltage regulator cuts it out at higher speeds. Most alternators are already somewhat overdriven, and will put out plenty of juice at high idle, so unless you expect to spend a lot of time running a winch or a snowplow at slow idle speed, there's nothing to be gained from additional overdriving.

So why are generators so common on motorcycles if rpm is an issue? They are generally run at crankshaft speed with redlines 10k and higher?

Based on my motorcycle experience, generators do have issues. As you pointed out, they generally have lower output at idle. The permanent magnets get weaker over time, especially is subjected to too much heat.

 

[langer1]

So why are generators so common on motorcycles if rpm is an issue? They are generally run at crankshaft speed with redlines 10k and higher?

Based on my motorcycle experience, generators do have issues. As you pointed out, they generally have lower output at idle. The permanent magnets get weaker over time, especially is subjected to too much heat.

Most motorcycles have alternators even though the manuals call them Generators, if theres diodes or a rectifier it's an alternator no mater what the book calls it.

 

[lawsoncl]

Most motorcycles have alternators even though the manuals call them Generators, if theres diodes or a rectifier it's an alternator no mater what the book calls it.

The definitions of alternator versus generator seem to vary. The best explanation I've seen of the difference is whether the spinning part or the stationary part generates the voltage. If the voltage generating windings are spinning, they are generators. At one time, they were called generators if they used magnets for the field instead of windings. On all the older motorcycles I've worked on, there was no field coil and they used spinning permanent magnets. So I guess under the first definition they'd be alternators.

Checkout http://www.rowand.net/Shop/Tech/AlternatorGeneratorTheory.htm for a long winded explanation.

 

[langer1]

The definitions of alternator versus generator seem to vary. The best explanation I've seen of the difference is whether the spinning part or the stationary part generates the voltage. If the voltage generating windings are spinning, they are generators. At one time, they were called generators if they used magnets for the field instead of windings. On all the older motorcycles I've worked on, there was no field coil and they used spinning permanent magnets. So I guess under the first definition they'd be alternators.

Checkout http://www.rowand.net/Shop/Tech/AlternatorGeneratorTheory.htm for a long

winded explanation.

A shorter explanation is a ALT is a type of generator but mainly the term Alternator came from the fact that the output was AC (Alternating current) thus Alternator.
Both Generators and Alternators can have permanent magnets.