Dash Volt Gauge

[ParadiseMJ]

88 MJ Pioneer, 4.0, AW4.  Now that I've finally gotten my AW4 driveability issues under control I'm ready to tackle yet another gremlin. The MJ is running GREAT.  I've put a good 1500 miles on it in the last month without a single hiccup (knocking wood).

 

So, here's my problem. The gauge on my dash says I'm running at about 10 volts. As I drive from home to work, in the early AM (always dark) the gauge reads right off at 10 volts. If I use any of the accessories (extra lights, heater fan etc.) the needle goes down a couple volts. The closer I get to work, 22 miles, the gauge goes up a little.

On my way home, without lights - maybe just radio or wipers, the gauge says about 12 volts. I have no power windows or locks. Sounds easy, right?

When I put the meter to the battery, idling in the driveway, it says 13.5 or so. With engine off, says 12.5 or so.
The battery is strong, lights are bright (for a Jeep) and everything runs as it should. I've done the instrument panel ground upgrade, I've done the sensor ground upgrade, I have a 2 ga. cable for an engine to firewall ground and an auxillary ground from battery to fender, the taillight bucket ground is clean and tight, the radio has an extra ground wire.

Could this be as easy as the just the gauge going wonky or is there a deeper problem??  And IF it's just the gauge...how do I tell without tearing stuff apart.

[Comanchemodder]

 

When I put the meter to the battery, idling in the driveway, it says 13.5 or so. With engine off, says 12.5 or so.

 

 

 From these volt measurements at the battery, I would conclude the Alternator is good. These volts are exactly as expected.   I assume the voltmeter used to measure the battery is accurate of course.

 

I do not have a in-dash voltmeter on my truck, but have your tried to measure the voltage at the in-dash meter leads using your hand-held meter?  This would verify if the in-dash unit is accurate or not.

[ParadiseMJ]

 

 

 From these volt measurements at the battery, I would conclude the Alternator is good. These volts are exactly as expected.   I assume the voltmeter used to measure the battery is accurate of course.

 

I do not have a in-dash voltmeter on my truck, but have your tried to measure the voltage at the in-dash meter leads using your hand-held meter?  This would verify if the in-dash unit is accurate or not.

 

Yes it's accurate...unless BOTH of them are off by the same amount as I used two meters to confirm the readings.  I got basically the same response "on another forum" (to catch a reading AT the gauge). 

 

Along the same vein...WHERE does the gauge get the reading...in other words, from the alternator? at the battery? coil? start relay? Where does that gauge reading get it's info.?

[HOrnbrod]

Sounds like you might have the Renix "floating" ground problem. I had a similar problem with my 91 HO. The meter sensing + lead was tied to the PDC (Power Distribution Center) lug. A set of Jon Kelly's mains cables included a much larger positive cable from the battery to the PDC and cured the spastic voltmeter sensing problem with mine.

 

Frome his site:  http://www.geocities.com/JeepI6Power/faq.html

 

RENIX FAQs
Since RENIX carries its own set of oddities, I thought it deserved its own section of the FAQ page. So, here are questions I've gotten so far...

R1] You've got two "optional" mains cables listed for 1987-1990XJ and MJ - what are those?
Those are "optional" for a reason - not everyone will need them. How do you know if you do? Keep reading...

R2] What's this "chassis ground" for? I don't know why AMC neglected this one, but it was an actual good idea from when ChryCo engineers started revising things! On RENIX, the chassis is grounded through the engine block - by way of the ground strap from the firewall to the cylinder head, and then through the cable from the engine block to the battery. This can be fraught with failures - and this is the reason that the firewall ground strap is known as "The RENIX Killer."
Installing this additional ground provides a more direct ground path for the chassis - which is important, because this is also the primary ground for the Engine Control Unit and for some key electrics! This is a ground I'd suggest everyone add as part of a mains installation - it can't hurt, and it's very likely to help!

R3] How about this "alternator case ground?" If you've got a RENIX rig, the engine is grounded toward the rear. The alternator is mounted up front. This is a relatively long ground path for high current output, and could cause you trouble (even with a stock alternator - I've seen it happen!) Since RENIX uses a Delco CS-130 alternator, there is provision for a direct ground (which is not easily done with the later ND alternator - but the engine ground is closer.)
If you've upgraded past, say, 140A - this ground is a good idea. If you've noted "floating output" at cruise or part-throttle without changing the electrical load, you're also a likely candidate for a "floating ground" that this cable can correct.
There is a cast boss on the back of the alternator case that has been drilled and tapped to accept an M8-1.25 screw. If you order the alternator ground for RENIX, I'll provide the screw as well - you fix one end of the cable to the alternator with the screw provided, and the other end goes directly to the battery - terminal. So much for the floating ground...