International Full Size Jeep Association
Home Forums Reader's Rigs Tech Library Trail Stories FSJ-List
International Full Size Jeep Association  

Go Back   International Full Size Jeep Association > Tire Kickin' > General FSJ Tech

Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 07-20-2021, 11:31 AM
Brynjminjones's Avatar
Brynjminjones Brynjminjones is offline
258 I6
 
Join Date: Jun 11, 2017
Location: Derby, England
Posts: 440
1991 Voltage Issues

My 1991 GW has always had low voltage at idle, but recently it's started bothering me again so I want to do something about it!


It's fine when there's not much electrical load, but it struggles once you've got a few things running.
The thing that kills it the most is the air conditioning. With headlights and AC on high it will drop into the red at idle, and won't go above 12 even at highway speeds.

I've been around and cleaned all the grounds I can find. I have a solid ground from battery to engine block, and one to the radiator support. There's also one from engine block to frame side of the motor mount, plus I added one from battery to the passenger fender.
The headlights are running a separate relay system to take the current away from the switch.


Despite the low voltage the battery always charges well, but it worries me when stuck in traffic with the AC or heater on as the voltage drops so low that everything slows down.
Fortunately I have no battery drain issues - it will sit for months quite happily and fire right up.


Is it possible that the original alternator (78 amp 12SI IIRC) simply can't handle that load at idle, or is there something else I should look at?
__________________
1991 Grand Wagoneer - Hunter Green. All stock. Rebuilt 360 with Melling MTA-1 cam.

1998 Cherokee (XJ) 4.0
1997 Grand Cherokee (ZJ) 4.0
1974 Ford F100 390
Reply With Quote
  #2  
Old 07-20-2021, 11:53 AM
MysticRob MysticRob is offline
327 Rambler
 
Join Date: Nov 26, 2019
Location: Boise, ID
Posts: 624
Quote:
Originally Posted by Brynjminjones
it will drop into the red at idle, and won't go above 12 even at highway speeds.
Sounds like you're using the stock gauge, which is already suspect in my mind, so I'd hook up a DVM and verify at the battery terminals, then go from there.
__________________
--Rob--
1988 Jeep Grand Wagoneer / Baltic Blue & Tan
2008 BMW 535xi Wagon / Deep Sea Blue & Tan

My build thread:
https://ifsja.org/forums/vb/showthread.php?t=189245
My Howell TBI Install How-To:
https://ifsja.org/forums/vb/showthread.php?t=189877
Reply With Quote
  #3  
Old 07-20-2021, 01:07 PM
rang-a-stang's Avatar
rang-a-stang rang-a-stang is offline
Administrator
 
Join Date: Oct 31, 2016
Location: Camarillo, CA
Posts: 4,315
Quote:
Originally Posted by MysticRob
Sounds like you're using the stock gauge, which is already suspect in my mind, so I'd hook up a DVM and verify at the battery terminals, then go from there.
100% agree.
__________________
79 Cherokee Chief (passed CA smog check Sept 2020)
(Cherokee Build Thread)
11 Nissan Pathfinder Silver Edition 4x4
09 Mazdaspeed3 Grand Touring
Reply With Quote
  #4  
Old 07-20-2021, 01:14 PM
letank's Avatar
letank letank is offline
AMC 4 OH! 1
 
Join Date: Jun 03, 2002
Location: San Francisco
Posts: 4,014
clean the contacts on the alternator plug, use deoxit or you favorite contact cleaner


the stock was 85amp for my 85 with AC. Currently running a 78A because the store did not have the 85A when the alternator clunk out during a trip, but my AC is non op... voltage is good with headlights and heater at idle... (ammeter bypass done, running a voltmeter)



Now I jinxed it... the alternator is over 10 years old...
__________________
Michel
74 wag, 349Kmiles on original ticker/trany, except for the rust. Will it make it to the next get together without a rebuilt? Status: needs a new body.
85 Gwag, 226 Kmiles. $250 FSJ test lab since 02, that refuses to give up but still leaks.

See Ouray 2013, Engine bits and Fuel and brake lines, and Body work
Reply With Quote
  #5  
Old 07-20-2021, 01:58 PM
tgreese's Avatar
tgreese tgreese is offline
 
Join Date: May 29, 2003
Location: Medford MA USA
Posts: 11,491
The diodes in the Delco alternators are quite reliable. That said, if one of the six diodes fails open in the rectifier bridge, the output will be reduced by 1/6th. There is also a voltage reference (zener diode likely) in the regulator that is compared to the battery voltage to set the field current and adjust the voltage across the battery. The plug mentioned above includes a sense wire used to monitor the battery voltage. I kinda think a dirty connector there can only boost the output, since the extra resistance will require a higher battery voltage to compensate.

The Delco unitized alternators are easy to take apart and put together. The TSM has a lot of diagnostic info from back when they repaired these units rather than replacing them. You can still buy the service parts. Warm up your multimeter!

Regarding the factory gauge, the chemical potential of a lead-acid battery is 12.6 volts. It should read close to that (maybe a little above) with everything off. The voltage regulator should maintain a constant 13.5 or 14 volts across the battery when the engine is running. This is the sum of the chemical potential and the charge current times the internal resistance of the battery. 16 volts is too much. 12 volts is too little. Any voltage drop due to load should be small compared to those numbers.
__________________
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
ECO Green: '15 FCA Jeep Cherokee KL Trailhawk

Last edited by tgreese : 07-20-2021 at 02:06 PM.
Reply With Quote
  #6  
Old 07-21-2021, 05:45 AM
Brynjminjones's Avatar
Brynjminjones Brynjminjones is offline
258 I6
 
Join Date: Jun 11, 2017
Location: Derby, England
Posts: 440
Thanks everybody for the advice - I don't know why I always forget to verify with a multimeter!

I spent some time going over it yesterday and did discover that the resistance wire to the alternator had snapped at the connector. I repaired the connection but it's made no real difference, except it starts charging immediately after starting the engine now. I don't think it had been snapped long as I checked it about a week ago!


I've spent some time today getting some proper readings for me to report back. I'll attach a picture below to explain roughly how I've been reading the gauge.


Here goes:

Engine off (no load):
-Across battery terminals - 12.35V
-Alternator post to negative battery terminal - 12.35V
-Gauge - 11V

Idle in park (no load):
-Across battery terminals - 14.00V
-Alternator post to negative battery terminal - 14.13V
-Gauge - 13.75V

Idle in gear (no load):
-Across battery terminals - 13.40V
-Alternator post to negative battery terminal - 13.85V
-Gauge - 13V

Idle in park (high load):
-Across battery terminals - 13.70V
-Alternator post to negative battery terminal - 13.95V
-Gauge - 12.5V

Idle in gear (high load):
-Across battery terminals - 12.35V
-Alternator post to negative battery terminal - 12.40V
-Gauge - 11V


It seems from this that things aren't nearly as bad as I thought, but I'm wondering if the idle in gear figure is still a little low.
What would you all expect to see?

Waggy Voltage Gauge by Brynjaminjones, on Flickr
__________________
1991 Grand Wagoneer - Hunter Green. All stock. Rebuilt 360 with Melling MTA-1 cam.

1998 Cherokee (XJ) 4.0
1997 Grand Cherokee (ZJ) 4.0
1974 Ford F100 390

Last edited by Brynjminjones : 07-21-2021 at 08:21 AM.
Reply With Quote
  #7  
Old 07-21-2021, 08:02 AM
MysticRob MysticRob is offline
327 Rambler
 
Join Date: Nov 26, 2019
Location: Boise, ID
Posts: 624
I haven't been able to test mine at idle in gear, but will do today if I can then report back. I know mine in Park at idle was putting out 13.7V without a load though.
__________________
--Rob--
1988 Jeep Grand Wagoneer / Baltic Blue & Tan
2008 BMW 535xi Wagon / Deep Sea Blue & Tan

My build thread:
https://ifsja.org/forums/vb/showthread.php?t=189245
My Howell TBI Install How-To:
https://ifsja.org/forums/vb/showthread.php?t=189877
Reply With Quote
  #8  
Old 07-22-2021, 03:39 PM
letank's Avatar
letank letank is offline
AMC 4 OH! 1
 
Join Date: Jun 03, 2002
Location: San Francisco
Posts: 4,014
Looks good, idle in gear is usually a bit lower because of the added load , the idle is lower , specs to be around 700 rpm... you can bump the idle a bit...
__________________
Michel
74 wag, 349Kmiles on original ticker/trany, except for the rust. Will it make it to the next get together without a rebuilt? Status: needs a new body.
85 Gwag, 226 Kmiles. $250 FSJ test lab since 02, that refuses to give up but still leaks.

See Ouray 2013, Engine bits and Fuel and brake lines, and Body work
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Dual alternator setup Subic Jeep Off-Road FSJ Tech 32 12-22-2010 03:31 AM
Electrical bugaboo / low voltage KensAMC General FSJ Tech 2 11-29-2009 07:52 AM
What should my voltage gauge read at idle? ClovisMan General FSJ Tech 8 10-01-2009 08:00 PM
Alternator and voltage drain ?'s Creech General FSJ Tech 5 07-31-2006 04:32 PM
GM HEI vs. Motorcraft/Duraspark ignitions (very long) Sycho15 General FSJ Tech 11 09-06-2002 07:46 AM


All times are GMT -6. The time now is 05:19 AM.


Powered by vBulletin Version 3.5.4
Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.
corner corner