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Old 10-07-2019, 08:15 PM   #1
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Dash Volt Meter Reading ~10V; BATS Read 12V

I have a 1998 Discovery (Cummins and Allison 6 speed) with ~16K miles; 6K put on this year.

The voltmeter (analog/needle) reading on dash went quickly from normal mid range to lower range just above red area during a trip yesterday. I estimate 10V and noticeably lower then normal. I drove it 250 miles with a few stops and no other indication of poor voltage and the rig operated and restarted several times with no issue or abnormal conditions. Even after the 4 hours of driving it starts fine. There was no rain when the reading dropped nor has the rig seen any rain for months on several major trips.

The Voltmeter reading on the dash fell into red zone ~8V with headlight lights on but returned to the low ~10V with lights off. The flashers and wipers caused the needle to move slightly, but more noticeably they caused the speedometer to fluctuate slightly during the cycles. The speedometer reading increased with full lights on. The speed read normal with headlights off according to GPS.

At home, I check the batteries after overnight, several restarts, cycled the house and main batteries. No change. The batteries read fine 12.2 with a external volt meter and also indicate full green at the panel inside. the batteries are less then 2 years old.

I assume the voltmeter is not taking a direct battery reading. Does anyone know the wiring for the voltmeter or where the signal to the meter comes from directly? I am looking for a potential ground issue from that device, fuse, the device itself or line from battery to the device. I was not sure which wire from the battery was actually the feed for the meter reading. I am also open to anyone who has the answer or experienced a similar issue! Thank you in advance. Tom
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Old 10-07-2019, 08:55 PM   #2
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With the engine running what voltage does your handheld voltmeter read at the battery terminals?
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Old 10-07-2019, 09:20 PM   #3
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12.2 volts is a discharged battery. They should show 12.6 volts, engine off with no shore power.

First test is, as mentioned, is battery voltage at the batteries with the engine running. It should be 13.+ volts.

If not, see if you can get to the + cable on the back of the alternator and see what you have there. Again, it should be over 13 volts, with the engine running.
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Old 10-08-2019, 06:23 AM   #4
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Twinboat and Arch H, Thanks. I'll try the battery volts running tonight and let you know.
I expected the voltmeter to read current battery voltage with key on/engine off like normal. Usually goes from 0 to mid range on meter with key on then, right?
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Old 10-08-2019, 06:47 AM   #5
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What we are trying to determine is whether or not the alternator is providing it's normal power to the batteries when it is running.

Have you removed the battery cables and cleaned the terminals?
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Old 10-08-2019, 07:57 PM   #6
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Went out tonight and the batteries were at 11.6 and no noticeable change when the engine was running.

The alternator was also outputting at 11.6V. With the Generator running the volts to the batteries was over 12. So the Gen would charge them if I got into a jam. I noticed a burnt electric smell also at the alternator area which I actually smelt at one of my stops.

So looks like a new alternator is required.

Anyone have a recommendation for the replacement? Maybe an upgrade or size recommendation? I'll search the sponsor vendors also.
Thank you for the great response and support. I'll confirm the fix.
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Old 10-08-2019, 08:11 PM   #7
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A lot of folks go with the Delco 28SI alternators.

Here is a link to a supplier who can help if you give hime your alternator model #.

https://www.qualitypowerauto.com/ite...lternators.htm
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Old 10-08-2019, 09:22 PM   #8
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Looks like you've found the problem, good for you!
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Old 10-15-2019, 03:17 PM   #9
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Alternator wiring

Well, I pulled the old alternator off and one of the leads going to the spade connector was corroded and broke off. (This was the wire from the spade to the hot lug. The other spade wire goes into the harness via a freightliner PN 28022-000 pigtail.

What I also discovered was a broken white wire that goes into the harness (same point as the pigtail mentioned above, but I do not know if it was actually connected to the alternator or anything. The connector end is gone off the white wire. Does anyone know if this wire should be connected. There is a small lug next to the main lug out of the alternator. Maybe it goes there.

I need a little wiring diagram if anyone can detail the wires on back of the alternator. This is a Delco alternator (1998 Diesel Fleetwood Discovery/Freightlinerw/Cummins). I have a few pics and can see the large ground connected, the large red and one spade wire connection.
Thanks.
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Old 10-20-2019, 01:21 PM   #10
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Issue Resolved; I did need a new alternator.

I did purchase the Quality Power alternator (150 amps vs the 130 Amp Delco). It has an extra small terminal, but is not used. It also has a jumper in the spade terminals that needs to be removed. The bushing used by QP is ever so slightly larger than the Delco and I had to grind it (I tried like heck to press it out) in order to get the alternator on the mount. I never would have been able to install this alternator if I was not at home with a grinder!

For wiring, the number one spade goes to the harness via a 4-6 inch connector; and the number 2 spade terminal goes to the Battery lug on the alternator with the main power wire. The spades are numbered. None of the small terminals on the alternator are used. The harness from the alternator spade connections (06-28022-000) is no longer manufactured by Freightliner, so I purchased a simple Delco connector from NAPA, off the shelf, and rewired it with the old wire for the harness and a new wire for the BAT terminal connection. I am not certain where the diode is to prevent voltage feedback to the ignition switch, the wiring diagram shows one, but the Freightliner harness diagram did not.

For folks reading my post, the white wire from the harness is for the oil pressure (I broke this wire when I removed the alternator and had to crimp on a new connector). So disconnect the oil pressure switch to prevent brakeage if removing the alternator. I had to cut the main wire bolt off the alternator due to heavy corrosion and a twisting lug. Otherwise I would never had this issue

I did have issues with flashing dash lights, gauges going backwards on shut down, fuel pump running when main batteries and aux turned on, and not being able to start without shore power and the AUX Bats on. These issues are contributed to a bad main ground connection from the main batteries. Simple fix with cleaning and a tightening! Thanks to the group for the support, hope this may help others in the future.
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