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Alternator problem or voltmeter problem?

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BFC Avatar
BFC Ben Clark
Allegan, Michigan, USA   USA
1980 MG MGB "Kilr B"
So... I finally got my ‘80 B roadworthy after sitting for about 8 years with the PO and 2 while I tinkered with everything. As a part of getting it roadworthy I replaced the stock Lucas alternator with a higher capacity GM unit and installed a Smith’s voltmeter. I also replaced/rerouted a lot of wiring that was bad or got put on relays.

Except... when I start up the car when cold the voltmeter is reading about 13v, rising to 14.5v once I get moving. When I stop the car and go to restart it when hot, the voltmeter goes up to about 12.5v - 13v and slowly falls to 12 or even 11.5v (seems to depend somewhat on what electrical gizmos are running -lights, radio, radiator fan) over the next 5-10 minutes.

At first, I assumed that the voltage issue was because it was 94 degrees and I was restarting the engine with the radiator fan already running and somehow affecting the alternator. Last night, however, it was cool enough that the fan didn’t kick on, and I went to restart the car and I had the same issue with the voltage dropping, indicating the alternator wasn’t charging.

The voltmeter is wired to a green circuit behind the dash, not directly to the alternator. I haven’t had any problems starting the car though, so maybe the alternator is charging and I am just not getting a good reading?

I’m good with electrical issues, but this one has me baffled. Do I have a bad wire to/from the alternator, a bad alternator, or am I being misled by my voltmeter?



I don't know. It's always smoked like that/made that sound/done that.

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HardlySlept Avatar
HardlySlept Chris Gatti
New Berlin, New York, USA   USA
1980 MG MGB "Blythe"
Any chance the belt is slipping? Is your idle speed very low?

BFC Avatar
BFC Ben Clark
Allegan, Michigan, USA   USA
1980 MG MGB "Kilr B"
Good question! The belt seems to be adequately tight and idle is about 950-1200 rpm. I haven’t heard any squeals or seen other signs that the belt is slipping.



I don't know. It's always smoked like that/made that sound/done that.

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Ex-Calif Avatar
Ex-Calif Gold Member Dan D
Cincinnati, Ohio, USA   USA
1968 MG MGB GT "Bart - Yellow And Naughty"
1977 MG MGB "Red Betty"
2012 Jeep Liberty "Tank"
2014 Hyundai Accent "Skate"
Check the alternator output to ground when the reading is low on the dash gauge. If there is a disparity it's likely high resistance somewhere to the gauge.

If the gauge is piggybacked randomly into the green circuits you definitely can see lower system voltage although I never see less than like 13.1 even with all the load on including wipers, heater fans etc.

I have the stock 35amp alternator.



The goal - Reliable summer driver interspersed with mechanical tinkering...
Motto - "Driving fifty in the twisties..."
On Mods - It's your damn car - Do what you want. Haters gonna hate...
On SUVs - Drive your B like a soccer mom is texting her friends about how she wants to kill you...
Red Betty - http://www.mgexp.com/registry/GHN5UH418165
Bart - http://www.mgexp.com/registry/GHD4U146898G

BFC Avatar
BFC Ben Clark
Allegan, Michigan, USA   USA
1980 MG MGB "Kilr B"
The GM alternator doesn’t have a discreet ground out lead (IIRC), but I can check voltage output off the positive. I should have done this before posting. ::slaps forehead::

The parasitic draw on the voltmeter is entirely possible. The entire wiring harness was cut in two and spliced with cheap butt connectors behind the glovebox by a DPO. The dumb current owner has done his best to avoid similarly poor choices, but there’s a chance I did something foolish. smiling smiley



I don't know. It's always smoked like that/made that sound/done that.

HardlySlept Avatar
HardlySlept Chris Gatti
New Berlin, New York, USA   USA
1980 MG MGB "Blythe"
I just went out to my '80 MGB and measured voltage in a few ways. I connected a decent DVM at the fuse box terminal, brown wire, that is 'hot' all the time.

Engine off: 12.4 Volts
Cranking: Drops to 10.0
Fast idle (1300 RPM) during warm up: 14.4
Fast idle (1300 RPM) with lights, wipers and heater blower on: 13.8
Normal idle (900 RPM) no accessories running: 13.9
Normal idle (900 RPM) with lights, wipers and heater blower on: 12.7

Stock alternator with stock pulley.

TonyV Avatar
TonyV Tony V
Rutherford, NJ, USA   USA
1974 MG MGB
Have you confirmed the output of your new alternator with a multimeter at the alternator? Using a cheap cigarette lighter style voltmeter could help verify the readings at your newly installed Smith’s voltmeter. When I installed the GM CS130 alternator on my '74, it was recommended that I run a second, heavy gauge wire from the new alternator to the starter to help carry the increased output available from the upgraded unit. MORE HERE I'm not implying that this is your issue but it couldn't hurt.



Edited 1 time(s). Last edit at 2017-10-09 10:01 AM by TonyV.

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Ex-Calif Avatar
Ex-Calif Gold Member Dan D
Cincinnati, Ohio, USA   USA
1968 MG MGB GT "Bart - Yellow And Naughty"
1977 MG MGB "Red Betty"
2012 Jeep Liberty "Tank"
2014 Hyundai Accent "Skate"
Sorry - I meant check the alternator output (with the red lead) to ground (with the black lead) - but you figured that.

Yeah - There are so many dirty connections in the averagely maintained MG that 12-13 volt readings are common.

One of the reasons to have the VM is to identify stuff like this.

Recently helped a guy with some electrical. I checked green circuit voltage behind the console and it was like 10.6-11V at rest. I cleaned a bunch of bullet connectors and the fuse box connectors and when I was done we were seeing 12.4.

He became a believer and I left him the task of cleaning a bunch more bullet connectors. He probably won't. He'll probably just call me when the headlight switch burns out from too much resistance. - LOL...



The goal - Reliable summer driver interspersed with mechanical tinkering...
Motto - "Driving fifty in the twisties..."
On Mods - It's your damn car - Do what you want. Haters gonna hate...
On SUVs - Drive your B like a soccer mom is texting her friends about how she wants to kill you...
Red Betty - http://www.mgexp.com/registry/GHN5UH418165
Bart - http://www.mgexp.com/registry/GHD4U146898G

mgblestyle Avatar
mgblestyle Philip Shave
Olympia, Washington, USA   USA
Ben, you've already had some good advice. I installed the AC Delco 96 amp alternator in my 1980. Voltage will often be 14.5 at start up, dropping to 13.8 after a few minutes of running. This alternator should make full voltage at 1000 rpm idle. Should never show 11.5 while the engine is running. As others have noted, check voltage at the hot terminal (heavy gauge brown wire) at the back of the alternator--from there to a good ground. I added a 10 gauge wire to the starter just as insurance. Might also check grounds at battery and engine and positive connection at battery. Phil

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BFC Avatar
BFC Ben Clark
Allegan, Michigan, USA   USA
1980 MG MGB "Kilr B"
Chris-
Thank you so much for the reference readings! Extremely useful.

Tony-
I’ll have to use a multimeter because I ...um... sort of replaced the cigarette lighter with a USB charger. Good idea to check a purple circuit rather than the green one. Less junk attached to that circuit.

Dan-
Bullet connectors are Satan’s children. I loathe them. The only known British electrical weirdness I have left is from the bullet connector to the right front bright headlight. I feel like I should leave it in so American car guys have something to point at. Spade connectors aren’t perfect, but at least they rust together rather than vibrate apart. I’ll check the voltage off the alternator when I get home this evening.



I don't know. It's always smoked like that/made that sound/done that.

BFC Avatar
BFC Ben Clark
Allegan, Michigan, USA   USA
1980 MG MGB "Kilr B"
Phil- that is the range I expected - 13.5- 14.5v wi he idle at about 1000 rpm.

Two of you have suggested looking at or augmenting the wiring from the starter to the alternator, so I'll look at that when I take the voltage reading. I had to replace the starter and a hunk of associated wiring, so there's a chance that caused the issue. I'd prefer the issue be with wiring I can fix easily than inside an alternator that I cannot.

(Side note: If I knew now what I didn't know then, I would have replaced the wiring harness straight away rather than patch and bypass the damaged parts)



I don't know. It's always smoked like that/made that sound/done that.

BFC Avatar
BFC Ben Clark
Allegan, Michigan, USA   USA
1980 MG MGB "Kilr B"
I finally got a chance to run around town enough to test the alternator.

At idle, right after starting cold: 14 volts
While cruising, before first stop: 14-14.5 volts

At idle, after the engine warmed up (fan running when I went to start it up): 13.2 - 13.5 volts
While cruising, after engine warmed up: 14 volts - dipped a bit when I turned on the headlights

So... I couldn't replicate the problem, but its cold enough now that the fan shuts off once I get moving.

I checked the wiring at the battery and at the alternator and starter. Everything appears in order.

I think the low voltage I was seeing on the voltmeter is a combination of two things:
1) current draw from the fan, other gizmos
2) bad connection somewhere on my green (fused ignition) circuit that powers the voltmeter.

I'll take a more thorough look at the wiring over the winter. Right now, there's only a few weeks left to drive it!



I don't know. It's always smoked like that/made that sound/done that.

ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
Ben,
Someone might have the spec current draw for each fan, I would suggest checking each fan's current draw, it may be that one (or both) of the radiator fans are failing, in that process they are drawing more than the spec current which could cause other problems.

BFC Avatar
BFC Ben Clark
Allegan, Michigan, USA   USA
1980 MG MGB "Kilr B"
In reply to # 3608801 by ClayJ Ben,
Someone might have the spec current draw for each fan, I would suggest checking each fan's current draw, it may be that one (or both) of the radiator fans are failing, in that process they are drawing more than the spec current which could cause other problems.

I replaced the stock pushers with a single puller fan about 2 months ago, as one of the last things I did to get the car roadworthy again. If I recall, the stock fans draw about 6-9 amps on startup, and drop down a bit once they're at speed. I think the rating on the new fan is around 8-10 amps. It does an excellent job, but I may want to confirm that it isn't drawing more than that.



I don't know. It's always smoked like that/made that sound/done that.

ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
Sounds like total amperage is close to the same. A lot of folks add a relay for the fan(s) and pickup the current load from the starter primary where the heavy wiring from the battery connects. Just a thought.

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