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GameboyRMH GRM+ Memberand MegaDork
3/12/15 12:28 p.m.

Just went to get a battery checked and get the warranty certificate and found that I was getting 12.9v across the battery terminals while running even at high revs (with electric radiator fan spinning)...so much for the warranty.

The battery discharge indicator is turning off as the engine starts up. Could charging to just 12.9v damage the battery? I dropped in a new one on Saturday night so I don't want to risk this one.

Fixing this problem by repairing or replacing the alternator will probably cost me $75~$125, but I already have everything I need to swap in a high-output Mazda alternator except a custom mount. If it's safe to drive like this for a few more days I could get that mount made for under $50 and do the swap, saving me the expense of swapping in another Sammy alternator (which I'm just going to replace later anyway). And I need to save all that I can after replacing U-joints on two different driveshafts (which I think now need balancing), a clutch cable and throwout bearing, and then the battery all in the last couple of weeks.

GameboyRMH GRM+ Memberand MegaDork
3/12/15 2:33 p.m.

Gotta drive it again pretty soon...nobody knows? I'd guess anything over the resting voltage of a lead-acid battery, 12.6v, should be sustainable given sufficient charge time.

itsarebuild HalfDork
3/12/15 2:39 p.m.

That would be my guess too

alfadriver GRM+ Memberand UltimaDork
3/12/15 2:42 p.m.

Turn on a/c (which should turn on the radiator fan on a cold engine), crank up the fan, and turn on the lights- what's the voltage then?

If it's still that low- the regulator for the voltage seems bad.

If it drops to under 12V, it may be worse.

Unless it's a smart charging system.

GameboyRMH GRM+ Memberand MegaDork
3/12/15 2:43 p.m.

AC? Smart Charging? Factory electric fan? This is a Samurai we're talking about here

I don't have a voltmeter here but I'll turn on everything and see if the discharge light comes on.

psychic_mechanic Dork
3/12/15 2:52 p.m.

If you don't have any long trips planned you should be able to get by if you put a trickle charger on it at night. I did that with an old honda I had for a while with a questionable alternator.

I'd check your battery at rest with the voltmeter too, I've seen a couple that had bad cells from the factory. If you had a bad cell it could be dropping your voltage to 12ish instead of 14ish.

GameboyRMH GRM+ Memberand MegaDork
3/12/15 2:58 p.m.

I've had a bad cell before and the vehicle was unstartable, so I'm pretty sure that's not what's happened.

Edit: Here's when that happened, and the last time I changed the battery:


1988RedT2 PowerDork
3/12/15 3:17 p.m.

12.9 is too low. At the lower end of the charge rate of 2.3 volts per cell, you'd like it to be 13.8 volts.


kb58 Dork
3/12/15 5:03 p.m.

Agree with post above. That low, it's likely that the car is mostly or entirely running on the alternator and the battery isn't charging.

dean1484 GRM+ Memberand MegaDork
3/12/15 5:22 p.m.
kb58 wrote: Agree with post above. That low, it's likely that the car is mostly or entirely running on the alternator and the battery isn't charging.

This is not entirely true. Alternator amperage output is is also important. Why you get alternators with higher amperage output when you are adding electrical demand to a system.

dean1484 GRM+ Memberand MegaDork
3/12/15 5:27 p.m.

Very likley the voltage regulator in the alternator is going bad. I would see if there is a local place that rebuilds alternators and ask if they would sell you a new voltage regulator or possibly the local pick and pull has an alternator you can disassemble and just put has the voltage regulator leaving the yrs of the unit in the yard. Probibly get it for a couple dollars.

gearheadE30 Reader
3/13/15 7:32 a.m.

You should be fine for a few days. Batteries don't charge well when the potential difference between the alternator and the battery is that small, but it's a Samurai, so you don't have a whole lot of parasitic loss to deal with anyway. A modern car, it wouldn't surprise me if a day or two of sitting would result in slow cranking and difficulty starting.

GameboyRMH GRM+ Memberand MegaDork
3/13/15 7:41 a.m.

A new voltage regulator might be a little cheaper than the bracket, but it costs downtime that I don't have. The bracket's being made now, I should get it tomorrow, then I can install the new alternator on Sunday morning.

I'm pretty sure the battery is charging, because for some reason these days the engine now needs a helluva lotta cranking to start first thing in the morning...it would be long dead if it wasn't charging back up from that.

Edit: BTW I did the "turn on everything" test and the discharge light stayed off.

iceracer PowerDork
3/13/15 9:28 a.m.

The "discharge light" merely tells you that the alternator is connected to the system. It doesn't indicate if the alternator is charging at full voltage.

So even if the light is off, if the alternator is not charging 14.3+-V the battery can still be discharging.

Streetwiseguy PowerDork
3/13/15 12:03 p.m.

Theres an awful lot of European cars built in the 80's that don't see much more than 13.5 volts. Japanese, I`d say 12.9 is low. Could be regulator or corroded loose connection, or an inacurate voltmeter.

dean1484 GRM+ Memberand MegaDork
3/13/15 12:08 p.m.

I would also go after all the grounds. The low voltage may be contributing to the longer than normal starting. Especially if the voltage drop while starting is consistent. Could be getting a weaker spark.

GameboyRMH GRM+ Memberand MegaDork
4/6/15 2:02 p.m.

UPDATE: I'm live-posting now from the shop where I just finished swapping the alternator. Yes the old one lasted all this time, and it had bad bearings too.

Now everything is great with the new install except that I'm now getting nearly 15v between the battery terminals when running. Apparently this is a common problem when doing alternator swaps on a Samurai. From what I'm reading, it's because the dash indicator circuit doesn't have enough resistance. Anyone know how much I need to add and where? Searching around I've seen people suggest anywhere from 100ohm to 10kohm. I happen to have an assortment of resistors with me that I bought for use with the car's data logger. HALP!

spitfirebill PowerDork
4/6/15 2:14 p.m.

You sure are hard to please Goldilocks. Too high, too low...

I got nothing else.

pres589 UberDork
4/6/15 2:28 p.m.

The control circuit for the regulator is done via the dash light wiring? Doesn't sound right. Would need a wiring diagram for the stock vehicle to get a better idea. What is the new alternator normally installed in?

GameboyRMH GRM+ Memberand MegaDork
4/6/15 2:30 p.m.

This is a Mazda BJ alternator usually used in a Protege.

Dr. Hess
Dr. Hess MegaDork
4/6/15 2:37 p.m.

I would put it on a charger overnight and drive it during the day for a few days until you have the bracket. I mean, you live on an island. How far can you go before you run off the end?

pres589 UberDork
4/6/15 2:41 p.m.

So on a VFR, when the monitor wire to the Regulator/Rectifier is cut, the R/R thinks that there is no voltage getting to the stator windings, and will run the stator excitation to full. You get 15+VDC out of the rectifier and eventually a cooked battery and maybe some melted connectors. It sounds like the new alt, and here I am assuming that this is an internally regulated alt like a modern GM product and not some externally regulated alt because I don't know the cars in question, isn't getting proper voltage input to the monitor circuit in the regulator.

So after many assumptions I think that is the problem. When I get home tonight I'll try to dig up a wiring diagram for each car (somehow?) and give it a think.

GameboyRMH GRM+ Memberand MegaDork
4/6/15 2:46 p.m.

Yep it's a new and internally regulated alternator. I'm getting around 14.95v between the terminals.

pres589 UberDork
4/6/15 2:53 p.m.

If the alt is internally regulated then I'm not sure what you can do to fix the thing. Again, wiring diagram would help, as I'm curious what role the dash light plays in this (if any).

Dr. Hess
Dr. Hess MegaDork
4/6/15 2:55 p.m.

Wait. You're getting 15V out of the alternator, but 12.9 at the battery? Where's the rest going? You need some time with a volt meter. Measure across the battery. Measure ground to batt +. Measure across the output of the alternator. Measure alternator output to ground.

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