Bizarre - New Alternator WTF

[bruss01]

1978 Scout 345 727

old alternator went bad - took to local parts place, they tested, confirmed bad on all test criteria. Not too surprising since it's 30+ years old.

Got a new one. Upgraded to 12 si 105 amp. Put it in, everything seems normal except...

When I run anything electrical in the truck (lights etc) the alt gauge dips a little, and dips more with every addition of load. I would expect (patience here, years since I had a vehicle with an alt gauge) for it to dip momentarily, then recover as the regulator compensates for the added load, bringing the dial back to center. That doesn't happen.

I have broken out the digital vom and done some basic testing. With the engine off, voltage between the two battery terminals is 11.99 volts (brand new optima battery). Same between the two battery clamps, same between the + terminal and the engine block. All ok so far. When I started the truck and let it idle, it was reading 11.84 volts... Hmmm it shouldn't be going down in volts, so that's not good. With the engine running, when I touch the major terminal on the alternator (has the big wire coming out) connecting to the - terminal on the battery, I expected to see somewhere between 12 and 14 volts. Instead, it is around 74 volts. wtf?!! That's not even possible, is it? Tried it several times, switched to a different digital vom, same result. Really puzzled now, and a little confused over whether I'm testing the right way or not. Funny thing is the truck seems like it is running just fine like this, except for the alt gauge on the dash indicating a discharge. With that discharge, I expect that if driven enough the battery will eventually go dead.

Anyone have any advice?

 

[Eric VanBuren]

You have an open in the circuit between the alternator and the ammeter. Most likely you fried the terminals in and wire leading to the bhc. Because the alternator's output isn't making it to the battery the alternator can't "see" it's output voltage. Since it is seeing only 12v it keeps cranking up it's output voltage try to get a reading of ~14v on it's sense wire. Usually that peaks in the 18-24 volt range, I've never heard of 74volts.

The bhc is marginal at best for the base 37 amp alt and the stock wire marginal for the optional 61 amp alt. So that 105 amp probably made your stock wring fry. Either that or your original alt was fine and the open was your problem all along. It is slso possible that the old alt died because that open made it run at max voltage for too long.

So other than the alt's voltage peaking at 74 volts that is exactly what I'd expect to happen with an open between the alt and ammeter.

 

[bruss01]

you have an open in the circuit between the alternator and the ammeter. Most likely you fried the terminals in and wire leading to the bhc. Because the alternator's output isn't making it to the battery the alternator can't "see" it's output voltage. Since it is seeing only 12v it keeps cranking up it's output voltage try to get a reading of ~14v on it's sense wire. Usually that peaks in the 18-24 volt range, I've never heard of 74volts.

The bhc is marginal at best for the base 37 amp alt and the stock wire marginal for the optional 61 amp alt. So that 105 amp probably made your stock wring fry. Wither that or your original alt was fine and the open was your problem all along.

Eric,

thanks for helping, or trying to. Some of what you're saying doesn't resonate because I don't see, hear or smell anything frying, and the vehicle has not been operated except for a few minutes in my driveway for testing. Like I said, the old alternator was tested at the local parts house (removed from truck, taken in, and run on their tester) and they say it was bad on all counts.

I've been reading the shop manual and that testing calls for things I don't have (ammmeter) and something I've never even heard of (a "carbon pile", whatever that is). Not much help there so far. I did the testing where you read (with key on) voltage from all 3 alternator terminals to ground. All 3 registered zero, so I can't figure that one out. Maybe my vom is wacky, I don't know.

 

[bruss01]

Side question, what is dielectric grease, and how does it work?

Is it a conductor or an insulator?

 

[Eric VanBuren]

The quick test to verify that you indeed have an open in your main power feed is to run a minimum 8g wire from the b+ stud on the alt to the battery +. Then start the engine and check your voltages again, you should see voltage go up once it is started and stabilize around 14v at all points in the system. The ammeter will still swing to the d side when you turn on loads. That means the power being used in the vehicle is coming from that ammeters stud that connect to the battery via the b+ stud on the starter, verifying that indeed there is an open between the alt and ammeter.


Also the stock ammeter is not up to the task of being used with a 105 amp alternator.

I don't know how dielectric grease "works" other than yes it is an insulator, also prevents corrosion and spark plug boots from getting stuck to the plugs.

 

[bruss01]

I just took some photos of the bulkhead connector. Nothing there looks "fried" or bad. I disconnected everything from the bhc and aside from what looks like black grease inside the multi-spade, everything looks reasonably in shape for a piece of 32 year old automotive equipment. Of course I will do a bit of clean up (does carb or brake cleaner dissolve the plastic plugs?) and make it "nice" as I can, but this doesn't look like a real problem so far.

 

[bruss01]

the quick test to verify that you indeed have an open in your main power feed is to run a minimum 8g wire from the b+ stud on the alt to the battery +. Then start the engine and check your voltages again, you should see voltage go up once it is started and stabilize around 14v at all points in the system. The ammeter will still swing to the d side when you turn on loads. That means the power being used in the vehicle is coming from that ammeters stud that connect to the battery via the b+ stud on the starter, verifying that indeed there is an open between the alt and ammeter.


Also the stock ammeter is not up to the task of being used with a 105 amp alternator.

I don't know how dielectric grease "works" other than yes it is an insulator, also prevents corrosion and spark plug boots from getting stuck to the plugs.

Thank you eric, I will try that using jumper cables, since I figure they have to be at least 8 gauge... Although at times I have been tempted to use a 12 gauge on this thing...

Since I am only running the stock accessories so far, and the battery is fully charged, I don't think there should be anything too severe being demanded of the stock amp gauge as of yet. It should be able to handle sitting in the driveway and occasionally flipping on the stock headlights for a minute of two for testing... I would think.

 

[Eric VanBuren]

Since you have the later style bhc it is not as suspect. To check the wiring since you already have the spade terminals disconnected grab your ohm meter. Make sure the battery ground is disconnected. Take and ohm reading between the 2 terminals on the bhc, the alt b+ stud and the loose terminals, and the battery +cable and the loose terminals. In a perfect world you should have 0 ohms between them. Of course after 30+ years some extra resistance is likely but you still should have continuity.

 

[bruss01]

since you have the later style bhc it is not as suspect. To check the wiring since you already have the spade terminals disconnected grab your ohm meter. Make sure the battery ground is disconnected. Take and ohm reading between the 2 terminals on the bhc, the alt b+ stud and the loose terminals, and the battery +cable and the loose terminals. In a perfect world you should have 0 ohms between them. Of course after 30+ years some extra resistance is likely but you still should have continuity.

Eric,

well, a promising sign! Running a jumper from the alt bat post to the battery + terminal, I get a reading of 13.4-13.5 volts between the two battery terminals with the engine running.

I cleaned up the old terminals at the bhc since I was there, it doesn't seem to have made any difference. Wire brush on a dremmel and some rubbing alcohol, they look shiny now. Not even going to fiddle with the multi-spade at the moment, think it'll be ok. I will do the testing you recommend, if I can get the ohm reading on my vom to work for me. Newfangled gizmos, can't get anything but a 0.0 reading out of it, must be operator error.

 

[bruss01]

since you have the later style bhc it is not as suspect. To check the wiring since you already have the spade terminals disconnected grab your ohm meter. Make sure the battery ground is disconnected. Take and ohm reading between the 2 terminals on the bhc, the alt b+ stud and the loose terminals, and the battery +cable and the loose terminals. In a perfect world you should have 0 ohms between them. Of course after 30+ years some extra resistance is likely but you still should have continuity.

Ok, back... Had to run to radio shack for a new vom.

Alt b+ to bhc - good, zero ohms.
Bhc wire to battery + - good, zero ohms
across two bhc connectors - no connection!!!!


Does this point to a problem inside the cabin, possibly with the alt gauge or it's connections? Could that result in some or all of what we are seeing? The reason I ask, is that I did have the gauges out at one time, and I am now concerned that I May have made a mistake in how the amp gauge is connected. Can we potentially take this variable out of the equation by verifying that they are correct?

 

[JohnnyC]

I have broken out the digital vom and done some basic testing. With the engine off, voltage between the two battery terminals is 11.99 volts (brand new optima battery). Same between the two battery clamps, same between the + terminal and the engine block. All ok so far. When I started the truck and let it idle, it was reading 11.84 volts... Hmmm it shouldn't be going down in volts, so that's not good. With the engine running, when I touch the major terminal on the alternator (has the big wire coming out) connecting to the - terminal on the battery, I expected to see somewhere between 12 and 14 volts. Instead, it is around 74 volts. wtf?!! That's not even possible, is it? Tried it several times, switched to a different digital vom, same result. Really puzzled now, and a little confused over whether I'm testing the right way or not. Funny thing is the truck seems like it is running just fine like this, except for the alt gauge on the dash indicating a discharge. With that discharge, I expect that if driven enough the battery will eventually go dead.

Anyone have any advice?

First things first. Your brand new battery is almost completely discharged. Your reading of 11.99 is way to low to get any accurate readings in your electrical circut. Either place your batt. On a trickle charger or find a temporary batt. That is fully charged.

quote:
originally posted by eric vanburen
since you have the later style bhc it is not as suspect. To check the wiring since you already have the spade terminals disconnected grab your ohm meter. Make sure the battery ground is disconnected. Take and ohm reading between the 2 terminals on the bhc, the alt b+ stud and the loose terminals, and the battery +cable and the loose terminals. In a perfect world you should have 0 ohms between them. Of course after 30+ years some extra resistance is likely but you still should have continuity.

Ok, back... Had to run to radio shack for a new vom.

Alt b+ to bhc - good, zero ohms.
Bhc wire to battery + - good, zero ohms
across two bhc connectors - no connection!!!!


Does this point to a problem inside the cabin, possibly with the alt gauge or it's connections? Could that result in some or all of what we are seeing? The reason I ask, is that I did have the gauges out at one time, and I am now concerned that I May have made a mistake in how the amp gauge is connected. Can we potentially take this variable out of the equation by verifying that they are correct?

Second, if your measuring ohms (which is resistance) you should always have some number readout on you multi meter, if there's continuity. .1-.2 should be max. Even on brand new vehicals there will be some resistance.

If I were you, I would start in making sure all of your terminals are clean, tightened, and corrosion free. Also make sure all of your grounds posi's are connected properly. There might be a possibility that if you bought a remaned alt it could be bad.

Then I would disconnect your alt guage and properly hardwire your remaining guage wires together. Ihc decided that it would be smart to have all of the vehicals power ran through one central location. (not good when its in the dash with no insulation.)
I would then purchase a volt guage and attach it to a switched power source and properly ground it.

The biggest problem with the scouts electrical system is that it is old. Study the wiring diagrams carefully and anytime your connecting or disconnecting electrical components disconnect your ground cable.

 

[bruss01]

first things first. Your brand new battery is almost completely discharged. Your reading of 11.99 is way to low to get any accurate readings in your electrical circut.

That's interesting, but it would be way more interesting if you said what a 12 volt battery is supposed to read at... 12.0, 12.1, etc...

Second, if your measuring ohms (which is resistance) you should always have some number readout on you multi meter, if there's continuity. .1-.2 should be max. Even on brand new vehicals there will be some resistance.

My digital vom is reading volts ok, but for some reason I can't get it to register ohms... Not sure if it's defective or operator error. I ran to radio shack to get a traditional analog vom with the idea "keep it simple, stupid". This analog meter registers zero ohms. Now, whether the reading is actually 0.00000 or 0.00001 or 0.000002, I really couldn't say. Best I can recon by the needle is "zero".

Then I would disconnect your alt guage and properly hardwire your remaining guage wires together.

I appreciate the attempt at helping, but without specifics, it really does not do much good. "do it the way it is supposed to be done" is not helpful when I don't have a readily accessible resource to explain what that is... Whereas "the big wire coming out of the unit, connect that directly to the smaller red wire, taking care not to..." is a lot more helpful.

Johnnyc, I got the impression that you were trying to help, but please realize that the instructions, the way you have phrased them, are of next to no use to me.

 

[Eric VanBuren]

The fact that you have no continuity between the 2 bulkhead terminals means that yes the problem lies in the cab.

The main power feed is 4 distinct sections of 10ga wire, the ammeter and the battery cable.

Battery cable to starter
starter to bhc
bhc to "c" or "+" side of the ammeter
ammeter "c" side to ammeter "d" side, this is the point where the vehicle gets it's power.
Ammeter "d" or "-" side to bhc
bhc to alternator

since you can turn on the lights, start the vehicle ect and when you turn on loads the ammeter moves to the "d" side we know that the system from the battery through the ammeter to it's "d" side. It also means you have it wired correctly up until that point including the connection to the ign sw, lights ect. You seem to indicate that the wire is intact from the alternator to the bhc is good that leaves the wire from the bhc to the ammeter "d" side.

A fully charged "12 volt" battery should show a nominal ~12.6 volts when charged.

 

[ornelles]

Easy way to check your ohm meter is touch the leads together, if it reads zero thats good, when you separate the leads should go to infinite, thats good. Depending on what scale you have it set for it May show zero even if there is some small resistance, point 2 ohms or less need to be read on the smallest scale.

 

[bruss01]

the fact that you have no continuity between the 2 bulkhead terminals means that yes the problem lies in the cab.

The main power feed is 4 distinct sections of 10ga wire, the ammeter and the battery cable.

Battery cable to starter
starter to bhc
bhc to "c" or "+" side of the ammeter
ammeter "c" side to ammeter "d" side, this is the point where the vehicle gets it's power.
Ammeter "d" or "-" side to bhc
bhc to alternator

since you can turn on the lights, start the vehicle ect and when you turn on loads the ammeter moves to the "d" side we know that the system from the battery through the ammeter to it's "d" side. It also means you have it wired correctly up until that point including the connection to the ign sw, lights ect. You seem to indicate that the wire is intact from the alternator to the bhc is good that leaves the wire from the bhc to the ammeter "d" side.

A fully charged "12 volt" battery should show a nominal ~12.6 volts when charged.

Thank you eric! I will try to check that out today. I really appreciate you walking me through it this way.

 

[bruss01]

more findings!!!

I took the dash pannel off and removed the alt/fuel gauge from the dash. There are 3 heavy gauge wires connected to the alt gauge, one on the c side and 2 on the d side. The one on the c side has continuity to the battery spade on the bhc. Neither of the other 2 had any continuity to the remaining spade on the bhc... Until! I started tracing wires and found one of the heavy gauge wires was spliced into and was routed through a toggle switch on the dash!!! Obviously the work of a previous owner!! With a flip of the switch blam we have continuity between one of the d side wires and the alt spade on the bhc. Wtf?!! I cannot imagine why this switch would have been installed, except as a poor man's "anti-theft" system to cause the battery to run down eventually and strand the unwitting thief. Maybe there was a short somewhere draining the battery and this was a "cut-off" to prevent that?

Questions... First, do I have the wires routed properly. Second, what earthly reason could there have been for the in-dash switch? Any speculation on what might happen if I simply remove it from the circuit, routing the wires as original?

Thanks again for all the help guys.

 

[Eric VanBuren]

It is hard to say why the po installed that switch, if it was on the other wire the one from the c side to the bhc then it would work as a kill switch. The other thought is they had a drain issue and though that would cure it.

Either way you need to remove it and do a proper splice to eliminate it. I highly doubt that sw is rated for the current of a stock alternator let alone a 105 amp unit.

The 2 other large gauge wires on the d side are correct. One supplies the fuse box items that are always hot like lights, horn ect. The other supplies the ign switch and every thing that is controlled by it, heater, wipers ect.

 

[bruss01]

Awesome, eric. Appreciate your advice. Since I turned the switch "on" and hooked everything back up, I am seeing the alt gauge reading slightly to the "c" side when running. Once I am satisfied that leaving that switch on does not cause the battery to go dead I will make some time one of these days to remove it and restore the original wiring.

Again, thank you very much for the advice and assistance.

 

[JohnnyC]

Bruss I apologize for not being clearer in my explanation, here are some diagrams that May help out. Also the repair manual is your best friend when it comes to messing with these dinosaurs.

I would remove the switch and resplice them together. The switch has no real purpose other than ruining an expensive battery and alternator.

 

[Eric VanBuren]

You're welcome.

The only drain that sw would stop is a bad diode in the alternator. I'd recommend getting that switch out of the circuit sooner rather than later. The common toggle switches you get at the local parts stores are usually rated at 20 amps or much less. So you May fry it because once you are running and the alternator is charging all the power used by the vehicle as well as any charging of the battery will pass through that switch.