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Old 06-17-2020, 02:55 PM   #1
mschultz373
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Default Alternator and fuses weirdness

i am (was?) planning on a 10hr drive tomorrow, so I was checking out my 85 240 to make sure everything was a go. fluids, etc. this was only a few hours ago, and my battery cold was around ~12.4V, idling was ~14.4V. so everything was checking out.

i wanted to get a quart of oil to be safe, so I drove the autozone - all good. get back in my car, start driving to go get the tires filled, and i notice my transmission overdrive is 'on' - which, i had not chosen to engage - and the battery light in dash is on. none of my running/brake/hazard lights were working. i turn off overdrive and drive home.

my battery voltage with car running was ~12.2V. so alternator was not charging. battery cold was ~12.5V. checked the alt and all the wiring to it is good. so I check the fuses, and fuse #9 and 13 are blown:

9 = hazard warning lights
13 = instruments, turn signals, fuel injection system relay

so i replace the fuses. currently, everything appears to be working normally and looks normal on the dash. but my battery is ~12.4V cold, and 13.77V running. so my suspicion is - my alternator is going out?

But my lingering question is - why did fuse 9 and 13 blow? and what would've caused all this to happen, and why would my alternator suddenly be crapping out? how could it go, in a matter of hours, from a totally fine reading to a below-spec reading?
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Old 06-17-2020, 04:01 PM   #2
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alright it's my alternator crapping out. battery is ~13.3V car idling; revving with lights + radio + ac full blast on, it gets up to 13.4V.

battery voltage is present at terminal B+. but i am getting nothing with ignition on at terminal D, the smaller spade terminal adjacent to the main battery terminal on the alt. i am getting battery voltage at the terminal on the main engine harness from the battery, however. does this indicate a fault with the brushes inside the alternator, or?

any thoughts on if I can make the 10hr drive home? i don't really wanna get an autozone alternator to replace the original. if i can make it home, i'll be able to order a good quality one.

Last edited by mschultz373; 06-17-2020 at 05:07 PM..
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Old 06-17-2020, 05:23 PM   #3
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Fuse 13 supplies a bunch of stuff, including the instrument cluster. If fuse 13 is bad, then the D+ connection to the alternator doesn't get voltage which means the field coil in the alternator isn't getting voltage. Without fuse 13, the alternator won't charge.

Seeing 13.4V on the battery means the alternator is at least partially working (it could be fine depending on temperature and if the battery's charged).

IIRC, fuse 13 also supplies the voltage to the rear passenger's seatbelt warning light in the center console. If this light falls down, it can short out and blow fuse 13.

Starting with a good fuse 13, I'd try wiggling all the engine compartment wires and see if fuse 13 survives. Have you replaced the original, potentially falling apart, engine compartment wiring?
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Old 06-17-2020, 05:29 PM   #4
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Fuse 13 is now good and working. the D+ terminal problem persists. engine harness is providing battery voltage but the nuts on the alternator read <2V varying.

i am getting 13.4V on alternator with a good battery (12.4V static) and ambient temperature in the mid 70s.

I've replaced main engine wiring harness but not the engine compartment wiring at all. i just recently hard-wired my tail lights, though, so maybe a turn signal light wire shorted out and caused fuse 13 to blow.

will my alternator make a long trip? best guesses on that?
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Old 06-17-2020, 05:31 PM   #5
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I departed from 240 ownership in the previous century, so not particularly up on the details. The D+ / 61 terminal on most alternators is connected to the charge light to do double duty. When the alternator fails to generate voltage internally current flows from the battery through the charge light and through the alternator connection causing the light to come on telling you the alternator is dead. The connection has a second function. Most alternators require a bootstrap current to get them started. That bootstrap current flows from the battery through the charge light and powers up the alternator field winding with a few milliamps of current which is just enough for the alternator to create a little voltage which the internal regulator can then use to supply more field current which boosts the output voltage which supplies more field current which ..... until you reach 14 volts or so.

With the car not running if you switch on the ignition to run is the charge light lit up? If so, that means that the alternator should be getting its bootstrap current. With the ignition switch in run; but, engine off pull the wire off of the D+ spade connector. The alternator charge light should go out and you should measure 12 v on the wire. If you do, that is an indication that your alternator is dying. The fact that you are getting 13.4 volts indicates that the alternator is probably getting its bootstrap current. Either your regulator is on the way out or the brushes are failing. When the brushes fail the effective resistance of the field circuit goes up and the regulator is not able to supply enough field current to generate 14 volts which would be typical.

So, your alternator is sort of working. As long as the voltage remains above 12.4 volts you are not depleting the battery. How long the alternator is going to continue to sort of work is a risk assessment only you can make.

How long you can go on a fully charged battery with a non functioning alternator depends on what the electrical load is like. No radio, no heater / air conditioning, no running the headlights or long periods of stop and go traffic with lots of brake lights, no stopping and restarting of the motor and starting with a fully charged battery that is in good condition - you could probably do it. Battery not so good, maybe not.

Last edited by 142 guy; 06-17-2020 at 05:41 PM..
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Old 06-17-2020, 05:41 PM   #6
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Quote:
Originally Posted by 142 guy View Post
With the car not running if you switch on the ignition to run is the charge light lit up? If so, that means that the alternator should be getting its bootstrap current. With the ignition switch in run; but, engine off pull the wire off of the D+ spade connector. The alternator charge light should go out and you should measure 12 v on the wire. If you do, that is an indication that your alternator is dying / dead.
this is exactly what I am getting. battery voltage is present from engine harness terminal that would drop onto D+ spade.

charge light illuminates with ignition on. removing the D+ terminal from alt yields the oil light only.

thanks for the updated info on the volt reg/brushes. it sounds like an entirely new alt is NOT necessary as of yet, given current info.
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Old 06-17-2020, 05:41 PM   #7
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In a bind, you can just "slap" 12V to the exciter wire and the generator will wake up and start charging. You only need to do it ONCE per drive cycle. So if for some reason, you are on that road trip and you lose that 12V to the alternator, just do a "manual excite" with a jumper wire from your favorite 12v source, such as the B+ wire on the generator.
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Old 06-17-2020, 05:43 PM   #8
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Your fuses are blowing from a short to ground somewhere.
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Old 06-17-2020, 05:47 PM   #9
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In a bind, you can just "slap" 12V to the exciter wire and the generator will wake up and start charging. You only need to do it ONCE per drive cycle. So if for some reason, you are on that road trip and you lose that 12V to the alternator, just do a "manual excite" with a jumper wire from your favorite 12v source, such as the B+ wire on the generator.
so basically jump from B+ (large alt plug with constant bat voltage) to D+ (spade connector)? Apologies for my confusion here....

for the sake of condensing the info, with ignition ON
B+ = 12.4V
D+ = ~2V
terminal 61 (2 screws that holds plastic plate in place - that's the volt regulator?) = 5.2V


Quote:
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Your fuses are blowing from a short to ground somewhere.
right, I think it was from the turn signal hard-wiring grounding out against the metal chasis in back. i wrapped the spade terminals in electrical tape to mitigate this risk.

Last edited by mschultz373; 06-17-2020 at 05:56 PM..
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Old 06-17-2020, 07:40 PM   #10
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Quote:
Originally Posted by mschultz373 View Post
so basically jump from B+ (large alt plug with constant bat voltage) to D+ (spade connector)? Apologies for my confusion here....

for the sake of condensing the info, with ignition ON
B+ = 12.4V
D+ = ~2V
terminal 61 (2 screws that holds plastic plate in place - that's the volt regulator?) = 5.2V
Huh??? What sort of alternator do you have? The standard small 55Amp Bosch alternators have a B+ stud and a recessed D+ male spade terminal. Terminal 61 is another name for the D+ spade terminal. The 2 machine screws that hold the voltage regulator in place both go into the alternator housing, which is grounded through a thick wire to the engine block.

If you battery voltage with ignition on, but not running is 12.4V, then when running, any voltage over 12.4V means the alternator is charging the battery. If the voltage drops below 12.4V when running with all the accessories turned on, then your alternator isn't keeping up.

When you went to the store and it misbehaved, was the tail light wiring shorting out? Have you had blown fuses, or dash warning lights coming on, after taping off the tail light wire?
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Old 06-17-2020, 07:40 PM   #11
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so my main question now is...

-should i replace alternator as a whole ?

or
- replace just the brushes or volt regulator?

any way for me to confirm the precise fault?
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Old 06-17-2020, 08:03 PM   #12
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Quote:
Originally Posted by mschultz373 View Post
so my main question now is...

-should i replace alternator as a whole ?

or
- replace just the brushes or volt regulator?

any way for me to confirm the precise fault?
Worn brushes can be confirmed by visual inspection.

If your alternator has the internal regulator, it looks like the brushes are integral to the regulator assembly. Complete regulator / brush assemblies are available from Rock Auto for about $30- $35. I don't know how much your local vendors are. A rebuilt alternator is going for 3 - 6 times that price.

If inspection shows that your brushes are worn to a nub you may have damaged the slip rings (which the brushes ride on) on the alternator. You pretty much need to disassemble the alternator to confirm that. In that case, new brushes/ regulator may provide no fix or only a temporary fix. A replacement alternator is required. If the slip rings are OK, then the brush / regulator replacement may do the trick. The rest of the alternator is pretty much reliable as rocks. The diodes can fail; but that is exceedingly rare.

What to do. If you had the time you could pull apart the alternator and make an informed decision based upon testing and inspection as per the service manual. If you are short on time and risk adverse, then you suck it up and install a new alternator. If you are not so risk adverse then you could try a new regulator / brush assembly and see if that fixes it. If your retirement plan consists of buying lottery tickets then just assume that your sort of working alternator will hang on for the 10 hours it take to get you wherever you need to go.
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Old 06-17-2020, 08:16 PM   #13
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so basically jump from B+ (large alt plug with constant bat voltage) to D+ (spade connector)? Apologies for my confusion here....

for the sake of condensing the info, with ignition ON
B+ = 12.4V
D+ = ~2V
terminal 61 (2 screws that holds plastic plate in place - that's the volt regulator?) = 5.2V
I am with Bobxyz on this.

D+ is terminal 61. With the ignition switch on and you have D+ at 2 volts (which is roughly the forward bias voltage of the internal diode) and your charge light is lit, then you are successfully applying field flashing to the alternator.

I don't know what that 5.2 volts is. If as Bob says the plastic plate is your voltage regulator assembly those screws should be at 0 volts. If they are not, then that will seriously mess up the operation of the voltage regulator. The regulator measures everything relative to ground potential. Check the grounding of the alternator. If the alternator requires a separate ground cable to the block as Bob indicates, is that cable securely connected. The alternator housing on my 140 pivots on a large lug that is integral to the engine block so does not require a separate ground conection.
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Old 06-17-2020, 09:57 PM   #14
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D+ is terminal 61. With the ignition switch on and you have D+ at 2 volts (which is roughly the forward bias voltage of the internal diode) and your charge light is lit, then you are successfully applying field flashing to the alternator.
I may have just measured some useless thing then... apologies for my confusion.

but in terms of applying field flashing - that means my alternator is still partially working for the time being? fundamentally, if it is NOT below the battery voltage static - i.e without car on - then the alternator is doing its job?

but at 13.4V with the car idling, the bottom line is that a new alternator is basically in my future? regardless of the brushes or regulator? shouldn't the voltage, ideally, be 14V at least?
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Old 06-17-2020, 10:09 PM   #15
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any way for me to confirm the precise fault?
Years of experience, a meter, and a test lamp.

No, the voltage does not have to be 14V. Modern cars with duty cycle regulated generators show 12.6V sometimes on the charging system. 14+ sometimes.

And you asked about the "manual stim" of the generator. Yes, jump from B+ to the exciter spade. This is assuming something happened to you on the road and you needed to get home with a blown fuse for the cluster.

To find what is blowing your fuses you will need a WIRING DIAGRAM. Then disconnect items on the circuit one by one until the fuse stops blowing and then you have found the branch of that circuit that has failed. I dont know what is on your circuits, but I know that pinched wiring in the floor console between the seats is a common short.

Another common short is where the wiring passes under the engine for the oil pressure light and the exciter wire. Those wires short to ground all the time due to heat and oil saturation.

Last edited by ZVOLV; 06-17-2020 at 10:18 PM..
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Old 06-17-2020, 10:14 PM   #16
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thanks for the info ZVOLV and everyone. appreciate you and your time.

Last edited by mschultz373; 06-17-2020 at 10:19 PM..
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Old 06-17-2020, 10:20 PM   #17
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I am gonna go drink beer. I have been working on cars all day. I diagnosed and fixed about 6 of em. Done.

Check the brushes. 2 screws. Run another ground wire. And run a new exciter wire. Run a new oil pressure wire.
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Old 06-17-2020, 10:20 PM   #18
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I myself would never try a 10 hour trip with my brick midbehaving like yours is.
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Old 06-17-2020, 11:32 PM   #19
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alright further diag.

voltage regulator brushes = 8mm and 7mm. good.
batt static = 12.45V. ~75% charged.
batt @ idle = 12.8V going up to 13.2V after start.

volt drop test on alternator = 310mV. Not good!
D+ terminal, ignition on = 2.23 V
B+ terminal, static = 12.45V

i went for a 15min drive, no issues nor shorts nor fuses blown. remeasured.

batt @ idle = 13.45V
volt drop on alternator = 269.5mV
D+ terminal car ON = 13.65V, car OFF ignition ON = 1.89V


so, it looks like the alternator ground wire needs replaced. volt drop should be <200mV, right?

my hypothesis is that, driving earlier today, one of the spade terminals on the turn signals shorted out to the body of the car, blowing fuse 13. after I noticed and was driving, i flipped on accessories and lights to see what worked. flipped on hazards with fuse 13 blown led to fuse 9 blowing.... i taped up the turn signal terminals to mitigate risk of grounding out.

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Old 06-18-2020, 12:44 AM   #20
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Your hypothesis for the blown fuses sounds reasonable. With this, your alternator charging voltage at the battery is OK, but on the low side. It sounds like you've done some voltage drop tests and there are some bigger than expected drops. If you want to do the full drop testing, try this when idling with headlights and fan on:
- start with the voltage right at the alternator, B+ post to bare metal on alternator
- next check bare alternator metal, or B+ post, to the engine block -- this would show the drop through alternator ground wire
- check engine block to battery - post -- this would be the battery to engine block ground cables
- check battery posts to cable clamps - just an inch away but will have a drop if the clamps aren't tightened
- check the battery + post to starter stud -- this is the big battery to starter +12v cable
- if you can get to it, check the starter stud to B+ on the alternator, which completes the voltage loop

If you see big drops on any section, try cleaning/tightening those connections.
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Old 06-18-2020, 01:21 AM   #21
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but in terms of applying field flashing - that means my alternator is still partially working for the time being? fundamentally, if it is NOT below the battery voltage static - i.e without car on - then the alternator is doing its job?

but at 13.4V with the car idling, the bottom line is that a new alternator is basically in my future? regardless of the brushes or regulator? shouldn't the voltage, ideally, be 14V at least?
Yes, the alternator is charging the battery. You do not need to mess around with field flashing because that is not your problem. If you want to try an acid test, with the engine running switch on the headlights, 4 way flashes, the heater fan, radio and any other electrical loads that you can think of. Watch the voltmeter. If the voltmeter drops to the battery voltage you know that the alternator has limited ability to supply charging current. This could be an internal alternator problem or it could be a wiring problem. If the alternator is able to maintain the voltage at 13.4 volts with all that load you are probably OK to go.

The 13.4 volt operating point is lower than ideal and may be a sign that the regulator may be out of spec, or damaged or that you have a nasty wiring problem. I don't think the 13.4 volt operating voltage confirms that a new alternator is mandatory. Since your brushes were not worn down a new regulator might fix the problem; however, I am more inclined to believe that you might have some wiring problems. I had an 1987 745 T which had the Volvo biodegradable wiring. I thought the charging system wiring was separate from the engine wiring harness. If you have the original charging system wiring you may have some issues there.
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Old 06-18-2020, 09:47 AM   #22
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to be clear, which I wasn't, the volt drop I measured was alt case to negative battery - testing the alt ground wire. i'll perform the others this morning, but my plan of attack is to clean up or replace the alt ground wire.

i replaced the engine wiring harness with a dave barton one last month.

is my voltage at D+ terminal OK? Or too low? Bentley says it should get battery voltage, which makes me think it should be 12.4V instead of ~2V?

Last edited by mschultz373; 06-18-2020 at 09:57 AM..
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Old 06-18-2020, 10:39 AM   #23
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here's the full volt drop measurements:

idling in P w/ fan on full, AC on, radio on, lights on
battery = 13.35V
battery + to alt B+ = 174mV
B+ to alt case = 132mV
alt case to engine block = 285-300mV
engine block to batt - = 15mV
batt + to starter = 61mV
starter to alt B+ = 113mV

battery posts to clamps were <15mV
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Old 06-18-2020, 10:43 AM   #24
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300mv drop from the alt to the block is a lot. Try running a redundant ground..


What's the issue with the charging system?
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Old 06-18-2020, 10:47 AM   #25
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yeah, i'm going to clean up the alt ground wiring points first and then see what the measure is.

the other volt drops are in spec, i.e. under 200mV.

i was just asking what voltage at alt D+ ought to be - if it's supposed to read as battery voltage (~12.4V) or if my read of ~2V is acceptible.

thanks for your continued time and help yall!
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