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soldave

Alternator going out again?

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Out for a drive today with the heating and wipers on, and noticed the voltmeter in the car dropping under 13v. Not done that for a while so it got my attention; I turned them both off to test and it crept back up.

Got back home and pulled out the multimeter to do some very basic testing across the battery (I know not the most effective, but humour me). Did some measurements of the voltage; first with everything off, and then with the fan, heater set on full, lights and hazard lights on. With all the accessories turned off, voltage was 15.3v at the battery and showing a couple of needlewidths above 13V on the guage in the car. With accessories on, the multimeter was showing 13.2v but the guage in the car was in the red, just above 11v.

I also looked at the rear lights. With accessories on there was quite a noticeable dimming of the rear lights when the hazard lights came on; with accessories off there was still a little dimming but not as pronounced.

Does it sound like the alternator's on its way out? I did replace the Lucas one with an eBay special about 2 1/2 years ago, but it's done fine until now.

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Hi Dave.Whats the battery voltage with the engine running.ie the alternator charging the battery.It should be at least over 13.8v.If you rev. the engine the battery volts should increase.It's worth checking the alternator belt.If the alternator is doing its stuff then you have a faulty battery. 

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You mentioned the lights at the rear are dim, are you getting dimming at the front?

It’s worth checking the condition of the earth connections as well

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55 minutes ago, alan.phillips said:

Hi Dave.Whats the battery voltage with the engine running.ie the alternator charging the battery.It should be at least over 13.8v.If you rev. the engine the battery volts should increase.It's worth checking the alternator belt.If the alternator is doing its stuff then you have a faulty battery. 

Those voltages were when the engine was running. With all accessories on, revving the engine did raise the voltage to a peak of around 14.95v. Alternator belt doesn't seem to have excessive play in it.

30 minutes ago, Straker said:

You mentioned the lights at the rear are dim, are you getting dimming at the front?

It’s worth checking the condition of the earth connections as well

Lights at the front shown. Doesn't seem to be dimming there (or it's very faint) which would probably suggest an earthing issue on the rear lights.

 

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Hi Dave,

Fully charged, good battery at rest, should measure at 12.6 volts. When the engine is running, this measurement should be 13.7 to 14.7 volts.  15.2 does seem high.  Check the alternator itself.  With the engine running find the terminal that has the wire to battery and place probe on that terminal and one on a nearby earth point, don't use the alternator case to ground, see what you get.  If the alternator is fine the reading should be between 13.7-14.7 volts.  if still reading 15.2 then, yes the aternator could be failing. 

Most alernators have a diode pack which can be removed and replaced. So there may not be a need to replace the whole thing.

Roland

 

 

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Car charging systems are often pretty basic, but if the voltage across the battery is >14.7 then you probably have a problem as said above. Extended periods of charging at 14.7V will cause gas(hydrogen) to be created, and the battery could also get hot, if the charging source/alternator can supply lots of current. Usually, once the voltage has reached 14.6-7 V then the charger should taper off to about 13.7, which is perfectly safe for extended periods. Sometimes, if a battery has been severely discharged, and has been jump-started. it may not hold charge until it has gone through a proper charge cycle. I had this recently on a battery, and it had been dead flat, and jump started, but after about half an hour I stopped the car and it was still almost completely flat still, so had clearly gone into a high internal resistance condition. I charged it at a constant 1 amp for about 36-40 hours(it was a 36Ahr battery I think), where it finally tapered off to about 100mA at 14.6V, then I reduced to 13.7V for a few hours to fully charge it. Now it seems fine. (Admittedly, I have a power supply which allows for constant voltage/constant current operation, so a bit lucky I guess). But any decent battery charger should do that sort of thing automagically, possibly allowing for variable current charging.

Not sure where the voltage meter takes its 0V/GND from, but when there is current flowing, as you had with fans and lights on, it doesn't take much resistance to cause a variation in earth voltage around the car of a few volts. Say 20A was flowing with lights and heater fan, then only 0.1ohm will cause a 2V drop. (V=IR).

Your volt gauge may just be reading 2V low, as it has roughly 2V difference for both high and low current. So, that is possibly a calibration issue.

The dimming of hazard lights will be an earth resistance issue, but may not be possible to pick up easily with a normal multimeter, as low resistance measurements are difficult, and you may be only looking for 1 or 2 ohms or even less. But worth a go.

And finally, the alternator regulator may be damaged. I don't think it should put out over 15V for any length of time, certainly if a battery isn't to be damaged. But I am not an automotive engineer, so it may be normal for some instances, but I haven't seen it when working on my cars.

Good luck with the detective work.

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Right - a little more testing today. First off - took the battery to Halfords to check the condition of the battery; good news is that they said it's in perfect condition. Battery should have a CCA of 380, actual value 380. I also checked the electrolyte in the battery and (I think) it seemed all as it should be - pics are below. So we can scratch off a problem with the battery as the main issue.

I also did another bit of testing voltages from cold (after the battery had been on the conditioner all yesterday afternoon and overnight). Started the car with and cold with no accessories the voltage across the battery started at around 13.7V, and then continued to increase to around 15.1V where it stayed constant. Put on fan, heater, lights, hazards and tested again and it had gone down to a stable voltage of 12.5V (revving did increase it). Voltmeter readings inside the car were 13.5 and 11V respectively, so it looks like that gauge is reading around 1.5V less than it should be, although that's not really the main issue here.

I can understand the requests to check for a bad earth (especially when it comes to the rear lights, and I probably agree with that diagnosis, as the front lights are hardly dimming), but would a bad earth be causing that high a voltage across the battery with no accessories on?

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A bad earth link would reduce the voltage across the battery, as the alternator would see say 14V, but there would be a voltage drop across the 'bad' earth. The drop would vary with current, but the battery is only there for transient current draws, the alternator should supply the current normally. So in equilibrium, when battery charged and alternator supplying the load current, any bad earth between battery and alternator will not be evident and the voltage across the battery will be essentially the same as the alternator.

Short answer, voltage seems too high = faulty regulator.

But, have you verified your multimeter? It is possible it is faulty. Do you have another to compare with?

 

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I think you’ve got a combination of problems...... sounds like a regulator issue + an earth issue somewhere as well 

Definitely check the performance of the multimeter... I’ve been misled by faulty test  equipment several time

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Did another quick test of voltage output from the alternator instead of across the battery. 15.1 volts with accessories off and 12.8 with accessories on, using a chassis earthing point.

My diagnosis continues, but will include some sort of testing of the multimeter itself too, just in case!

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Assuming that your multimeter is accurate, the alternator (15.1v ) is chucking out too much, and indicates the regulator needs changing.

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Yeah it is looking that way. If it's confirmed, I'm wondering if I should get the latest eBay alternator rebuilt or the older Lucas one.

Lucas one might have better quality internals  (although needs rebuilding) but I'd need to modify the wiring a little. It has spade connectors only at the back whereas the current one connects through one spade connector and two ring connectors. Only a small mod but the less fiddling with wiring the better for me! Only chance would be if the Lucas alternator can have a threaded fitting to it so I can attach the ring connectors directly.

None of that probably makes any sense!

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1 hour ago, Trunnion 74 said:

Dave, have you got make and model numbers for both alternators?

Old one that I thought was Lucas, sticker come off so no.

 

Current one: https://www.ebay.co.uk/itm/LOTUS-ECLAT-ESPRITE-ELITE-UPGRADE-65AMP-BRAND-NEW-ALTERNATOR-1973-onwards/281723431091

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Decided to take some guidance on the bad earth front and take off the earthing strap which runs from the driver's side of the engine down to the chassis. Bit of a pain to get off with a stiff bolt in the chassis partially blocked by a pipe going into the power steering rack (I believe). A bit of lost skin and swearing and I got there in the end.

Now the earthing strap does look like it's seen better days! But I did some resistance checks across it and it was showing as having 0.8 ohms of resistance. That doesn't seem to me to be so bad. But it does look like it could do with renewing so that's what I'm going to do. Got my eyes on this (https://www.ebay.co.uk/itm/170-Amp-Car-Battery-Power-Earth-Cable-Choose-Colour-Length-Terminal-Hole-Sizes/222662654414), if 170 amps would be OK. Not quite sure on what amperage I'd need, so if that is way below what I'd want then please do let me know.

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Don't forget ohms law, V=IR so for example 0.8 R @ 40 A = 32V, which is of course impossible

When cranking, say 150A(a guess) you don't want to lose much, but typically the voltage across starter drops to 8V maybe(battery internal resistance plus external resistance(cable and connections), hence why the ignition ballast resistor is shorted by the ignition switch. So, say you don't want more than a volt loss across the cable @150A, you want R = V(1)/I(150)  which gives about 7milliohms(.007 ohm), and no 3 1/2 digit multimeter can cope. 

Hence reason why a very small bit of corrosion can really stuff things up.

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14 minutes ago, Clive59 said:

Don't forget ohms law, V=IR so for example 0.8 R @ 40 A = 32V, which is of course impossible

When cranking, say 150A(a guess) you don't want to lose much, but typically the voltage across starter drops to 8V maybe(battery internal resistance plus external resistance(cable and connections), hence why the ignition ballast resistor is shorted by the ignition switch. So, say you don't want more than a volt loss across the cable @150A, you want R = V(1)/I(150)  which gives about 7milliohms(.007 ohm), and no 3 1/2 digit multimeter can cope. 

Hence reason why a very small bit of corrosion can really stuff things up.

Got it. Thanks for doing the maths there.

Does a 170a amp ground cable sound like it would do the trick? Would probably be happier with a cable over a strap, for reasons I can't explain!

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I would think the strap is used because it has fine strands, whereas the cable is less flexible, so it is more likely to fracture and break more quickly than the strap.

170A should be enough I think, but don't forget, the battery is rated at 320cranking amps, so the more current capacity the better.

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5 minutes ago, Clive59 said:

I would think the strap is used because it has fine strands, whereas the cable is less flexible, so it is more likely to fracture and break more quickly than the strap.

170A should be enough I think, but don't forget, the battery is rated at 320cranking amps, so the more current capacity the better.

That's a good call actually, on both strap and amperage. Will look into both straps and cables.

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Don't forget the earth fixings and contact areas, I'm guessing your Eclat has a galvanised chassis, galvanic oxidation is an awful conductor, so clean up the area and use knurled copper locking washers. 

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21 minutes ago, Steve V8 said:

Don't forget the earth fixings and contact areas, I'm guessing your Eclat has a galvanised chassis, galvanic oxidation is an awful conductor, so clean up the area and use knurled copper locking washers

Good point. Those copper locking washers aren't so common to find, but I'll do some hunting tomorrow and find some. Will also clean up both contact areas when the new cable goes on.

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Popped into the garage after work and cracked open the multimeter (does a multimeter open? Anyway, I digress). Did some resistance checking of wires. First, positive cable which runs from from alternator and eventually to battery. Result: close enough to zero for me to be satisfied.

Then I looked at the negative wire from the alternator. 20 ohms of resistance running from alternator connector to the negative battery terminal. Checked a little further; around 10 ohms from alternator connector to coil, and 20 ohms between coil and negative battery terminal.

Should I be getting this much resistance between these areas? Seems pretty damn high unless there's a special reason for it.

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The alternator has a negative? I recall mine using the engine as the negative return path. The alternator has a thick(or possibly 2) wire(+ve) and a thin wire for excitation which goes to the dash.

Those numbers are very wrong. negative everywhere should be essentially 0ohms resistance from anywhere in the car.

Do you have a wiring diagram?

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15 minutes ago, Clive59 said:

The alternator has a negative? I recall mine using the engine as the negative return path. The alternator has a thick(or possibly 2) wire(+ve) and a thin wire for excitation which goes to the dash.

Those numbers are very wrong. negative everywhere should be essentially 0ohms resistance from anywhere in the car.

Do you have a wiring diagram?

I do. Actually When I said negative I should have said the thinner wire. Apologies - made a mistake there. Was still giving me some sort of continuity through to the battery terminal though, which does question my abilities!

Tomorrow I'll do some more testing or resistance across individual wires, to see what I can find. Will probably start going negative battery terminal to where it grounds out, and then work from there.

Edited by soldave

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The thinner "exciter" wire goes to the dash light doesn't it?  In which case you might expect some resistance from the bulb in that circuit.

Pete

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