Alternator diode? New rectifier and regulator sorted it! Or maybe not!!

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cappers

cappers

Well-Known Member
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3.5 V8 (ex 1992 Disco 1) in my 90.
At night the battery warning light has started to come on weakly after a few miles driving. Not always and doesn't flicker when it does. Not visible in daylight.
No issues with headlights or using wipers, heater etc. Dash voltmeter indicates charging ok. Starts from cold after a couple of cranks.

Tested the battery today, values read at the battery with expected value in brackets:
1. Engine off: 12.6V (12.2-12.7)
2. Engine on: 14.3V (13.5-14.5)
3. Engine & ancillaries on @2K rpm: 14.1V (13.1-15.0)
4. Diode test - engine on @1.2K rpm: 0.006V AC (0.000)
So possibly a diode as there is a tiny voltage flow?

A new alt. is expensive; I could get a rebuild kit to replace the diode pack but I've not done anything like that before so before I try I wonder if anything else could be causing this?

A while back I had issues with a loose starter motor pulling the exciter wire off the solenoid but I have checked and all seems secure.

Posted on LRUK but apart from diodes no other suggestions.
Any thoughts appreciated.
Cheers.
 
Change the diode pack, regulator and brushes as they all fail over time. Most likely diodes but its all so cheap and some of the bits come as a pack so just do the lot. Once the alternator is on the bench its a 1/2 hour job..
 
^^^^What rob says.
Plenty of 'how to's' on youtube.
Starter for ten Lucas A127 alternator.
Check whether your alternator has the main wire bolted onto a terminal or a push-on connector like the one in the link as you will need to order the appropriate rectifier.
The brush pack on those can be replaced without any further dismantling.
A few of the bolts have very small heads - can't recall the sizes offhand - so you might need to buy some shiny things if you haven't got them already.
 
Drove to a nearby auto electrician who specialises in classic cars.
He did some checks with a fancier bit of kit than mine and said he thinks the alternator is fine despite the very low voltage on the diode test.
He suggested it's a wiring issue between the warning lamp and the alt. If it was a diode there'd be other signs but it is charging correctly and within limits.
He said he'd gladly take mine to refurb or get me a replacement but that it'd be best to go and double check the wiring including behind the dash before I do anything else.
As I wrote before, she starts fine, the voltmeter behaves normally and having lights, heater, wipers, HRW and radio on doesn't have any abnormal effect.
Have to drive somewhere tonight and I'll see what the little lamp is doing.
Thanks for the input to date.
 
A week of night drives and all seems ok now. Charge light goes out on start up and stays out, not even a glimmer :) Voltage meter indicating charging and loading as normal, plenty of power on cranking, NAD.

I removed the contacts and wires to the alternator and starter solenoid, cleaned and sprayed contacts with electrical cleaner, checked all wires were wholesome and reconnected it all. Didn't get round to checking the wiring to the charge light itself.
My thoughts are it was a poor connection at the solenoid considering the earlier problem I had with a loose starter motor:rolleyes:
 
Well it is still happening and if I'm not mistaken it's brighter!
Measured the battery again, values slightly lower than before but still in spec.
1. Engine off: 12.3V (12.2-12.7)
2. Engine on: 13.6V (13.5-14.5)
3. Engine & ancillaries on @2K rpm: 13.6V (13.1-15.0)
4. Diode test - engine on @1.2K rpm: 0.007V AC (0.000)
I've checked every connection from ign switch to alternator to starter solenoid and all seems ok. The light comes on with the ignition and then comes back on after a couple of seconds after start up, dim and does not vary in intensity with accessories on but flickers slightly.
Done daily 20+ mile runs the past three days, starts fine, analogue voltmeter generally in the middle, digital values as above.
Any ideas? The only thing I haven't checked is the instrument warning light itself.
 
Charged battery and getting 12.6V now. Light still on weakly, exciter wire and connections checked and ok but voltage doesn't rise much on running ~13.7. and over the past few runs has varied between 12-14V.
So I will be fitting these on the weekend.
IMG_2253.JPG
 
A small amount of AC at the battery terminals is acceptable.
Have you done tests on your earth points? Ie measure volts at each end of earth wires
 
kermit_rr said:
A small amount of AC at the battery terminals is acceptable.
Have you done tests on your earth points? Ie measure volts at each end of earth wires
Click to expand...
I haven't as I didn't think there were any earthing issues, eg starts and runs ok etc. I have noticed that the charging is erratic and seeing voltages between 12.8 to 14.2 whilst running.
I'll replace the rectifier and regulator and if still having issues will start wire tracing but I hope this will sort it :rolleyes:
 
capo said:
I haven't as I didn't think there were any earthing issues, eg starts and runs ok etc. I have noticed that the charging is erratic and seeing voltages between 12.8 to 14.2 whilst running.
I'll replace the rectifier and regulator and if still having issues will start wire tracing but I hope this will sort it :rolleyes:
Click to expand...
Fingers crossed! Did you measure voltage between the alternator terminal and the body?
 
kermit_rr said:
Fingers crossed! Did you measure voltage between the alternator terminal and the body?
Click to expand...
No, measured at the battery and subsequent readings according to the digital voltmeter in the dash. The old analogue gauge tends to sit in the middle most of the time.
I can't do the rebuild or other tests until the weekend but will recheck all the earths and voltages then.
Cheers for the advice.
 
Took alternator out, dismantled and replaced regulator and brushes and the rectifier pack, both of which looked a bit rough so worth the effort.

Then I noticed that the new rectifier is wired to take a Euro plug (1st pic) whereas the original has a stud labelled D+ for the warning lamp. So I need to get an appropriate Euro plug and rewire it.

If I do that what about B+ and W wires which go to posts? Leave as or wire into the plug?

Any suggestions gratefully received.
Screenshot 2021-07-17 at 15.06.16.png


Screenshot 2021-07-17 at 15.10.04.png

alternator wiring.jpg
Cheers.
 
Ordered a 3 Pin Universal Alternator Wiring Connector Plug For Lucas Bosch Alternators (7.99 inc p&p).
Should get it by 23 July so Landy is off the road for a bit.
upload_2021-7-17_16-29-41.gif


s-l96.jpg
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upload_2021-7-17_16-29-41.gif
 
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Found the correct older style 3-pole rectifier on eBay :mad:
As I've ordered the Europlug I might as well just change the wiring than buy another rectifier to replace the new rectifier I've soldered in and then have a non-fitting spare :rolleyes:
Fool me for not checking the connections :p
View attachment 242986

EDIT - Change of plan. Found the correct three stud rectifier on eBay as above and have ordered it as I don't want to mess with the wiring.
Screenshot 2021-07-17 at 19.00.46.png

Prefer to keep it as it is as there are feeds for the headlight light relays and two condensers/capacitors (?) on the studs and I've no idea where they'd go.
 
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Correct 3-pole rectifier arrived this lunchtime :) Soldered it in, reassembled the alternator with a new voltage regulator and put it back into the 90. Had charged the battery up and she started on the button :D
Charge light came on with the ignition then off once started and stayed off. Voltmeter read 12.7 off, 13.6 ignition on, 13.6 on with accessories @2K.
I'll keep an eye out but I think it's fixed.
£18 for reg. and rec. plus a couple of hours dismantling, repairing and replacing versus £80++ for a new alternator :cool:

If anyone is think of repairing or refurbishing their Marelli/Lucas A127 with the three pin plug holder I have a spare rectifier (RL-01H) which is no use to me and non-returnable. F.o.c. just P&P and a small donation to LZ. PM me if interested.
 
:mad::mad::mad:
After replacing the voltage regulator, brushes and rectifier pack, the battery light goes off on starting and at idle but comes when revved and gets progressively brighter as the revs increase???!!! Still dim and only visible at night. The battery voltage is approx.13.5 and up to 13.8 when running, having accessories on does not affect the voltage.

It has been suggested that there could be a fault in the exciter wiring from the ign switch to the dash warning lamp to the D+ post on the alt and is earthing somewhere to get a glow.

Will try a wire with an inline bulb from the battery + to the D+ post to see if the light still comes on which suggests a faulty alt, if it doesn't it's a wiring fault.
I recall that the foil circuit film on the warning light panel was a bit creased, perhaps that's an issue or the bulb not in place fully?
 
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