No charge, no ignition light

XJ
1

Hi All,

Chasing down a no-charge situation and have conflicting feelings about my diagnosis.

Backstory.

I just replaced the master cylinder and power steering belt. Everything went reasonably smoothly. On my test drive the car started to ‘feel’ odd and I notice the voltage gauge was reading very low. I turned off the AC and drove home. I shut the car off in the driveway and the battery was dead and would not turn over. I charged the battery and the car now starts and runs although it is not charging.

  1. Voltage gauge (in dash) reading +/- 12 V when ign on not running and engine running.
  2. Volts at battery 12.4 when running, same as not running.
  3. Ignition warning light never on. Not with the key at ign and not when running.
  4. Ignition warning light bulb has continuity (assume it works).
  5. Voltage across Ignition Warning Light terminals is +/- 7.5 V when key at Ign.
  6. Small brown ignition wire on Alt is 12V when ignition is on. 0V when ign off.

So my diagnosis is a dead alternator as I am not getting a charge and the warning light circuit seems intact.

My conflict arises from the lack of an Ignition warning light and the odd timing of just having completed some work.

The PO told me the Alternator is recent and it is one of the shinier pieces in the engine bay but I do not know any more than that.

Any thoughts?

Cheers,
Jordan

2

Sounds like you aren’t getting power to the field wire that runs to the back of the alternator. It is a smaller diameter wire and separate from the larger brown wire that runs off the large spade terminal. Check to see if you didn’t disturb it or that it didn’t fall off when you changed the belts.

3

That was my first thought as well. I have checked the small brown wire (with black trace) and it is plugged in with 0V when off and 12V when ign on (item 6) so I believe this is functioning correctly.

1 Like
4

The warning light is grounded through the alternator windings, Jordan. With 12V on the brown/black from the warning lamp; ‘no light’ implies there is no ground through the alternator. Verify lamp lit with the wire grounded

It does then imply break in the alternator wiring or failed brush contact. The brush assembly is most(?) likely faulty - and replaceable…

Normally, when the alt is running, producing voltage, the 12V from the alt meets the 12V on the light - and with 12V on both sides of the light, there is then no light…

Frank
xj6 85 Sov Europe (UK/NZ)

5

One terminal should have 12v (same as the Small brown ignition wire on Alt - it’s the same wire) and the other should show ground.
Connect the Small brown ignition wire from the Alt to the ground, the Ignition warning light should light.
If it does then the problem is either the Alt or it’s ground connection.
If it doesn’t the problem is the light, the wiring or the ignition switch.

6

Check that the engine is tied to ground (by applying an additional cable from battery neg to a good contact on the engine) and then see if the ign light comes on before cranking. If it does, check the main engine to chassis ground cable.
I wish I had done that before replacing my alternator needlessly !

Cheers, Mike
1973 DDS S1

7

GEt out your VOM.

  1. Measure volts at the battery with the engine off. Record. 12 v + or -?
  2. Start the engine . Measue vots again. 13+ or -? Rw the engine, volts increase ?

Iyght tel you sonething.

Repeat, but, at the alternar big beown wire.

Oughta tell you something.

imade a patch cord years ago. connects my VOM via the cigar lighter. I read vots at vrius loads and rom’s.

tie, i do t agai. I am nt doing uch drivig. I have to charge the battery eer so ofte. Failing battery? perhaps, Like me, it i old. Failed alternator, i sure hope not.

elcronics are having a hissy at my house. TV, new/old PC/ Old Pc?

Newish chroe book. Smart phoe?

Ugh.

CHJ

1 Like
8

Indeed it does! I believe you have it. I will explore brush replacement.

Any thoughts on installing a smaller drive wheel to increase output at idle? With AC, lights, radio, and wipers the car is not very happy at idle.

Agreed.

Tested and confirmed. Nice suggestion.

12V engine off, 12V engine on, same reading with revs. No charge situation. Like the idea of the VOM in car. The OEM gauge is slow to react (very dignified) but not very helpful for diagnostics.

I’ll pull the Alt and look at rebuild options.

Thanks again for everyones suggestions.

9

Found!!!

used but working unit/ David Boger @ erydayxj. good guy. good stuff. Reasonable prices.

CHJi

10

[quote=“Jordan-xj, post:8, topic:436973, full:true”]

It’s not a good idea, Jordan; it high rpms the alt may then exceed its safe rpms - and little time is spent in idle anyway…

You may have had an iffy alt for some time, and with a low battery the alt voltage will be low even with a well functioning alt. With a high current drain necessary to charge the battery alt voltage will inevitably drop - and, particularly in idle, as the alt output is not at max…

Solve the alt problem and see what happens…?

Frank
xj6 85 Sov Europe (UK/NZ)

11

This Alt is 75A and that’s all it can do. If you are not happy with that you need more Amps.
A smaller pulley is not a good idea.

This thread is for the V12, but I think it’s the same.

12

Makes sense. Unfortunately, I spend a fair amount of time at idle and very low speeds (30km) in my home area.

Yes, I can see this being an issue. If the factory system is functioning well, should it produce 14V (charge) at idle?

Since I am working on this, I would prefer to only do the job once. I have installed a GM 10si on another old car and the idle output was a game changer!

I am not convinced I need any more amps, I just want more of them at idle!

13

Well, that’s the thing…
At idle an Alt will produce about 30% of its capacity.
So the 75 Amps are reduced to a pittifull 22 Amps.

14

Ah! That is interesting. I did not realize the output was related in this way. I was under the impression that low rpm output had been (magically??) improved through design or control mechanisms sometime in the 80’s!

Thanks for this insight, I will chat with my local Alt shop about options.

15

Jordan,

you’re right in that at one point there was a - though not magical - improvement: that was the changeover from classic dynamos to alternators … but it occurred alreade during the 1960s. Dynamos basically produced no voltage at idle while alternators do. Still, at idle - in particular at low idle - the output (amperage) is marginal, of course.

Yet, with an alternator you will have 14 V at idle and enough amperage to supply your lights, fans and wipers. What else have you got on a Jag requiring that high amperage? There is no electric oil pump, heated seats and all the other stuff modern cars are bloated with. My car (admittedly without AC) has a 45 A alternator and is doing fine.

All that being said: a Jag needs to be driven … seriously! I wouldn’t even start my car if I knew I wasn’t doing more than 30 kph - this is my bicycle speed:-) I bet that even in moderate suburban cruising or gentle B road sweeping you’ll have plenty of amperage, charging and capacity.

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)

16

Aye. I recall that my 63 Corvair had a generator/dynamo. And then my 65 had an alternator.

My first car 23 T ford ad a gnerator/alternator! Three brushes. One was adjustable. Move one way to ncrease the amps. It had an mmeter. 15 amps!

In a drk cirner of my shop, I have some old Ford generators/dynamos.

Back in he early das, the current wwas barely regated. Only an on/off solenoid and engine speed.

so, on og highway crises, it was possible to cook a battery by overcharge. So, we turned on the ights to eat soe ams.

Thos early regulators could be adjused . Crudely by bending tab.

The neat part was that a gnerar could be tested easily on h bench. Aply 6v. If continuty was good, t would motor.

hey came apart easy and cud be renewed as well. New bearig and bush. new brushes. wipe the cmmutatorr and un dercut

Worked a bit.

My geerato would cease production from ime to tie. Ilearned that i could reove a cover and wipe the commutator with gas wetted rag. Presto, amps back on the eter…

CHJ

17

Nothing dramatic on my car. I do have a small amplifier with a subwoofer in the trunk but it is not much of a draw. I do have AC and my experience is that with everything running (AC, blowers, radio, lights) that the car idles poorly and hesitates on acceleration. This may be an indication of poor alternator performance. This may also be an indication of poorly performing points ignition.

Agreed!!! I just can’t help myself and use the car for school runs and shopping occasionally. I was on a 1000km rally earlier this year and the car was glorious on gentle sweepers if a little nerve-racking in tight mountain roads.

18

Only when the alt output, amperes, exceeds the current drain, Jordan…:slight_smile:

While common users; lights, AC, wipers etc have sort of fixed, and limited, power requirements - the battery’s ability to draw, and deliver, current is almost bottomless. Ie, a depleted battery will demand a lot of current, exceeding alt delivery - and the voltage will drop below alt’s peak/regulated voltage…

However, as long as charging voltages is higher than battery voltage, the battery will be charged - the amount of charging current is crudely proportional to the voltage difference between the alt and battery. Nominal voltage of a fully charged battery is 12,8V - at 12,4V battery less than half charged. And a faulty battery is less willing to deliver current - and may swallow more current than it can use…

Voltage decides how much current is pushed through a user. As long as there is ample current capacity, and even a depleted battery has plenty, performance/power, of users will vary only with voltage.

In short; fully charge the battery, and check, over time, whether your driving situation depletes the battery. Which may be caused by an iffy battery or alternator - but of course also by a higher drain than an idle can support…

To have lower fans, light or wiper actions in idle is an irritant rather than an objective problem - and changing pulley may not solve a problem that is elsewhere…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)

19

Do he math:
ECU: 1A
Ignition: 1A
Fuel pump: 8A
AC: 2A
Blowers: 3A
Headlights: 10A
Sidelights: 2A
Instrument lights: 1A
Radio: 1A
Amplifier: 5A (at low volume)
Various relays, clock, climate control: 2A

That’s a total of about 35A while your 75A alt is producing at best 25A at idle.
If your battery is marginal everything is starving and the ignition has not enough power to make a good spark.

20

This is sound advice. My XJ6 is the first car I have ever tried to return to stock. The idea is to get everything back to factory spec and slowly make changes based on an authentic baseline (sway bars top my list).

Good math! Thanks, this puts in perspective.

I hope to get some time to pull the Alt off of the car this afternoon and take it to my local Alt Shop. If they have a larger amp replacement I will take it, if not, I will ask for a repair and move forward confirming battery health.

Thanks again for the help.