Wacky alternator or something else?

I replaced my alternator last summer with one of the GM-style ones John’s Cars sells. It worked ok for a while but it’s been getting more and more wacky. Battery light has been flickering, occasionally there are obvious voltage drops (blowers audibly slow a little, headlights dim slightly). But also sometimes the voltage goes high. Fairly new Die Hard Gold battery has been leaking around the base (at its vents I think) as if it’s being overcharged. All seems to come and go randomly regardless of RPM.

I ordered a NEW Bosch Alt from Rock Auto as the price wasn’t bad so we’ll see if that helps.

I’ll check and make sure all the connections are clean of course.

Any ideas? Can Less than perfect connections cause over voltage?

Have you actually checked battery voltage with a voltmeter, Martin; the usual routine - the dash gauge is very slow reacting. You have routinely checked the alternator belt, of course…

That said; it may be an iffy voltage regulator - or maybe failing diodes. A working replacement, as ordered, will likely fix the problem…

xj6 85 Sov Europe (UK/NZ)

Yes I know the dash gauge isn’t particularly trustworthy and I just haven’t had a chance to wire up a proper meter. I’ve seen the dash meter deviate pretty far from the “13” mark in both directions. I had trouble with the belt squealing when I first put it in last summer until I ran it up to “pretty darn tight”. It was quiet for a long time after that but started again this fall. I’ll be sure to replace the belt as well.

…‘pretty darned tight’ may be overkill, Martin - it may stress the alt bearings…:slight_smile:

A slipping belt may of course cause voltage variations, but slipping belts tends to squeal - so it’s likely something else. Likely fixable, though identifying the exact fault may pose problems…

xj6 85 Sov Europe (UK/NZ)

Hi look at your starter relay them Lucas are bad…. I’ve information how to wire it up …

It sounds to me it’s very intermittent

Could be a combination of 2 things your light control Lucas relay …

What would the starter relay change?
And the voltmeter might not show the voltage at the battery but it is a very simple construction and trustworthy. Of course if there’s something with the wiring that’s a different story…

If the alternator drew too much power the belt might not be able to cope, but if it’s within reason just tighten it to spec, don’t force it and find the problem elsewhere…
Nothing but trouble with replacement alts… nothing but trouble with the original but at least they can be fixed without opening a new can of worms!


it all sounds like a shot voltage regulator and excessive voltage. While the onboard VOM is highly dampened it will react clearly to lasting (+ 5 secs, I’d guess) changes - and that is what you want to know. A very short voltage spike won’t do much harm. The onboard VOMs are certainly not precision instruments, but they do react to changes well enough.

IMHO, you might just purchase a drop-in replacement part for less than 100 EUR and be set for some time. If you’ve still got an original Lucas alternator, you can buy replacement regulator parts, brushes and bearings and take the time you buy with the spare to recondition the original.

The belts shouldn’t be overtightened: one width of a thumb over the longest distance with normal pressure … Did you check you’ve got the correct belt to begin with? An overtightened belt might eventually damage the alternator’s bearings and cause a seized alternator, but shouldn’t cause excessive voltage.

Good luck


75 XJ6L 4.2 auto (UK spec)

, all I can say is this when them starter relays fail them Lucas units were always bad any Lucas relay is bad I always get good quality myself relays I change over them….… them Lucas stater relays certainly
can cause issues down the road as regards with voltage…. If it fails could be getting transmitted issues like voltage resistance… I had a similar as problem as for to a solution it worked well…… your always best for to buy quality as regards to automotive parts… it controls more then the stater…… there’s just a good solution method sometimes that’s all… more then likely a bad unit alternator can can cause problems such as flickering lights…. Also when you go to quality you’ve a great car looks it’s a pleasure I respect your comment but I’m only giving your man help by mentioning could highly be possible on the stater relay…. Taking to much voltage …. When they fail…. You can get good quality replacements no question but original otherwise is best… & repair If possible……

Sorry I can’t follow you. If you’re saying they fail often the lucas relays are very good, the silver cans especially, the plastic cased ones from the 70s are not as good but not worse than say bosch.

Hi I know they are good certainly…
I’m saying they can give trouble when they go , Note
also you’ve a more powerful current flow with the 80A Morden relay type … 14VDC80A … better starting all together….

Also very reliable relay … the 80A

The current flow is restricted by the resistance in the starter solenoid, David - and the wire between the relay and solenoid.

Properly working relays have little or no influence on the current flow - whatever their makers…

xj6 85 Sov Europe (UK/NZ)

Almost certainly a failing voltage regulator. Hopefully you’ll know more after the replacement alternator is installed

Checking connections, yes, of course.

Over-voltage would be a regulator fault. Presumably both your present GM-style and the replacement Bosch have internal voltage regulators so I’m hard pressed to think that problems any external connections would cause over-voltage…but I’m just two sips into my first cuppa coffee.

Replacement alternator, clean connections, proper belt tension. The see what happens. The charging systems on these cars are dirt simple. Personally I wouldn’t go too far into the weeds with more esoteric possibilties, relay replacements, etc. at this point.


Ok thanks, that sounds like the correct answer. Only problem is I crashed it into the weeds yesterday and lightly smashed the radiator mounts damaging the fan and knocking it off the clutch though somehow not damaging the radiator, oil cooler or trans cooler.
So I’ve gotta sort that mess out once the weather gets a little warmer.

I likely have any parts you would need. a 1986 xj6 parts car

Ooooooo thanks for that. I’ll let you know when I get into it.