1990 5.3 v12 overcharging
 

Hello all! Hoping for a little help here, got myself a nice shiny xjs! She is a 1990 5.3 v12, now after a good 2 hour drive home she was faultless! Today however..... not so much! Started her up and the volt meter on the dash was just over the red at the bottom so undercharging but no warning lights? So shut her off and shot inside heading for the volt meter! Checked the battery and it was a steady 17.3 volts!!! Hmmm so started the old girl up and the dash meter was just under 13 now in the normal.... so went back in the boot and hooked up the volt meter and at idle she is putting 21.2 volts into the battery!! Im not sure if I’m doing something wrong with the meter so will take a pic of it later and add on here, I had a car do this once before where the voltage regulator knackered up and it was putting 19v in the battery, but that put EVERY warning light on and most electrics in the car wouldn’t work and blew a load of fuses, but the jag is fine, everything works, no blown fuses no raging inferno from a roasted battery, so I’m a little stumped! Going to check the volt meter on my other car tomorrow to rule out the volt meter being at fault, Any input would be greatly appreciated! Thanks Pete

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the meter is on 200V , maybe try 20V???

Could be the voltage regulator is bad in the alternator. I wouldn't drive it like that. Changing the alternator isn't the easiest thing in the world, but doable. Easier if you don't have an air pump.

I would also go through and check all the connections at the alternator , and the earth cable at the battery and body. Make sure all the earths are clean and tight. A further test might be to check for a voltage difference between the body and the engine block. There shouldn't be one, but could be with a bad engine ground strap.

If this reading is directly across the battery terminals then the reg is almost definitely the problem.

If you run it like this you will boil the battery and it could explode if the vent does not work then you'll have a mess in the boot to clean up. I'd pull the alternator and have it rebuilt.

You can get the alternator out from underneath, it's a real PITA job as all the belts have to come off as the alt is the flat belt closest to the block.

I suspect that voltmeter of yours is out of calibration or just plain broken.

I think the alternator is shot. I would not drive it like that until fixed; but worth checking the voltage with another meter first. If you have to change the alternator, post again for pointers.

I don't think that alternator is even capable of producing 20 V at idle. You would need to rev up for such a voltage.

Thanks for the response everyone! Yeah the reading is across the terminals on the battery, I put the meter on 200 as it wasn’t reading on 20 as the car is producing over 20v, I agree that 20v is mega high at idle (around 500rpm) i checked the volt meter against my wife car as I know that is perfect and readings of 12.7v on the battery and 14.2v running, I have bought a new volt meter today just incase, am I right in assuming that if it’s the voltage regulator this can be repaired separately as the alternator is working ok? Or has the regulator been toasted because the alternators gone nuts and started running mega high! Is it possible someone has installed a 24v system and installed a 12v battery by mistake? I’m not sure what goes into installing a 24v system I’m assuming it would require a complete rewire and would yield no benefits over a 12v system but just a thought I had.

It still can be bogus reading. For instance, if there is a 20 V impulse coming from alternator it can give 20 V reading on the voltmeter, while the effective voltage is in allowed range 14 V or less. That's the trouble with electronic voltmeters. Old fashioned voltmeters with a needle didn't have such an issue. And no offense, but all SI units which are named after a person are uppercase. Volt is named after Alessandro Volta, thus the SI unit is V, not v. Lowercase v stands for velocity.

Thanks for the info! I didn’t know that, i shall ensure all my future are fully correct as per your instruction, you have a good point that my meter may be picking up on a voltage impulse sadly my new meter is also the inferior electronic type so will see what happens.... also just a note putting ‘no offence’ before saying something offensive does not make it any less offensive... haha but no offence taken, I’m in fact greatful as I have genuinely learned something! Thanks please feel free to correct my grammar too as I’m sure that is terrible too!

An alternator generates 3 phase AC and has rectifier diodes to convert the AC to DC then it has a regulator to keep the voltage between certain values approx 13-15V.

Alternator voltage will be at least 3 volts higher than max voltage required so 20V at idle is about correct if the reg fails. There is nothing wrong with your meter and you were correct using the 200v range as your meter does not auto range.

You will need to remove the alternator and have it fixed.

One thing that does kill alternators is heat and if they get gummed up with oil the regulator can not cool correctly and overheats and dies.

Great thanks for the info, I’m glad I had the meter set up correct, have had a quick look and the alternator is very dirty/oily so I think your right and it’s overheated, I will check the voltage now with my shiny new meter and see what it says.... but think will have to address the oil leak situation too, the whole front and underside of engine is very oily and it had a advisory a 2 years ago for oil leak around belts area, so may need to look into that too

Hi Pete

Is your Oil Cooler leaking?

As I am in the UK and with our Temperate Climate (apart from just a few days in the Summer!) I decided to eliminate mine altogether

Where the Oil Temp seems to maintain itself at 144o/c which some say is the perfect Temperature?

OB
144 degrees Centigrade? That is too hot for oil.

Hi Greg

What would you say is the perfect Temperature?

From my days designing industrial equipment, the recommendations were between 75-95C. 140 is way too hot, that will oxidise the oil rapidly and shorten it's life considerably. How were you measuring that temp? Was it the bulk temp of the oil in the pan?

Hi Jagboi

Yes the Bulk Temp in the Oil Pan with a laser hand held Thermometer and I also change the 10/40 Semi Synthetic every year regardless of whether it needs doing or not, including the Filter

110°C is considered optimal engine oil temperature. The oil has to get over 100°C to vaporise the water. Conventional oil will tolerate temp up to 120°C but start to break down north of 130°C. Synthetic will tolerate higher temps and needs to especially in turbo applications.

I bet you were picking up exhaust heat OB. I once put temp strips on my V12 engine all over it, as well as a proper probe thingy on the entry and exit points of the oil cooler. The hottest part of the engine was the cam covers at 90C. the oil temp at the filter and the sump as never over 85C. The cooler (bypass type and I was doing these measurements to decide if the full flow conversion was worth it) was taking 20C out of the oil temp between inlet and outlet, so I ceased to worry about the FF conversion!