Hello,

I’m hoping to elicit some help and ideas regarding an electrical problem that I’ve been having with my 1994MY XJ6(XJ40) with an automatic transmission. The problem is that under certain circumstances like while driving in traffic and/or while idling, the battery will get progressively drained. The problem has been misdiagnosed by several shops as the battery draining while parked but that is not the case based on my diagnostics.

The condition that occurs is that under moderate electrical loads and low rpm, the system voltage drops and the battery gets progressively drained.

I’ve worked on this problem very hard and thoroughly, and I’m almost at the conclusion that the installed alternator is not able to provide enough output power at a normal idle of 580 rpm. The installed alternator is a Denso brand remanufactured part # 210-0406. Supposedly, the alternator was replaced a year ago, before I owned the vehicle, and this problem predates the alternator replacement.

As part of the diagnostic process, I have carefully measured for any voltage drops along the ground and power circuit branches between the body connections, alternator, battery and the firewall pass-through. I’ve also cleaned and applied dielectric greased to connection points along these branches (including ground straps, etc.)

Lastly, I installed an ammeter in series with the power output terminal of the alternator, a voltmeter between the alternator output and ground, and a frequency counter from the spark trigger signal at the ignition (spark) module to ground. This allowed me to measure the alternator current and voltage and engine rpm under the problematic conditions. I was specifically careful to ensure that the alternator belt was not slipping and the batter was fresh.

I can reproduce the problem intentionally by idling the car fully warmed up with the transmission in park, heater blower on high, headlights on, rear defog on. This alone will pull the alternator output down to about 13.3 volts or less and 60 amps with the engine at 700 rpm. If I put the transmission in drive, the rpm drops to 580 rpm as expected, whereupon I can pump the brake pedal a few times to activate the accumulator pump. This causes the alternator voltage to drop to 13 volts or less and an alternator current of 50A. If I continue to pump the brake the alternator voltage will continue to drop and the current will stay at about 50A. If I do this for a while then as expected the battery will be significantly drained. Usually, the vehicle can be shut off and restarted immediately but if left overnight then it will not restart due to a weak battery.

So, I’m leaning towards an underpowered alternator but a too power-hungry vehicle could also be contributing.

So far, I have not been able to find the output specifications for the alternator operating at 580 rpm (or any low rpm for that matter). It is a 12V 120A alternator but that doesn’t say anything about what the idle specifications might be.

I also don’t have any data regarding the amount of current that the vehicle should consume under the conditions that I’ve described. Is my vehicle drawing too much current? As far as I know, it doesn’t have any aftermarket additions that I can find.

For the record, I also measured the alternator output under the same conditions as above but at 1500 rpm and the output parameters could be maintained at greater than 14V ( depending on the state charge of the batter) and around 66 amps. These seem like good numbers to me.

I have more measurements that I can provide if needed.

What do you think? Is this an alternator problem, an electrical load problem or a combination of both? Or something else that I’ve missed?

Any thoughts would be greatly appreciated.

Thanks,
Don
Vancouver, BC, Canada

 

First of all, what size (CCA) battery are you running? Have you had it checked by a battery specialist for its present condition/ability?

The batteries for these 94's in battery supplier's reference books are not good enough, you need a bigger sucker. Mine is something like 850 or 900 CCA. I ran a recommended smaller one for a year or two, but they just couldn't cut it. Fit the largest that will just squeeze into that tray in the trunk.

Your belt is definitely not slipping and the harmonic balancer isn't either, right?

Only other thing that springs to mind is this dielectric grease stuff ...dielectric grease is an insulator and if you have it between any connectors/connections, you're asking for trouble. It's ok to put on the outside of connectors/connections but NEVER inside. It will prevent electrical flow.

Larry (over on the Island!)

 

Hi Don,

Dittos on all of Larry's comments, especially the use of dielectric grease. Reading your account, the first thing that came to mind is high resistance in your battery power and ground circuits, and dielectric grease between cable connections can add considerable resistance. Corrosion is the most common source of high resistance, and occasionally a loose connection. I would suggest that your first step be to disassemble and clean the battery cable terminals and battery posts, the negative cable ground point where it connects to the right rear inner wheel well, the jump-start post on the firewall, the alternator, starter and the engine ground strap (both ends).

I can't recall if the XJ40 ECM manages battery charging voltage, but just in case it would be a good idea to clean the ground points referenced by the ECM: two on the intake manifold, and one on the firewall behind the cylinder head.

Off the top of my head, I believe the 120 amp alternators in the '93-'94 cars were made by Bosch, but the fact that yours is a Denso shouldn't matter if its output is sufficient (Denso was a primary OEM on X300s and later models).

Jaguar documentation states that the quiescent current on an XJ40 should be less than 38.6 mA after the car has had 60 minutes to fully go to sleep.

If you have a local independent shop with a higher-end battery/charging system analyzer (mine is a Midtronics EXP-800), they can quickly test the alternator for proper output and excessive AC ripple or a failing diode. They can also test the battery for conductance and actual CCA delivery versus its rating.

I owned a '93 for 16 years and my assessment is that the electrical systems in the late XJ40s were more demanding than the alternator and battery technology available at the time were able to support reliably, so you do need a very healthy alternator, battery and low-resistance wiring to keep the car happy.

I'll try to think of more ideas later. Please keep us informed.

Cheers,

Don

 

Hi,


That battery used during the diagnostic testing was a new 650 CCA. I've had to replace the battery several times because they have a hard life from being repeatedly discharged by this problem. The previous battery was a 750 CCA. In my mind, a larger capacity battery would not solve this problem because the underlying issue would not change. The battery would still get discharged under the conditions that I outlined. It would just take longer.


I was judicious in my application of the dielectric grease. It was applied after the connections were cleaned and reassembled. I also did follow up voltage drop measurements across the connections, wires and ground straps, while under electrical load. This verified that there were no high resistances in both the power and ground circuits between the alternator, battery, and power distribution fuse boxes. In any case, the current and voltage measurements that I gave in my previous post were done directly between the output stud and housing (ground) of the alternator so did not include any ancillary voltage drops in the vehicle.


The alternator was bench tested by my local alternator repair shop. It passed their test. Unfortunately, they really only tested it at full output at high rpm which doesn't reveal much about its behaviour at idle (580 engine rpm, 1740 alternator rpm) where the problem is occurring. I have not been able to find any specifications regarding the alternator's output capability at idle. I scoped the output to check for possible diode problems but all looked good there (see the attached picture) too.


On this XJ40, alternator output regulation is done entirely within the alternator. It is not controlled by the ECM - thank goodness.


Do you folks happen to have any idea how much power these vehicles typically need while operating under the problematic conditions that I specified? If I run my vehicle under those conditions except at 1500 rpm it draws roughly 66A @14V which seems high but reasonable to me. It should be roughly the same at idle but maybe a bit lower current since the fuel injection system isn't working as hard. What do you think?


I'll check the harmonic balancer again for slippage. However, I think it is not the laminated type as found in some other models.


I've got a call into Denso to see if they can quote me the alternator specifications at idle.


Thanks for your help.


Don

 

Harmonic balancer IS the laminated (vulcanized) type, I just replaced mine due to delamination!

Sounds like you are well-versed in electrics having done all the above checks already, so the only other things that might figure in the equation are the footwell fuse boxes and the big ol' harness stud within?

This battery drain while driving symptom occurred prior to the alternator replacement, so I'm assuming the original alternator (and even this one) might have been OK?

You already tried looking for a quiescent drain and the car sits within the tolerance Don mentioned, right? High quiescent drain is pretty common fault although I haven't had the problem (yet) myself, but if I park the car for a couple of weeks or more (say I go on vacation) I always disconnect the battery.

My 94 is driven (daily) with the climate control on auto, blowers on medium, CD player on and of course as we are in Canada, daytime running headlights and taillights are also constantly on.
I too encounter slow moving and stop and go traffic as construction and flag persons are everywhere. I do manage to get the revs up a bit on the highway, but I would say the majority of time on the road is spent non-highway.

Just thinking about your 580 idle speed ..I'll have to check but I think my car may idle more like 650? Not sure.

Larry

 

Don,

I can't see a scale on your scope to know whether the AC ripple shown is tolerable or excessive. Do you happen to know what the scale was for that reading?

I cannot recall seeing a "typical" current load specification anywhere in the Jaguar documentation, but I will try to remember to post one or more of the Jaguar electrical systems training manuals from my library on another computer.

Don

 

Hi Larry,

Yes, my mistake. The harmonic balancer is laminated. When I looked at it earlier, I didn't understand its construction so I assumed it wasn't laminated. Now I see that the inner core of the balancer that gets clamped down by the crankshaft bolt is solid and includes the outer most pulley, and the alternator drive pulley is laminated to the core. I tested it again this time being sure to mark all of the pulleys. I did not detect any slippage of the pulleys with respect to each other (see attached image). I was sooo hoping that would be the problem.

Yes, the alternator was replaced in search of this problem. I don't have the details from the shop that did the work though. It could be that the old alternator was of the exact same type so it may have been experiencing the same problem as the new one.

I have serviced the big harness stud.

The quiescent current drain on this vehicle is about 50mA after letting it sit for a few minutes.

Thanks,
Don

 

Hi Don,

The vertical settings on the oscilloscope for the alternator output ripple image was 1V/div. It was measured directly at the output of the alternator, under load, where there would not have been much smoothing effect from the battery but the noise suppressor (capacitor) should have been working. I also measured it with my multimeter and it was about .4Vrms.

I think what I really need to do is borrow a known good alternator and try it in my vehicle. Alternately (pun intended), it would be nice to know what the power requirements are from another identical vehicle.

Thanks again,
Don

 

DonB said:You said: Out of my area so not sure that 11.4 mA makes a difference, but maybe check drain after an hour?

My car is identical to yours other than being a VDP and as I'm also in the same region I should think conditions should be similar. I have no electrical issues (rapidly touches wood)

BTW Coast Auto wreckers on Mitchell Island scrap all the Jags in Vancouver - not suggesting you scrap yours but they would more than likely have a spare OEM alternator.

Larry

 

Hi Larry,

I've been thinking about this problem. On many other vehicles that I've worked on, they had an idle-up system whereby as electrical loads are added (like rear defrost, heater blower, etc.) the ECU would increase the idle speed to make the alternator happier about generating the electrical power to supply the increase in demand. I've not seen any reference to this in the XJ40 documentation that I've read. Do you know if this system exists on the XJ40?

I do see an input to the ECU from the climate control system that is used to bump up the idle when the A/C is on.

Have you noticed what your normal engine RPM is while in drive with everything off? Does the idle speed go up when you turn on the heater blower or rear defrost? Just curious.

Thanks,
Don

 

Don - I always run the car with A/C on auto (even with heat selected) with blowers usually running on medium at the same time. Rarely use the rear defogger but when I do I'm usually on the move and haven't noticed any variation in idle speed.

My car idles at about 625 or so and I'm happy with that, doesn't seem too high to me.

Maybe your car is idling too low to pump out enough juice to keep the battery happy? Just a guess.

I haven't run the car with everything off but I'll give it a go when I remember and see if my idle speed drops

Larry

 

Hi Larry,

I spoke to the Denso Tech Hotline today. They emailed me more detailed specifications, including an output characteristic graph, of the Denso remanufactured alternator that I'm running (Denso 210-04060). Based on this data, it looks like my alternator is not up to the job of running my vehicle at idle (with a few electrical loads on). The graph shows that the alternator is only able to put out about 50A at 1740 alternator rpm. This is exactly what I measured.

What alternator do you have in your Vanden Plas?

I'm thinking I might try a smaller alternator pulley as a test. This would move the idle point to the right on the graph which would provide more output current. This would be at the risk of potential belt slippage and excessive alternator rpm at high engine revs.

I think I'm making progress. I would still like to know what other people have for total electrical load?

Thanks,
Don

 

Hi Don,

You are definitely making progress! Trying a smaller alternator pulley is a great idea and just may work. If the belt adjuster doesn't have enough range, you should be able to buy a shorter belt to make it work. Gates and Dayco typically offer the same width belts with the same rib patterns in length increments of 1 to 2 inches. Your local parts store can look up the correct belt for your car, then look at the Gates or Dayco part number and find the next shorter belt part number.

The last time I checked I couldn't find a new or remanufactured Bosch alternator for a late XJ40, but another option might be to find a working used alternator from a '93-'94 car on eBay and if necessary have it reconditioned by an alternator/electric motor shop.

Cheers,

Don

 

Why did this car have the same problem BEFORE the alternator was changed? Was the one that was removed a replacement Denso as well?

What if your idle speed were higher .. (like 625), would that be enough to increase the output?

Larry

 

I may be wrong, but off the top of my head 400 mV of AC ripple seems to be on the high side for a modern vehicle (modern in terms of electronics). Have you checked the rectifier to see if a diode has failed? The capacitance of the noise suppression capacitor is probably much too low to provide any smoothing - I think it's just there to shunt AC EMI/RFI noise to ground.

But Larry's question about this car having the same problem with the previous alternator is a good one.... Something to ponder.

Cheers,

Don

 

Hello all,

The person that was helping me at the Denso Tech Hotline sent me the attached drawing today. I had asked him what the specified pulley was. 55mm diameter according to the drawing.

I have to put this project down for a couple of weeks (going camping in Utah!). But I will continue to post my progress after that.

Nice diagram. Everything you ever wanted to know about the OEM alternator (DBC6819).

Thanks,
Don

 

Hi,

I'm back to working on this problem.

Yes, the previous alternator was replaced in pursuit of this problem. It seems that this vehicle simply has too high power consumption at idle and the alternator can't keep up. Although, at idle (580 rpm) the alternator is operating in a regime where it's not designed to put out much power. If the alternator were spinning a bit faster, say 20%, the alternator would be in a much happier place (see the output spec graph).

That still doesn't answer the question of why THIS vehicle is having a problem. Maybe the brake accumulator is failing and so its pump motor is running too much particularly in stop and go traffic?

I wish I had a power/current measurement from another vehicle for comparison.

What model and type of battery are you folks running? Is it group size 49 or 48 for this vehicle?

Thanks,
Don

 

I run a maximum size battery, 850CCA - the largest that will fit in the tray.
Batteries specified in jobber's parts catalogs don't cut it for the 94 as far as I'm concerned, they are way to puny and small - especially with Canadian cars that have the headlights on all the time. (DRL's) -

Idle speed same with stuff on or off.

Keep in mind that if you are not a daily driver, prolonged periods of inactivity will run the battery down, and BTW I still think your idle speed is too low.

Larry

 

Hi Lawrence,

Do you happen to know the make and part number of the battery that you're running?

I agree that the idle seems too low. However, 580 rpm is the proper specification for a warm engine with the transmission in "D". The ECM controls the idle speed via the idle air control valve so there isn't really a correct way to adjust it since it is hardcoded within the ECM software. From the performance chart of the alternator, 580 rpm (1740 rpm at the alternator) is way down where the alternator doesn't put out much power.

Thanks,
Don

 

Sorry I don't know the battery code, The invoice just has the internal stock name AFAIK. Pretty sure the battery was a no-namer. I got it from
Island Batteries Inc.
Victoria, BC
(250) 385-8658
and had the luxury of the lads bringing various batteries out to the car to see if they fit. The one I got they told me they usually sell for Cummins diesel trucks, it was about $150 IIRC.

580 might be spec in D but my car sits at about 620. Just sayin. Where did you get the 580 number from? Was that spec for a Canadian car with DRL's?

Anyway, too bad about your car, flat batteries are a pain in the butt, that was why I went for the big one. The spec battery I had before never really had much gumption.

Larry