S3: Low (False) Volt gauge readings

E-Type
1

Very low volt gauge readings prompted me to upgrade the alternator to a 130A unit from a Land Rover Discovery 2 V8. Its NOT the amps that prompted me to use that alternator but rather its FORM FACTOR which made it rather easy to install. However, after the upgrade the volt meter still presented very low readings despite my Fluke multimeter telling me otherwise.

Without boring you with electronic detail, I found that

  1. the Volt meter was taking its reading from an inappropriate point - from an electronic engineering point of view; (old wires, corroded contacts, excessive number of contacts, worn ignition contacts all contributed to false readings)
  2. the amps drawn by the gauges themselves also influences the reading downwards.

I have now modified the point where the volt meter takes its reading from as follows:

  1. I installed a relay which is energised by the light green wire that previously went to the volt gauge;
  2. The two switch contacts of the relay is connected to a: the volt gauge and b: the permanent 12V “input” 3rd terminal cluster from the top, left side. The wiring diagram shows this as a brown wire but on the car it seems more white than brown. This wire comes directly from the double ended terminal close to the battery +. Please note this point is still not the most optimal point from an electronic engineering perspective. The MOST optimal point would be a dedicated wire coming straight from the alternator B+ terminal.

I also modified the earthing of the volt gauge so that it now has its own fresh, dedicated earthing point.

Now the readings correspond to my Fluke - without having had to re-calibrate the meter.

2

Hummmm…

130 amp alternator replacement. Was the original wiring upgraded to handle the additional amperage (doubled plus) of the new electric generation source? If no and something shorts out,… do you carry a fire extinguisher or have the Fire Department on speed dial? Another point. The “Voltage gauge” is not a direct reading Voltage gauge in the normal sense. It’s a Battery Condition Gauge.

Yes, 40 plus year old car with original wiring has issues. Grounds, corrosion and brittle wiring are a few of the major problem areas. Also, the antiquated Smiths Voltage regulator has caused more than enough of its share of problems. Just replacing that regulator with a modern electronic version make a big improvement, especially in reliability.

Happy Motoring,

Dick

'74 OTS
'99 XJR
1947 Stinson 108-1 “Voyager”

Growing Old is Mandatory… Growing Up is Optional…

3

Dick, would you explain? I never thought much of it, but I assumed it was a simple volt meter- maybe a slow acting one, but still just reading the system voltage. Does it have some type of electronics that analyzes the battery? (I am actually referring to the S2, not S3, but are they the same?) Thanks,
Tom

4

Tom,

Great question… It is not a voltmeter as found in American cars. That
gauge is not the way we do things here in “America” -i.e. American Muscle
cars . Early Jags had generators with an Amp gauge, showing the output (or
lack thereof). On changing to an Alternator Jaguar decided not to measure
the voltage emitting from the Alternator, rather taking a reading of how
well the battery is charged. The battery buffers the reading. That’s why
it’s not a fast acting gauge. A quick read of the Owners Handbook explains
what each segment and condition(s) are subject to that range. British
engineering. The S3 electrical system was well engineered for its day.
Original quick disconnect connectors are ravaged by time and the elements.
Today’s cars have sealing connectors eliminating most connection faults. Jags
live off their electrical system (electric cooling fans, ignition, etc.) and
are very dependent on the health of the battery.

I was also rather puzzled by your description of Form (wave form?),
referring to the Range Rover Alternator. The BUTEC (English version of
Leece-Neville licensed design) is a very robust alternator. Again, without specific
maintenance intervals, such as mandated on aviation components, failure is
usually the first requirement for maintenance/upkeep. Early BUTEC’s had a
3 amp Tri-diode (white “thingy” on rear top) which failed in quick
fashion. It was later replaced with a 5 amp version, a more robust unit and not
prone to failure. Jaguar issued a Service Bulletin to that effect.

The BUTEC regulators also had a high rate of mean time between failure.
That was mostly due to the quality of available parts (transistors, etc.).
I’ve found Leece-Neville regulators (5078) hold up better than the OEM
Butec’s. BUTEC’s are easy to overhaul and bench test as are the Leece-Neville
regulators.

Fuses don’t protect the equipment… They protect the wiring. So if you
have one of those large brown wires short to ground the wiring is only
designed to withstand 60 amps plus safety factor. When a greater amp output
alternator (130 amp, etc.) is installed it can easily overload the wiring
when a fault/short is encountered. It’s kinda like putting a penny in a fuse
box. The wiring will overheat and can lead to a nasty fire.

Hope this helps.

Dick

In a message dated 2/28/2017 9:14:01 P.M. Eastern Standard Time,
noreply@jag-lovers.com writes:

     _Tommd55_ (http://forums.jag-lovers.com/users/tommd55)

March 1
Dick_Wells:
The “Voltage gauge” is not a direct reading Voltage gauge in the normal
sense. It’s a Battery Condition Gauge.
Dick, would you explain? I never thought much of it, but I assumed it was
a simple volt meter- maybe a slow acting one, but still just reading the
system voltage. Does it have some type of electronics that analyzes the
battery? (I am actually referring to the S2, not S3, but are they the same?)
Thanks,
Tom

5

Nope. Not necessary. An alternator is a VOLTAGE SOURCE not a CURRENT SOURCE. This means that it will provide a constant voltage, and deliver current up to 130A, IF THE LOAD DEMANDS that much current. The ONLY load on the car that MIGHT ever demand 130A is a dud battery. The wire between the alternator IS adequate for all normal situations and IF the battery shorts out, the battery will burn long before anything else.

I do carry a fire extinguisher, but our fire dept is not worth calling anyway.

This is NOT correct Dick. The Volt gauge is EXACTLY that - A volt gauge. A very crude Volt gauge. The ideal volt gauge should NOT draw ANY current, this one draws 80 - 130mA, which is why the current drawn by this gauge (and the others) actually affect its own reading. But in my case its readings were surprisingly accurate when tested on the bench with a high quality variable power supply.

Done. :grin:

6

Dick, with respect, but again I have to counter the statement. With standard wiring, the volt gauge is simply “measureing” the voltage after the ignition switch and after a fuse (which also supplies current to various other electrical circuits) and having 20+ electrical contacts in the circuit between the battery terminal and the actual gauge. Each of these contacts (resistance) is contributing to a voltage drop caused by current drawn by the four gauges and all other electrical equipment that may be drawing current through both the fuses in fuse unit #19 as per the wiring diagram.

At first I simply moved the Volt gauge’s measuring point to where the the White/Pink (WK) wire terminates onto fuse unit #19 and this already improved the gauge’s reading, but this still suffered from voltage losses through the poorish electrical contacts of the ignition switch.

The term “FORM FACTOR” I used, refers to the physical design of that particular alternator. It was the position and nature of its mountings that made it so easy to mount on the E.

The fuses in this car don’t mean much to start with. The reason being they are situated too far “downstream” (meaning too far away from the power source). Fuses should be located in the immediate vicinity of the power source (battery) to minimise the length of wiring running in the car that is still unfused. Fuese panels on modern cars are typically situated very close to the battery so that all wiring thereafter is fuse protected against shorts to earth.

I should add, that I might even re-install the Butec at some point, just to see if it will carry the load if all the lights, radiator fans, and A/C is on at night - just to satisfy my curiosity. And even if it does, I will again fit the Bosch alternator simply being a more common item and having a better chance of repair in case of a failure when far from home.

7

Sorry, one more thing. The reason the Volt gauge (and the fuel, oil pressure and water temp) are not fast acting is because they all work on the principle of a wire wound around a bimetallic strip, relying on the heating effect of the wire to warm up the metal which causes it to bend which causes the gauge to move. This heating effect obviously takes time.

This is what they look like inside:

tn_20170301_101014_Richtone(HDR).jpg1800×1013 320 KB

8

Dear Dick,

Do you want to quantify how and why the antiquated Smiths voltage regulator causes more than its share of problems?

Every etype has an identical bimetallic-based water and oil gauge, most old British cars have the same set of Smiths instruments and even XJS’s have bulb failure modules working on this principle. I don’t see anything in any other forum about these causing ANY problems.

The reason the battery voltage gauge conventionally reads low is that it is at best misunderstood:- it measures what it is connected to, which is the instrument cluster feed at the green wire, not anything remotely close or related to the battery. This green feed also supplies the drop down gauges and so the “true battery voltage” less the voltage drop associated with the current consumed by the whole drop down panel is what is reflected on the gauge.

The s3 electrical system most certainly wasn’t even engineered. It was/is a legacy item from the s2, which is mostly the same as a s1, which draws on the xk150 and ultimately has its roots in a wartime design from the 1940s. The xk120 design, which had far fewer and lwer current usage components, may have sufficed then with its simple blanket fusing, but having just a clutch of simple 35amps fuses was woeful even for the 1970s. Any sensible owner should measure what the current demand is per circuit and substitute in lower values if they really want some protection. The white wire isn’t even fused and there is no great need to run the (unfused) twin fuel pumps through the ignition switch. All of the inline fuses prove that they just patched an already antiquated design.

kind regards
Marek

9

Assuming the meter is indeed a straight volt meter (and, Dick, if it is not, more specific details would be appreciated) then I am not sure the reasoning to change the wiring on the volt meter. Yes, if it were hooked directly to the alternator B+, one would know the alternator output, but not what voltage is charging the battery, nor through the car system. In some respects, the farther down the electrical system this volt meter gauge is located, the more information it provides in that now not only does it provide information on the alternator, or the battery, but all the possibly poor connections on the way to the meter, which may also be feeding other components on the way. So when you noticed your original problem of low meter reading, had you initially checked at the battery or alternator and found that voltage was okay, but the meter was not, that was, to me, indicating the alternator was not the problem but a poor connection between the gauge and the alternator. It is true that a good diagnostic voltmeter should draw minimal amps to least affect the circuit one is testing, but in this case, the slightly higher amp draw again is an advantage, as only with the slight amp draw will it help to point out the poor connection in the system.
Tom

10

Dear Tom,

As soon as the car gets any rpm, the alternator IS powering the battery and the rest of the car. There are no other higher voltage power sources and mysteriously, the battery doesn’t run down on a running car.

Check out why the idiot light is lit on a non-running or low rpm but running engine and why it goes out on a running engine.

kind regards
Marek

11

Marek, I agree with what you are stating. Apparently I am not being clear on my point. What I am trying to say is if the gauge were connected to the alternator directly, then we should be reading that voltage- say 14.8 volts engine running, and one would assume all is okay, because we would assume the voltage at the battery is 14.8 v. However, if there were a poor connection between the alternator and the battery, the battery voltage may only be 12.8 v, a marginal situation. But, per the gauge, we would not know that. If the gauge were connected to the battery, we would know a problem exists. With the gauge connected, as it is, through various control wiring, including the ignition switch, the gauge is actually not only checking the alternator output, but, to some degree, the ignition switch and the other various wires to the gauge. Now based on that theory, to thoroughly check the system, one would need a gauge connected directly to the battery, the ignition switch, the fuel pump, the coil, etc., etc. Obviously not practical, so I am indicating the factory location is a compromise to check “some” of the car system, and not only the alternator.

As you know, a battery could run down on a running car if the alternator failed. Eventually the fuel pump, coil, etc. would drain the battery, but I know that is not what you meant. However, in the unusual case where there was an odd wiring failure between the alternator and the battery, in theory the battery could continue to start the car, the alternator would keep the running voltage at 14.8v, but maybe that 14.8v would not reach the battery, never recharging the battery. Eventually, the car would not restart. Don’t ask me what that poor connection would look like:)

And to confirm, the indicator light (E-types have no idiots???:slight_smile: ) should be on when the rpms are too low to have the alternator charge over about 12.6v.
Tom

12

Dear Tom,

I’m glad that’s clear. Philip’s advice was good - the conventional “low” reading is misleading because it measures the wrong thing and he corrects that. (Otherwise it’d be a case of “low and lower”.)

1/ I guess we all want to measure and know that our alternators run at 14-14.4v.
2/ The battery should be able to start the car. If 1/ is true, then the battery will not have any other role and won’t be asked to do what isn’t designed to do, i.e. power the car.

In this respect, the old ammeter made some sense, in that it told you which side of a even keel you were on, with respect to where the electrics were getting their power from. An uncorrected voltmeter simply obscures what the real score is.

kind regards
Marek

13

Here a few pics to show what the Volt gauge displayed when powered from my bench power supply at different voltages:

12V:

13V:

13.5V

14V:

Well, I’m not interested in a “wiring integrity meter”. If something does not work due to bad wiring or contacts, I’ll eventually find out what the problem is. I want to know what the alternator is putting out and therefore, as stated in my first post, measuring from the B+ terminal WOULD be the most optimal point, but that requires installation of an additional wire, whereas the Brown wires terminating on fuse block #19 is an adequate 2nd choice.

14

I suppose I have always thought of that gauge as a general electrical system gauge even though it is labeled ‘Battery’. Like some other gauges (oil pressure notably) it is a change in behavior over time that seems more important to me than what the gauge reads when all is well.

As for protection (possibly including protection when an alternator with much higher potential amps is used) I have added a pair a fusible links to the my S2. One is on the non-fused ‘brown wire’ circuits, the other is on the non-fused ‘white-wire’ circuits.

I’m no EE - I just used a highlighter on a wiring diagram to appreciate how much of the harness is unprotected and decided I could improve on that. Don’t think it hurts anything, might even save me someday.

15

So to get a handle on whether the gauge is giving good info, why not just stick a multimeter across the battery and compare measurements between it and the guage? After all, there’s no really accurate graduations on the gauge so it’s really just an estimate.

16

a few years ago I bought a little volt meter built into a cigar lighter on ebay. without arguing voltage drop from this or that it basically gave me a system voltage, like the battery gauge already does. it was so boring I took it out and put the lighter back in. but if you are curious they come straight out of china for about 5 bucks… cheap entertainment.

Bob F

17

HI Everyone,
Interesting conversation from all electrical wizards…great input ! Seeing this is relating to elec issues, my question unrelated to alternator’s…What is the maximum recommended cranking amps battery I can install in my Series III OTS ? Would 1,000 CC be to much amperage for the vintage electrical system to handle ? Or isn’t the CC amps relative to the wiring…Easy Question ??
Thx, Todd

18

[quote=“TMK, post:17, topic:351389”]
…What is the maximum recommended cranking amps battery I can install in my Series III OTS ? [/quote]

Install the most powerful battery per unit cost that can cold start the car on a freezing day.

The car’s wiring loom won’t enter into the equation because the wire to the starter motor and the earth strap back to the chassis are both very substantial and thick pieces of cable - as thick as jump leads. What you mustn’t do is crank continuously for a long time. If it doesn’t catch, let the battery (and warmed up wiring) recover for 10-20 seconds and then try again.

The standard Lucas starter motor will turn over at 120rpm and a gear reduction motor at about 160rpm with a well maintained battery. The limitation on how quickly it fires up will typically be related to how well the fuel atomises and reaches the cylinders after taking its tortuous path through the manifolds and runners.

This can be deduced by firing up an identical specification engine which has fuel injection instead of carburettors - in this case it fires up almost instantly. My car is fuel injected, runs lpg (propane) and has a datalogging ability attached. This means that the fuel is already perfectly atomised at any temperature as it is gaseous and according to the datalogs, it fires up and catches on about the third cylinder to be sparked - that is half a revolution of the crank. From experimentation, the amount of ignition advance it can tolerate on cranking is quite wide, so the conventional carburettored engine’s Achilles heel will be how well it mixes the fuel into the air supply and delivers it to the cylinder rather than letting it condense on the inside of the manifold and runners. Given this, it means the v12 may have to crank for a while when cold so the more powerful the battery when cranking, the better. If it fires up quickly, then so what?. You simply have spare capacity.

I use an Optima Red Top.

kind regards
Marek

19

Thank you Marek for the in depth explanation !!! My current battery installed by the previous caretaker is approx going on 7 yrs and is a 650 CA. Using the AC with lights on the amp-meter goes into negative mode. Otherwise it’s fine. Starts on the 2nd or 3rd crank .
Again, Many Thx,
Todd

20

Why not wire all these points to the voltmeter through a selector switch? :smiley:
Be a great diagnostic tool.

I think you are implying that Jaguar changed from ammeter to voltmeter when they changed from generator to alternator. My 67 S1 has an alternator and an ammeter.

BTW, long ago from the service manual circuit diagram I checked what circuits did and did not go through the ammeter, and found several surprising circuits that did not. One that I remember was that the headlights through the light switch did but through the turn signal/flasher switch did not. Or vice versa.

Personally, I prefer the ammeter over the voltmeter.

Cheers,

Rod