HI, new to this so here goes, have a 65 jaguar s type 3.8 auto, been sat for years, i acquired it, cleaned out the carbs/fuel tank etc, starts and runs, had an aftermarket cold start system fitted battery /ignition is on negative earth. i have working ignition / headlights/ wipers/ fuel pump/ interior instrument panel lights. i changed the flasher relay and then the flashers worked, washer pump worked, brake lights worked , great all working as these are on same circuit ! instrument lights did not, moved bulbs around and some light up, so cleaned contacts , all lights except tacho light up including the gear change light, then suddenly it melts the no 3 fuse? one to flasher relay every time i turn igntion on. instrument lights still work but as per diagram on different circuit, so why fry the fuse when they started working. ammeter is running backwards and horns have been disconnected previously , possible only half the circuits have been switched properly to negative earth? causing incorrect flow through relays/circuits? have no clue where to start any help to figure out or trace what may be happening would be appreciated.

 

if the generator was polarized correctly for negative ground, no need to change anything except to reverse the ammeter wires.

go to JCNA.COM and search the library for Negative Ground.

then to do an actual re-polarizing of the generator, go to STAR AUTO ELECTRIC.COM for instructions on re-polarizing to negative ground.

exercise the instruments lights switch since they tend to get asleep in the off position. The switch has three positions, off / low / medium.

the circuits are very simple. If you need the factory service manual, or wiring schematic, go to my website and download it free.

https://classicstypeparts.webstarts.com

 

Some of the later clocks have a diode that needs to have its polarity reversed. I think it's in series with a resistor so shouldn't cause the problems you mentioned.

 

Peter it is well known that the clocks stopped working the minute you drove away from the dealership.

if dead, send it to Mike Eck at www.jaguarclock.com to get it working again. Amazing my '65 has a working clock fixed by Mike.

 

thanks for the reply, assuming it has been changed to negative earth as the battery is fitted that way and car starts, but dont think wires on ammeter have been swopped, dimmer switch does work ok, its the melting the fuse that controls the flasher relay,washer pump and brake lights that has me confused as it only started doing this after the instuments lights came on and these seem on a different circuit to the flasher etc according to the diagram, tacho wires to clock have been disconnected that also show its been swopped to negative earth but not completely as ammeter round the wrong way, would this cause problems of blown fuses on the flasher/ horn/brake light circuit?

 

thanks for the reply, assuming it has been changed to negative earth as the battery is fitted that way and car starts, but dont think wires on ammeter have been swopped, dimmer switch does work ok, its the melting the fuse that controls the flasher relay,washer pump and brake lights that has me confused as it only started doing this after the instuments lights came on and these seem on a different circuit to the flasher etc according to the diagram, tacho wires to clock have been disconnected that also show its been swopped to negative earth but not completely as ammeter round the wrong way, would this cause problems of blown fuses on the flasher/ horn/brake light circuit?

 

Anything is possible but I think it might be a ground wire or a hot wire misconnected somewhere. That's why fuses blow, a short or an overload.

all you need to invert is the ammeter wires after re-polarizing, and of course the battery cables, but that is it, nothing else, assuming this is an American car, because certain fuel pumps are positive ground only. But if the pumps are running fine, leave them alone.

I suggest you re-polarize, it only takes 10 minutes and all you need is a short piece of wire for a jumper following the instructions at Star Auto Electric. Besides, you must re-polarize the generator every time the battery is disconnected and reconnected.

How do you know the cold start carburator is aftermarket? These cars came with an electronic cold start carb.

Each horn has a purple and a black wire. The black wire goes to one of the bolts with a ring terminal, and the purple wire to the horn with a spade terminal. You can test the horns (claxon) easily with two wires at the battery. I'm sure you know that. One is a high note, one is a low note, both sounding together produce a Major Third Interval. (I am a musician too). in my car the horns were bad and I got a pair of Jaguar XJ-6 1985 which sound identical to the original horns. Order good used ones from David at www.everydayxj.com if you need them.

 

I would disconnect the circuit that you think is at fault and see if that stops the problem, assuming it does then you need to test the wiring and find out where the fault is.

I would expect the fault would be somewhere before the flasher relay as when the wiring after this is shorted you tend to find the relay clicks really fast as its a bimetallic strip that opens and closes as it gets warm when load flows through it.

The wiring diagram and a multimeter will be your friend, have a look at the loom for any areas where its been messed about with by a previous owner as those tend to be the most likely cause of issues followed by inspecting the loom along its length for anywhere that it would have chaffed through.

Happy hunting!

 

Swap over that Ammeter. Lucas Ammeters can play up big time. I kept my car positive earth at restoration & when we first started the car it internally shorted the correctly wired ammeter, Gauge glass immediately went white & the thing overheated. Had a little puff of smoke out of my newly rebuilt generator/dynamo. Shut things down quickly. Fitted another ammeter & all has been fine since.

On these cars there are just 6 units that are polarity sensitive. The clock, the windshield washer pump, fuel pumps (if original), the ammeter, the coil & the radio if fitted.

Good luck with tracing your gremlin. Note edited above now that I'm awake:

Polarising Generator/Dynamo. S Type RB 340 on right.




I keep a laminated one of these on my garage wall. High Res attached.

 

That is only four Glyn. Did you forget the Radio?
You also need to change the earth wire over on the original coil and distributor unless as I did you change to electronic ignition which will be + or - and you have to install the correct one for your polarity. .

 

Hi, thanks for all the replys again , think it may give me a place to start on, car runs on negative earth, so has been changed over, fuel pumps work, wash pump has been changed to new one so and works so assume on negative, clock is disconnected ,horns disconnected, ammeter works but backwards, melts fuse to indicator relay when i got the dash instruments lite up, was fine and all working before after changing the flasher relay. so i assume by this the horns and clock are still positive- why the got disconnected, so hopefully following helpful suggestions, most likley ammeter problem as its wired into ignition and follows through to dimmer switch aswell? i'll try swopping this over first , had to replace creeper switch aswell as this was leaking/would this cause a problem if i didnt wire this for negative earth ( put back as it was? ) any and all suggestions greatfully recieved

 

Although the ammeter needs to be swapped so it reads correctly it wouldn't make any difference to the operation of the wiring system. All the ammeter does is measure the flow of amps across it and if they are going into the battery its charging and out of the battery is discharging, if its the wrong way around it just reads backwards which makes no difference and wouldn't cause any fuses to fail, I don't even think this circuti is fused.

 

Looking at the wiring diagram above fuse 3 seems to serve a few systems, I would disconnect the output from the fuse carrier and then measure each circuit and see what the resistances are on them, in theory they should all be open circuit (assuming that the car isn't in reverse, indicator switch isn't on etc.) and the one which isn't would be the problem.

The fuse seems to serve the following on each wire:

1. Indicators which also serves the reverse lights on an automatic model.
2. Reverse lights on a manual model and the overdrive circuit.
3. Screen wash
4. Horn which also loops to brake lights.

If you can narrow down which one has the issue, then reconnect the other three and see what above doesn't work you will then know which circuit has the fault and can then start tracing it through.

 

many thanks to everyones help, looks like a wire by wire trace ...

 

It was the early hours of the morning & I was half asleep. The coil or radio was No 5. I don't remember. I think radio. Should be 6 as I've edited above if radio fitted.

Absolutely the CB & SW connectors should be reversed. Reverse the coil connections: if the coil is original, then it will be marked CB (contact breakers) and SW (switch). The SW terminal should now be connected to the distributor. In the case of the modern coil, the negative terminal should be connected to the distributor (points). This allows the spark to jump from the central electrode to the L shaped side connector on the spark plugs.

Sorry Rob. I must not answer questions at that hour of the night.

 

Absolutely correct as long as the Ammeter is good.

 

Well I forgot the windscreen washer pump and the fuel pumps, neither of which are OE on my car. It never had a radio - the soundtrack is the engine. The clock worked fine from 1963 to 1977, when the car suffered (literally) a body restoration. After that, the clock has happily sat at afternoon tea time.

The only way to find the short is eyeball, ohmmeter and elimination. At least it's a fairly simple and accessible system compared with a modern car. I had a similar problem a few years ago with my DD6 (XJ12 outside the UK). Someone had tidied the lead from the sensor on the gearbox to the speedo by wrapping it around an exhaust pipe. I guess he'd forgotten that exhaust gas is hot.

 

Rob.

Fortunately many radios of the era were switchable polarity. My NOS unit is.

Now that I am awake do my comments in the 2 posts above make sense? Did you experience any trouble with your windscreen wipers parking? The motor will run fine. I think the reverse parking will too.

 

wipers work fine, coil wires have been hacked about ( i mean carefully replaced by a previous owner ) cold start has a modern upgrade? stat for the cold start is disconnected, so are several wires and a newer solenoid type has replaced it/ probably 30 years old cut and soldered in badly to some wires in engine bay. today disconnected horn fuse goes , disconnected flasher fuse goes , washer pump disconnected fuse goes , have not taken off brake light switch yet so? but as it connected to the horn circuit? if that doesn't stop it and i don,t think it will it must be shorting threw another connected circuit? horrible feeling it going to be a complete rewire job...

 

A complete wiring job is close to impossible unless you are doing a ground up restoration. I replaced every wire & harness in my restoration. Harnesses thread through the centre sills that an S type has, up A & B pillars etc. etc. You know what's behind the dash & in the engine compartment. It would be a nightmare. You might have to enlist the help of an auto electrician though to safely sort your present problem. The S Type has many loop backs. Many terminate at the jumble of connectors at the wiper motor.

The reason I rewired was a friend restored a Mk2 ~ beautiful job. He knew his harnesses were in a bad way & the car burnt out due to a short.

Side harness example.

A Register member's car under restoration. Outer sills cut away. Centre sill has holes in it. Harnesses are obvious.