I managed to connect jump leads from a strong battery on a running car with Accidental reverse polarity while trying to jump start a flat battery in the dark on my XJ8.

Hence my year 2000 XJ8 has experienced 12v reverse polarity accross the terminals of an absolutely flat battery. Sparks flew and clearly much energy was exchanged for 5 seconds before I realised.

The car starts and appears to charge the battery. the lights work but the electric seats don't. I will start trying fuses for the seats when I have daylight.


I am worried about diodes in the alternator etc etc . But I am hopeful that the presence of the totally flat battery may have at least partially cushioned the effect by resisting the reverse voltage. Optimism ?


So: How much damage should I expect ???
and Where should I start looking ???


Thanks,

Graham

 

I did the same thing a few months ago. There was no damage done, other than to the jumper cables. They melted in two. I ruined a perfectly good set of 8 gauge jumper cables.

 

Did you also post this on the XJ8/R forum? This is the XK8/R forum.

 

Sounds as though you may be lucky, but I'd cross every thing you have two of, and take up praying to Allah! Computers & the like do have an aversion to such treatment.

Best of British,

Languid

 

Most of the electronics is designed with some reverse polarity protection, so I think you do have reason for optimism and your plan to check the fuses first sounds like a sound one.

 

Blew the radio in a similar situation but not in a Jaguar.

 

I always enjoy your hugely informative videos especially because you don't edit out mistakes.

However, I'm guessing this will be one we aren't going to see ..........

 

You may have gotten lucky and just blown fuses for the seats, and the other items. However, you may also have taken out many of the sensitive mini controllers, such as the seat controller. You are also lucky the air bags didn't deploy. I wish you luck, and I hope the car turns out O.K.

 

There's no such thing.

This is true. Are you sure the seats were working before? The modules to have a tendency to "go bad" due to corrosion of the circuit board over time. This might have happened while the car sat. If this is the case they might be repaired. Are all seat functions out?

Also be aware that modern alternators are not designed for the heavy continuous load of charging a bad battery, you could kill the alternator by doing so. Use an actual battery charger that plugs into a wall outlet. If you were just testing breifly to make sure the alternator was still functioning you are probably OK there.

 

Really?

Mountain 8661074 8 Gauge 12-Ft Booster Cables

 

Just bought an Everstart 8 gauge set from Walmart

 

Thanks everyone,

I;m glad I aposted on your XK board by mistake ! Your answers have been very helpful.

I have found one 5 amp fuse blown on the left kickboard fusebox and one 5am fuse blown on the right kickboard fusebox. I believe these were the seat controls. Also one 10 amp fuse blown. I beleive this was the radio.


I have just rememebred the engine bay fuse box and will venture there tomorrow IN DAYLIGHT.

The alternator creates 13.5 volts at the battery when the car runs with the totally flat battery. ...so I think my alternator etc is intact.

I have also found a 5 amp fuse blown in the boot / "trunk fuse box. It was on the row nearest the front of the car and it was the fuse closes to the centre of the car.
Question 1: WHAT WOULD THIS FUSE BE ?

Question 2:
Would you still be worried about diodes / Relays or is this all sounding hopeful ???

(I will say a prayer to the god of fuses.)

 

Graham,

I still have the Vehicle Care Manual for my 2000 XJ8L although the car has gone!

There's seven 5 amp fuses in the boot fusebox and you'll find they are numbered on the carrier next to each fuse:

2 = fuel filler flap
3 = Audio System
4 = Radio telephone, 'E' post lamps, navigation
14 = navigation
19 = RH side lamps, LH & RH number plate lamps
21 = LH side lamps, L & r side marker lights
22 = Audio / telephone

Can't remember the fusebox layout but you'll be able to determine which it is from the above list.

from another Graham

 

Check all the fuses thoroughly.

All the relays in Jaguars have internal diodes across their coils to suppress electrical noise from the coils. The diodes are normally 'reverse biased' so they do not conduct unless there are negative going spikes to suppress. As a result, when reverse polarity is applied, many of the diodes will conduct and may pull excessive current. This will blow some fuses and may have also damaged the diodes in some of the relays. If a diode in a relay fails open, you probably will never know it likely will be no big deal because the control modules usually have secondary spike protection on the circuit boards. If a diode in a relay fails shorted, it will continue to blow a fuse.

Therefore, if you find any circuits that continue to blow fuses, the first thing to check is for shorts across the coil of the relays on those particular circuits that are blowing fuses. Fortunately these relays are inexpensive to replace.

Download a copy of JTIS (see the FAQ sticky) and you will find both the wiring diagram an a map of relay locations if you need it. Most of the relays are in the fuse boxes.

 

The issue with cheap 8-gauge cables is that they cannot carry starting current without an excessive voltage drop...you may find you cannot start a car with them after jumping unless you charge the battery for a few minutes from the other car. They may also get dangerously hot while trying to start if excessive cranking is involved.

To quote another thread, they may have actually saved your bacon Sam, because they acted as a resistor to limit the reverse polarity current as they self destructed. So I guess they were perfectly good for something

 

Thanks again !!

(I'm assessing whether to just buy a new battery or whether to buy a car with blown engine as parts as there's one local right now. Hence would like to test a few things and don't have the ability to just use the car as the old battery is fully flat and I'm concerned about buring out alternator if i run it too long.)

Hence:
Question !:
Is there an easy way to test a diode ??? I have a good multimeter and would be keen to pull and test at least the obvious ones in the boot / trunk and kickpanels.


Question 2: on the left hand kick panel there was a fuse with a yellow protection cover over it. The protector cover gave me the feeling off .... "keep away" and hence I have not tested it. What would this fuse be for ??

Question 3:
Is there a way to test the relays and is there any need to test these ??


(PS my circa 8mm leads would do indeed fit with the quote above "you cannot start a car with them after jumping unless you charge the battery for a few minutes from the other car. " and electircally I could' understand why they weren't instantly starting the car from the good running car they were jumping from ! Sounds correct !)


Graham

 

Graham,

Re testing the diode, yes. Use the multimeter on a high ohms (resistance) setting, and read one way, then reverse the leads and retest. It's not foolproof, but you should get high impedance in one direction, and much, much lower in the other. If both are high, or (more likely) both low or both are close to each other, it's an unhappy diode.

Cheers,

Languid

 

Problem is he's got a relay coil across the diode so the resistance will be very low in both directions. If he can isolate the diode your method will work. If not the only thing I can suggest is a small (less than 12W) 12V bulb in series with the relay. Connected up the 'right' way round to a 12V battery the bulb will be dim but the 'wrong' way round the diode will conduct and the bulb will be brighter.

 

The correct resistance of the coil of the brown relays is approx. 90 ohms. You are correct in that this low resistance will mask whether the diode wired in parallel is working or not with a multimeter.

You can, however tell if the diode is shorted. If the coil resistance is under 75 ohms I would say the relay is highly suspect for a damaged diode. The coil resistance is measured between pins 1 and 2 marked on the bottom of the brown relays.

 

The joke is that there might be perfectly good jumper cables, or 8 gauge jumper cables, but not perfectly good 8 gauge jumper cables.