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mrfixitrick, Mechanic
Category: Car
Satisfied Customers: 143
Experience:  Over 35 years of full-time, hands-on, automotive repairs to all makes.
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Positive battery terminal gets hot. Why?

Customer Question

Positive battery terminal very hot. Alternator light is on, new alternator was installed.

Submitted: 8 years ago.
Category: Car
Expert:  mrfixitrick replied 8 years ago.

What vehicle is this?

Could you tell me more details about why and when the new alternator was installed?

Is the alternator light still on with the new alternator?

What is the battery condition and how old is it?

Customer: replied 8 years ago.

02 Jaquar x type. New alternator was installed yesterday, because it was not working. Light is still on, battery is not old. Don't know exactly how old, this is my son's car. Battery is in good condition though, it seems.

My husband just took the battery out of his car, which is newer, and the problem still remained. So it must not be the battery.

Expert:  mrfixitrick replied 8 years ago.

The voltage drop of 1.1 volts seems excessive and is likely part of the overheating terminal problem. Voltage drop=Resistance=Heat.

Follow that voltage drop, starting from the battery post and terminal should find a sudden voltage drop somewhere between the battery positive post and the alternator connection. There may be a bad terminal connection or fusible link, for example. Does that heavy positive wire to the alternator look at all overheated?

Especially check the voltage drop between the positive post of the battery and the wire terminal connector to it. (leave connections connected for these tests) If volt difference is more than .2 volts, it is excessive. Corrosion, scale or a loose connection will cause the terminal to overheat. This can also cause the alternator to put out excess voltage to compensate for the additional resistance.

Check the battery voltage with key off... it should be more than 12.4 volts. If less than 12.4 volts, then there is a problem with the battery cells, or the battery needs charging. We can likely assume, since the battery was swapped with a known good one, that this is ok. But keep in mind some alternators have a cut-out circuit that prevents charging if the initial battery voltage is too low.

The 14.4 volts you get when running the engine indicates the alternator is charging the battery. Does the new alternator still charge like this?

If a low battery was charged up with the new alternator, then it's possible the alternator overheated it's light indicator circuit. A low battery should be charged with a separate battery charger to bring it's voltage up to spec first. Using a new alternator to charge up a dead battery can rapidly burn out the new alternator.

It's possible a fuse has blown and causing the charge light to stay on, so checking of fuses especially if labelled "engine" or "charge" would be good to do.

Check out these things and get back to me at this same question. This question can be returned to by you at any time.

Here is more input for your alternator warning light problem...
Replacement of the alternator with an aftermarket (non-jag) unit may make the light come on. Also, if the light comes on, a fault code may be set in the ECU. Doing a computer ECU scan may help isolate the alternator light on problem.
If you can get a scan done and let me know the results, I can diagnose or advise further on this.

If you wish to look deeper into further diagnosis, here are a couple of interesting, but very lengthy, Jaguar procedures to deal with this seemingly simple alt light issue. (note these procedures are for X type, but will be similar to yours) note also you may have to register to the forum to view these pages:

Hope I haven't overwhelmed you with info...sometimes things aren't as simple as it seems!

Some folks have reportedly had the Jag dealer replace 3 alternators and a couple of batteries to try and solve the problem with this. On the other hand, another person simply tightened the 10 mm screw on the battery terminal that connects other wires to the battery terminal, and got the light to stay off. Try that first...I'm hoping for a simple answer!

Customer: replied 8 years ago.

My husband will not be able to try any of these until the weekend. Will let you know.

Sorry it took so long to reply. We have tried everything. Even yesterday, had the electrical system checked. All is ok, but light still on. Also, now the drivers' door auto lock doesn't work.

Have no idea what's going on.

Expert:  mrfixitrick replied 8 years ago.
The alternator light on the Jaguar works through and is controlled by the ECM (electronic control module).
Here are two procedures from above link at to check it:

Connect the ECM electrical connector EN16 and the generator electrical connector EN49.
Clear the DTC.
START and RUN the engine at 1500 RPM with no electrical load applied.
Does the generator charge warning indicator lamp illuminate within 30 seconds of the engine being started?
INSTALL a new generator.
TEST the system for normal operation.
If the DTC is repeated, INSTALL a new ECM."

Test #2
RUN the engine at idle for 15 minutes and then increase the engine speed to 1500 RPM for one minute with no electrical load applied.
Does the generator charge warning indicator lamp illuminate or is the DTC repeated?
INSTALL a new generator.
TEST the system for normal operation.
If the DTC is repeated, INSTALL a new ECM."

So, basically, a scanner must first be used to reset the DTC's(Diagnostic Trouble Code) through the computer ECM. You may need to go to a dealer or Jag specialist to have this done. According to above info, if a new generator with an ECM reset does not solve the problem, then the ECM should be replaced.

I would suggest you reset the DTC's, and if alt light does not stay out, find a known good ECM and replace yours with it to see if the light goes out.
Customer: replied 8 years ago.
I'll give him this info for this weekend. So Sorry for the delay.
Expert:  mrfixitrick replied 8 years ago.
No problem on the delay. You can return to this question when you need to.

I hope you find this last info useful, although the possibility of replacing the ECM to solve the problem is a costly solution.

Good luck, and let me know how it goes.