Electrical Issues - Basic Troubleshooting Guide
Information compiled by Greg Hind (p38a Forum)
Greg kindly put this post together a while back. It's proved to be invaluable for members in the p38 forum and so it's been re-jigged for use in the other Range Rover forums.

Since every week there is a thread on some kind of electrical question, I thought I'd write this up.

Common symptoms include:
- fault messages such as alternator or gearbox failure
- lights flashing even when switched off - e.g. Headlights and indicators flashing like a police car
- windows, sunroof not working and/or cruise control not working
- lots more examples of electrical things not working, fault messages, strange behaviours etc.

This is also a very useful pre-purchase test.

Later electronic dependent Range Rovers are very sensitive to voltage. You need a good strong alternator, clean earth points, healthy cables and a very strong battery. Testing takes about five minutes.

Battery Tests:
-Turn the ignition key to the on position but DO NOT start the car.
-Turn on all electrical accessories (max heater blower, high beam lights, demister etc)
-Leave accessories on for 30 seconds
-Turn off all accessories and turn key switch to the off position
-Within one minute, measure battery voltage.

If the battery voltage is less than 12.5, the battery is not healthy. The battery may need to be replaced, however there is no point doing this before you are certain it is being charged properly. Weak charging circuits kill batteries pretty quickly. If you do need a new battery, the general rule of thumb is to fit the biggest the car battery box will accept.

Check the charge circuit with no load:
-Start the car
-Measure the battery voltage at 2000rpm (not more, it is not good to spin an engine or a ZF automatic transmission with no load)
-Voltage should be 14.2 to 14.4 volts. Anything less provides only a partial charge
You can repeat the test at idle. The voltage should be similar.

Check the charge circuit under full load:
-Start the car
-Turn on high beams and demisters
-Hold at 2000rpm (not more)
-Voltage should be 14.2 to 14.4 volts. If it is, your charging circuit is fine and your battery should be charging properly.
At idle, the voltage with such a high load will be less and this test may not tell you much

Alternator Tests:
You can repeat these tests to measure the output of the alternator directly:
-Start engine
-Hold the negative multimeter probe on the alternator body and the positive on the terminal on the back. Measure voltages as above.
Be careful as there are spinning belts, large currents (potential for the probe to short things out etc).

Cable/Earth Tests:
Finally, check your earth cable and positive cable for voltage drop:
-Start the car
-Turn on high beams, demisters, max heater blowers, heated seats etc.
-Prepare to measure the voltage between the negative battery terminal and the alternator body. Yes, I know they are both earth points and you may think there will be no voltage but in fact there will. What we are measuring is the health of your earth circuit.
-Hold at 2000rpm (not more)
Voltage should be less than 120mv (0.12 volts). If it is higher, clean earth points and or replace cables.

NOTE: p38a only - For more details on this test, you need to read Technical Service Bulletin 860396. This can be found on RAVE 11 (NOT 16) http://rangerovers.net/forum/viewtopic.php?t=6299 see point 1 and download RAVE 11. Install and navigate to technical service bulletin section, Electrical, 1996.

Perform the same test on the positive cable.

Note that alternators dont always just fail, the can become irregular. They can test perfectly sometimes and not others. If you suspect this, rig up a cigarette lighter plug to multimeter adaptor and check it as you drive as often as you like.

As far as fixing it all, Id start at point 1 and if that component tests bad replace it. Stop working down the list when all tests are good
1. clean earths
2. replace cables
3. replace alternator (non OEM units reportedly can give a lot of grief so genuine is recommended)
4. finally replace battery

Forum Posting - Posting Results:
If you haven't yet solved your problem and want help, please copy and paste this information below into a new thread. Please fill in answers for EVERY POINT. This is a cut down version of the above tests.

1. Engine off, all accessories on for 30 seconds, then off. What voltage ______
2. Voltage across battery at idle (no electrical load) ____________
3. Voltage across battery at 2000rpm (no electrical load) ________
4. Voltage across battery at 2000rpm (everything electrical switched on) _________
5. Voltage at ALTERNATOR at 2000rpm (everything switched on)___________________
6. Voltage drop between alternator body and battery negative (YES negative). Measure at max electrical load and 2000rpm ______________ (millivolts please)
7. Voltage drop between alternator positive and battery positive. 2000rpm, max electrical load _____________
8. Check Voltage drop from Battery Negative terminal to 1st groundpoint on vehicle______________
9. Check voltage drop from 1st ground point to block__________
10. Check voltage drop from Alternator Housing to block__________

Also:
-What are your symptoms
-What have you done to fix it so far

Note: Check battery voltage before you start any testing.
Do not hold 2000RPM with the autobox models for longer than necessary.