Electrical Issues - Basic Troubleshooting Guide
Common symptoms
- fault messages such as alternator or gearbox failure
- lights flashing even when switched off eg headlights and indicators flashing like a police car
- windows, sunroof not working and/or cruise control not working
- these expensive batteries are not lasting as long as they should (eg one year instead of seven)
- lots more examples of electrical things not working, fault messages, strange behaviour etc
- most of these tests apply equally to all 12 volt cars but details have been specified for the P38 RR.
P38 Range Rover electricals 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 Test
- 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 get a really good one (I no longer recommend Optima BTW)
Charge the battery with a good 3 stage or better charger. A cheapie standard charger won’t do. This charger will more than pay for itself in battery replacements. I use a 16 amp 7 stage Projecta brand charger on 8 amp mode to charge my RR battery.
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. If it is 14.1 or less volts, it needs to be fixed.
You can repeat the test at idle 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
Check the alternator directly
You can repeat these tests to measure the output of the alternator directly. 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).
Check your cables 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.
For more details on this test, you need to read TSB 860396. This can be found on RAVE 11 (NOT 16) 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 don’t 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, I’d 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
Worksheet
This is a cut down version of the tests, see above for detail on how to complete each test.
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__________
11. Voltage from alternator positive to alternator housing
What are your symptoms
What have you done to fix it so far
If the rest of this document doesn’t give you the answers you need, start a new thread and include the results for each of the 11 tests in your post. Detail your symptoms and work done so far. Don’t skip any tests.
How to Interpret Tests
Don’t ask if X volts is OK when this sticky already tells you the range it must be in. Voltages MUST be in the specified range to be OK and if they aren’t you have found at least part of your problem. Eg where I say you need 14.2 volts, I mean it! 14.1 volts is not acceptable. Where 120mv is specified as the maximum acceptable voltage drop, I mean 121mv or more is NOT OK. (120mv = 0.120 volts). It is possible to get voltage drops down to 30mv or so for example and 120mv is the WORST allowable not an approximation.
How to fix it
There are 10 tests above. Here is how to fix each step
1. If less than 12.5 volts, charge with a really good 3 stage charger. The find out why the car isn’t charging the battery itself. Complete the rest of the tests.
2. Keep doing other tests
3. Keep doing the next tests
4. If this is 14.2 volts to 14.4 volts your charging circuit is currently working as designed – if you are having problems rig up a volt meter to watch while you drive or try a different battery.
5. If your alternator is producing 14.4 volts but your battery is receiving less, then you have a voltage drop problem in one or more of your cables. Keep completing these tests to find out where
6. If this voltage is less than 120mv (eg 70mv) it is acceptable. 120mv is the worst result that is acceptable and it can be improved by cleaning connection points or replacing the cable with a bigger one. 121mv or more (eg 206mv) is NOT acceptable and should be fixed. The fattest possible cable is best.
7. Same as point 6 except you are now testing the positive cable. The fix is also to clean or replace with a bigger better one.
8. There are two earth cables between the alternator and the battery and either or both could be part of your problem. This test helps you find which one has the problem. There is no harm in cleaning all connections or replacing all cables with bigger ones.
9. As above.
10. As above
11. This test tells you if your alternator is producing 14.4 volts or not. Strictly not essential to do this test as the numbers above let you work it out mathematically but it gives you a quick idea. For example if your alternator is producing 14.4 volts but your battery is only seeing 13.8 volts then you can see there is a voltage drop somewhere that needs fixing. However, if your alternator is producing less than 14.4 volts, fix all other issues above before replacing it as there is probably nothing wrong with your alternator.
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