P38 Range Rovers were advanced for their years - a lot of electronics in them that were new and scared off a lot of 'old school' mechanics where wire went from a battery, through a switch and/or relay and then on to whatever it was controlling.

The addition of a BECM (Body Electrical Control Module) which basically piled a processor, bunch of MOSFETs, relays and input/outputs was like a magic voodoo box to a lot of them, but in reality (and compared to the newer ones!!) the electronic systems on a P38 aren't that complex.

P38's are notoriously power hungry, and sensitive to voltage, especially from weak batteries - or the BECM not sleeping due to RF interference issues, which seem to be a common problem the world over. The most common 'telltales' of voltage problems are 'GEARBOX FAULT' on the dash, sometimes accompanied by other warning lights, 'ABS FAULT', 'TRACTION FAILURE', or 'ALTERNATOR FAULT' (which isn't a sign of the alternator failing in every case)

Below are some tests, and expected outcomes for them to help try and diagnose where your issues lie in the starting/charging system. If you have battery drain problems, then it is worth doing these aswell, to make sure that the charging system is working properly. Once that is ruled out then other avenues can be explored (I am sure there will be another thread soon on testing for BECM sleeping and battery drain!)

These tests are a simplified version taken from other RR forums, and also a TSB (Technical Service Bulletin) from Land Rover on the subject of battery drain and charging problems.

If you have electrical/starting/charging problems, please help up to help you and do these tests BEFORE posting, and include these in the first post with a description of the problem, and what you have done so far. It will help narrow down issues and give you a quicker resolution!

  1. Battery voltage, Engine off - before starting:
  2. Battery voltage, Engine started, idling, with no electrical load:
  3. Battery voltage, Engine running at 2000rpm with no electrical load:
  4. Battery voltage, Engine running at 2000rpm with electrical load (headlights/high beams, blowers on full, heated seats/screens on (where fitted)):
  5. Voltage drop from the alternator body to battery NEGATIVE post, Engine running at 2000rpm with electrical load. Use the DC millivolts range on your multimeter:
  6. Voltage drop from the alternator positive terminal to battery POSITIVE post, Engine running at 2000rpm with electrical load. Use the DC millivolts range on your multimeter:

Some explanation/results you should expect to see from the above tests;

  1. A fully charged battery, with no load on it should read approx 12.6V. With a power hungry RR attached to it, and the BECM awake, then I would expect to see 12.1V and above. Less than 12V means the battery is starting to go flat and whilst it might crank and start, the battery is either not being charged properly, there is a very heavy drain on it so it is never fully charged, or it's starting to fail. Try a decent battery charger on it and see if it will charge up again fully.
  2. This shows a basic voltage that you are getting back to the battery with the alternator spinning. Ideally at idle you should see somewhere from 13.8v-14.1v depending on what is running and the condition of the alternator/cables/battery. At this point, as long as it's above 12.6V (a fully charged 12V battery) then you are getting something put back into it.
  3. You should now be seeing full charging voltage - for a P38, this is usually between 14.1 and 14.4V. much more than that and you run the risk of overcharging and cooking a standard lead acid battery. These systems weren't designed to use some of the newer battery technologies which require a higher charging voltage (14.8V) if you have one of these batteries, then it will never fully charge as the alternator isn't set that high and it would be worth swapping the battery back to a normal one which is designed to have the lower charging voltage.
  4. This will help tell you if the alternator/battery cables are up to the job of providing enough power to charge the battery AND run any ancillary electrical devices in the vehicle. In a healthy system, you should see little, or ideally no voltage drop when the system is running under load. If you have a larger drop (voltage less than 14V) then there is an issue somewhere - either with the alternator or the charging cables. The next tests will help find out where.
  5. This test the voltage drop along the earth straps. In a good system you should see about 30mv. According to the LR TSB, up to 130mv is acceptable, but anything over 130mv is a fail and the strap needs to be replaced. Lower is better - as this is how much is being lost in the cable, and affects the current carrying capability of the cables aswell. Even if the cable visually looks fine, it can corrode internally and cause electrical problems and charging issues.
  6. This does the same, but for the positive terminal. Again 130mv is the MAXIMUM, ideally again about 30mv is great to see.. GEMS models are particularly bad as the charging cable from the alternator goes from the alternator, down to the starter motor and then back up to the battery - nearly 2m of extra cable to try and shove the power through. Also if the start motor gets oil dripped on it over time, the connections there can start causing problems. I recommend GEMS owners to either buy or make an additional charging cable to run directly from the alternator to the battery terminal. This gives a direct charging path and can help immensely on GEMS vehicles. (Later Thor models already had the charging cable run directly)

Thanks to Greg Hind for posting this up originally on one of the other forums. I have adapted/streamlined it a bit here and added my own observations!