p38 2.5 wont start

Derek Ward said:

Hello Old Bean
I fully agree that's what they say and indeed on 99 on the relay is operated by the starter relay but through a diode. The fuel pump relay has a permanent earth so therefore cannot have a polarity reversal. The same diagram also shows the relay being fed from the glow plug circuit, again through another diode. The third feed of course (once again through a diode) is from the alternator. This is on the diagram 1999 Models on - 4th edition. These diodes obviously stop one circuit backfeeding another.
I have 3 other variations - 1995/4th edition / 1996 5th edition & 1997 6th edition. They all show no circuit between the starter relay and the fuel pump relay. I agree with you that this doesn't sound right. Maybe these drawings are incorrect along with the description?
But there is no polarity reversal!

ps Am I in the running for anorak of the month?

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Datatek said:

I agree, I think RAVE is wrong, a polarity reversal would as far as I can see lead to the pump blowing instead of sucking. Needs a bit more looking into

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Don't think so. We are not talking a polarity reversal on the pump power leads but on the relay activation circuit. It matters not which way the current runs on the activation side, the relay will be pulled in no matter what. When the starter is engaged the + power is fed in and earthed by alternator. When the alternator makes current the current flows the other way just like when the ignition light is put out. Then the activation circuit is powered from the alternator. The actual circuit that shuts to run the pump is not affected. A bit surprised by your answer there mate.

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Last edited by wammers; Today at 09:53.

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Derek Ward said:

Which diagram are you actually looking at wammers? Maybe you have a different revision than us (bottom left hand corner of the drawing)?
On the earlier versions for 95/6/7, the excitation feed to the alternator is taken from the ign live feed in the fuse box via a diode D1 and current limiting resistor. This is taken to the junction of the diode D2 and the alternator D+. Current will flow through this diode and resistor combination to the alternator when the ign is switched on. When the engine starts, 12V will be available here and then this holds the fuel pump relay in. Perhaps this is where the notion of polarity reversal has come from. But there is no connection with the starter. The relay has a permanent earth through Pin 2 of C0574. It is not switched.

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Please note it says "Fuel pump relay is energised when starter is engaged". Or words to that effect. There does not have to be a connection to starter itself. I have looked at my rave and do have all the electrical revisions from 1995 on. Without going through it in minute detail the fuel pump switching system is the same throughout. With exactly the same description of operation in 95 as in 2000. The schematics are all basically the same just drawn a little differently. The P38 wiring has lots of areas where polarity changes take place to use the same cable for different functions in different states as far as i can see.

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Derek Ward said:

Aye, I know what it says. It's not the same throughout though. It changes for the 99 on where it is switched on from the starter relay. 95/96/97 show a connection that could be linked but is actually shown as 'Not Used'. I think this is quite possibly wrong and it is actually used so that the pump runs when the starter is on.
The only circuits that I can think of that use polarity change are the ones with motors such as windows and mirrors etc but that's all.

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wammers said:

To be honest Derek all i know is the pump does not start until the key is in position three and when the key is released and the engine is running the relay is held down by current directly from the alternator. .

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Datatek said:

I agree with that, but as far as I can see there is no polarity reversal, just a different power source.

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To be honest Derek all i know is the pump does not start until the key is in position three and when the key is released and the engine is running the relay is held down by current directly from the alternator. That is how it has worked from day one according to RAVE. If it is switched internally by the BECM on earlier versions i do not know. Trying to trace through the wiring diagrams is a fools errand who knows what positive negative switching is done inside the BECM. And as long as it works who cares.

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Derek Ward said:

Aye, fair enough wammers. It's just me being pedantic about drawings. I'm one of those sad people who actually enjoys tracing through them. As you say, as long as it works. What's on the agenda for the next 'discussion' then?

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Datatek said:

Sorry, I'm another drawing pedant. The text is wrong in RAVE.
Relay 12 the fuel pump relay has 3 possible 12 volt sources to energise it, via D3 from the glowplug timer unit output GK6 which operates the pump during the glowplug phase; via D4 and relay 16 the starter relay when the starter is operated; and finaly via D2 when the alternator starts charging.
As Derek said, the diodes are to stop back cicuits. The other side of relay 12 is hard wired to ground so there is no question of polarity reversals.
The three diodes form a logic "OR" gate funtion

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wammers said:

Well then Keith, if RAVE is wrong it's been wrong for a very long time. Maybe everybody should ask Land Rover for a refund.

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Datatek said:

Sadly there are a lot of mistakes in the the text of RAVE, at least in the bits I have read, the schematics on the other hand seem pretty accurate.
Mistakes in the text of technical manuals are not unusual as they are not written by the designer, the schematics on the other hand are generated by the designer and if the circuit doesn't work it gets changed whereas the text often does not.

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