reving up and gearbox fault?

my 1998 diesel p38 has a little problem that only happens every so often from hot some time it will start normally but some times it will start but revs hard the rev counter dose not work so i turn the engine off and start it again and it will start normally but flashes up gearbox fault then goes off but if i drive it after the message gose out it will only go into 2nd or 3rd gear but if i turn it of then start it again all goes back to normal until the next time it decides to play up can anyone help me please

poor connections on alternator ,battery etc

if its a poor connection from the alternator, battery etc how dose that cause it to rev up and cause a gearbox fault?

Hi Micky,i have exactly the same fault with mine and at least one other forum member has the same.I have replaced battery,alternator, earth cables etc,but still the same.Since December last year i have been turning the key on and off twice and only starting on the 3rd turn of the key and although it works it is only a work around im afraid. Still have not found a definite cure for it,so good luck if you find out what it is and please let me know the outcome as it is only a niggle but would rather sort it correctly if possible.

hi iam glad iam not the only one too course if i find the problem and cure i will let you know with pleasure mine only seems to do it now and again when the engine is warm

just an update i have checked the battery terminals alternator terminals ans starter turminals thet are all clean and tight i have put a larger amp battery on (the biggest i could get that would fit in the carrier) if anyone can suggest anything else i could try please reply i need all the help i can get with this problem this is the biggest one i have had with both this one and my previous p38

Seeing as the throttling is controlled by the ECM and it has a link to gearbox for reducing torque on gear changes, that maybe the direction to be looking.

Dunno if i am barking up the wrong tree but the problem always sorts itself with the 3rd turn of the key,whether you turn it on and off twice then start it on the 3rd turn(as i do) or start it twice then on the third start it is ok ???

It seems that -for no apparent reason- 3 is the magic number and maybe one of the BECM gurus can let us know if this 3 turns of the ignition key has a re-setting effect on something within all the electrickery and gismos on these cars.
If yours is the same as mine Micky then its.
Turn 1= high revs,no rev counter,alternator fault displayed on message center.
Turn 2= gearbox fault displayed on message center,car pulls away in 2nd or 3rd and stays there.
Turn 3= no faults ,car behaves perfectly.......till the next time

No doubt that 3 is the magic number ,but why ????????????

Check the output on your alternator is above 14.2v
Anything less makes the becm throw out weird fault codes ie alarm/gearbox fault.

yes rodntod mine is the same as yours but on the first turn i dont get the alternator fault
just the hight revs and no rev counter next time mine plays up am going to get my mate to plug it up to his diagnostic machine and get the fault code and see what the fault is. as soon as i find the fault and cure i will keep you all posted if some one dont beat me too it

RE8ELD0G said:

Check the output on your alternator is above 14.2v
Anything less makes the becm throw out weird fault codes ie alarm/gearbox fault.

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The alternator output cannot be above 14.2 volts on most P38s. That is the maximum charging rate. Excepting for the 1999 on 4.6 with the 150 amp alternator which is 14.5 volts. If you have a pre 1999 P38 that is charging higher than 14.2 volts, it is over charging and you need a new control unit.

where abouts is the control unit? i will check the alternator output voltage tomorrow it should not be above 14.2 volts and i assume it should not be less than 12 volt?

battery

micky22 said:

where abouts is the control unit? i will check the alternator output voltage tomorrow it should not be above 14.2 volts and i assume it should not be less than 12 volt?

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Nominally around 13.8 to 14.2. Control unit is in the alternator but can be changed reasonably easily. Any less than 13.8 when systems are being used will give you problems. 13.8 at idle just after starting and 13.8 to 14.2 at 2000 rpm with heater fans and head lamps on.

wammers said:

The alternator output cannot be above 14.2 volts on most P38s. That is the maximum charging rate. Excepting for the 1999 on 4.6 with the 150 amp alternator which is 14.5 volts. If you have a pre 1999 P38 that is charging higher than 14.2 volts, it is over charging and you need a new control unit.

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That info is exactly correct as long as the alternator is original and has not been refurbed or replaced with one with a higher regulator set point. As Wammers said in another thread, up to 14.8 volts is OK with a low maintenace battery.

Datatek said:

That info is exactly correct as long as the alternator is original and has not been refurbed or replaced with one with a higher regulator set point. As Wammers said in another thread, up to 14.8 volts is OK with a low maintenace battery.

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Yes Keith just so. You would think that battery makers and car makers would make this information better known. I am sure a lot of problems are caused because of the mismatch one way or the other, that not a lot of people realise is there. Even some garages maybe.

wammers said:

Yes Keith just so. You would think that battery makers and car makers would make this information better known. I am sure a lot of problems are caused because of the mismatch one way or the other, that not a lot of people realise is there. Even some garages maybe.

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Sadly I think it's all garages Tony, even some of the alternator overhaul shops have no idea why the regulator set points vary:scratching_chin:

ok thankyou for all this info i will cheack the voltage in a few mins if the voltage is above 14.2 volts could that also be the cause of my problem

Datatek said:

Sadly I think it's all garages Tony, even some of the alternator overhaul shops have no idea why the regulator set points vary:scratching_chin:

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Think you maybe correct Keith to be honest i only realised it in our discusson on the subject a few threads ago. Most people and it would seem battery suppliers, alternator refurbers and the like just think a battery is a battery and an alternator is an alternator.

micky22 said:

ok thankyou for all this info i will cheack the voltage in a few mins if the voltage is above 14.2 volts could that also be the cause of my problem

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If the voltage is in the range 14.0 to 14.8 volts that's will not adversly affect any of the cars systems. At 14.8 volts it will boil the liquid off a normal refillable battery but is OK for a maintenace free battery.
It would seem that 14.2 volts is the correct voltage for the P38 with the original alternator but could be as low as 14 volts at the battery, any lower and you may have problems especially if the car is used for short runs.