New alternator - wrong rev reading.

Changed the alternator today for a recon one. It's cured my intermitant rev counter but now it under reads. It ticks over around 500 rpm (which it obviously isnt) and 2k rpm feels like around 3k rpm.

All wires are present and correct.

Any ideas

Are the pulleys the same size? There's notalot in it but I believe there's two different pulley sizes for Disco alternators.

That's it, the pulley. Just been thinking while having my tea, thats was the difference between the two. The one on the re-con was bigger. I'll swop them over when I get chance.

Cheers.

I have the same problem, Lr changed the pully size (smaller)sometime in 1996 to give a better charge rate when the car is ticking over. because my car is a 96 model and I didnt have the chassis number to hand I took a guess......and picked the wrong one, It runs and charges fine but the tacho under reads but it doesnt seem to cause me any problems so its staying like it is cos when I tried to swap the pulley's over they wouldnt fit!!!

Oh there are also two sizes of belt, make sure you fit the right one to match the pully size or it will slip

AHHHHH, I just change the alternator about a month ago along with the timing belt on my 300tdi (I say I CHANGED... I meant I got someone else to change )

I have noticed that doing 70-80 on the motorway the rev's read much lower, it won't rev past 3,250, I cleaned the sedimentor and checked pipes and still won't rev past that mark, but having said that, it sounds like if it did rev higher it'd blow up

This could explain it??

stretchie_ said:

AHHHHH, I just change the alternator about a month ago along with the timing belt on my 300tdi (I say I CHANGED... I meant I got someone else to change )

I have noticed that doing 70-80 on the motorway the rev's read much lower, it won't rev past 3,250, I cleaned the sedimentor and checked pipes and still won't rev past that mark, but having said that, it sounds like if it did rev higher it'd blow up

This could explain it??

Click to expand...

Ditto.

Same on mine. I also occasionally get the needle jumping a little bit.

As it happens I still have my old alternator with the smaller size pulley on ... if anyone wants it, and assuming I can get the pulley off, pm me an address and paypal a couple of quid for postage (Paypal email address) ...

Start the bidding at a tube of Smarties ...

Right... New problem! Just about coming to the end of my tether with problem after problem, there isn't much more to go wrong.

When I turn the ignition on I have no battery light, still have oil light which goes out when I start up.
It starts and runs fine and is charging the battery ok. Any ideas?

Shall I just forget about it? Im off to Billing friday, got to tow the caravan from Barnsley.

daveyboy300tdi said:

Right... New problem! Just about coming to the end of my tether with problem after problem, there isn't much more to go wrong.

When I turn the ignition on I have no battery light, still have oil light which goes out when I start up.
It starts and runs fine and is charging the battery ok. Any ideas?

Shall I just forget about it? Im off to Billing friday, got to tow the caravan from Barnsley.

Click to expand...

Mine does that too. IMHO, forget about it, and I do an easy 500 miles per week, no probs so far.

try earthing the THIN blue wire that goes on to the back of alternator next to the main feed and see if you get a battery light on your dash.if you dont then the bulb has blown but if you do then the regulator in the alternator is knackered or the rectifier in the alt is feeding back which will eventually kill your battery.

The alty needs a small amount of current to start, on older cars this came from the battery light comming on only, if the bulb went so did your battery, the alty would not begin to charge.
On later rover models (not sure about the disco) they have a resistor in parallel with the bulb, so if the bulb blows current to the alty is still maintained when you switch on the ignition, its a kind of failsafe thingy.
You'd be better changing the bulb, then if something goes wrong whilst your driving you know about it, If the drive belt breaks on the motorway for example, you know you'll have no power steering and soon to have a bat flattery.

Thanks for the head up Colin & Dr Pepper

Dr Pepper - I wouldn't wish a bunch of useless tossers (sorry politicians) on our already hard done to service boys