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  #21  
Old February 25th, 2015, 02:38 PM
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My alternator pulley is around 55mm........
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  #22  
Old February 25th, 2015, 02:42 PM
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shayne young
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I'm gonna say that Daniel is 100% correct... If the idle speed is showing that it's 1100 rpm but the engine sounds like it's idling correct, then it's the wrong size alternator pulley..as mentioned, there is 2 different sizes, easy enough to switch out with 22mm socket..undo the nut before you take the belt off...

------ Follow up post added February 25th, 2015 02:45 PM ------

Actually had this the other day when I installed a tach into a vehicle that didn't have one.. Changed alt pulley and bingo..was spot on..

------ Follow up post added February 25th, 2015 02:46 PM ------

Ummm, I think you need to have the bigger pulley, should slow the alternator speed down..and hence the tach reading..
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  #23  
Old February 25th, 2015, 02:50 PM
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The problem is a lot more than the pulley size based on the apparent error. It is most likely a case of an alternator with a different number of poles. 6 poles versus 4.
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  #24  
Old February 25th, 2015, 02:50 PM
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shayne young
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Huh? How can the transfer case have anything to do with the tachometer reading?
Lower ratio box is going to make the engine speed faster at given speed range..alternator pulley is altered so that it keeps it in the correct voltage producing range.. 1.2 boxes were usually used in v8 engines but the 1.6 and 1.4 was for lower revving engines like the diesels and 4 cyl a.. I think it's not so much the ratio of the box as it might be the engine that was used with the box and the alternator to match the combinations..????...
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  #25  
Old February 25th, 2015, 02:51 PM
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Shayne. You have lost your mind. The tach is connected to the alternator. The transfer case case has zero impact.

edit: Just checked and the TDIs and V8s have the same tachometer.
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  #26  
Old February 25th, 2015, 02:59 PM
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I may be wrong, but I think what Shayne is saying is that if the transfer case ratio is not a match it will cause the RPM's to run higher, regardless of whether the right alternator is being used. This would happen even if the tach was not connected to the transfer box.
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  #27  
Old February 25th, 2015, 03:03 PM
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Quote:
Originally Posted by Red90 View Post
Huh? How can the transfer case have anything to do with the tachometer reading?
It must be a ratio ... TC dictates speedo reading ... alternator dictates RPM reading ... if they are mis-matched then you'd read incorrectly from one to the other.
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  #28  
Old February 25th, 2015, 03:04 PM
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As mentioned.. I don't think its so much the transfer case... But the engine size..4 cyl diesels and gas engines rev at different speeds than v8.... Therefor alternators need to be spinning at a different speed to be in the "sweet" spot of optimum speed for voltage production.. 4 cyl diesels and gas engines have a different transfer box than v8s...and also the engine crankshaft pulley plays a factor in the alternator speed...bigger pulley on the crankshaft is going to spin the alternator more than a smaller pulley, so therefor would need a bigger pulley to reduce the speed...and archive the same corresponding rpm..
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  #29  
Old February 25th, 2015, 03:04 PM
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Quote:
Originally Posted by RBBailey View Post
I may be wrong, but I think what Shayne is saying is that if the transfer case ratio is not a match it will cause the RPM's to run higher, regardless of whether the right alternator is being used. This would happen even if the tach was not connected to the transfer box.
Good lord.....

The engine runs the alternator, which sends the signal to the tach. It does not matter what is in the rest of the drivetrain. Low range, high range, engine on a stand. It will still read the same.

The drivetrain has NOTHING to do with the tachometer signal.

Alternator pulley size and number or poles is all that matters.
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  #30  
Old February 25th, 2015, 03:06 PM
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Quote:
Originally Posted by Red90 View Post
Huh? How can the transfer case have anything to do with the tachometer reading?
The tac pulley has everything to do with the transferbox.

1.4TCs which were fitted to Defenders came with a 49mm pulley thus raising the RPM reading to go with the TC gearing.

1.2TCs which were fitted to Discos came with a 61mm pulley which corresponded with the higher gearing.

Zack's pulley is smaller than it should be so the tac is reading higher than it should. If he fits a 63mm pulley the reading should be correct with his 1.2TC and stock diffs.

Science.
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  #31  
Old February 25th, 2015, 03:07 PM
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Quote:
Originally Posted by Red90 View Post
Good lord.....

The engine runs the alternator, which sends the signal to the tach. It does not matter what is in the rest of the drivetrain. Low range, high range, engine on a stand. It will still read the same.

The drivetrain has NOTHING to do with the tachometer signal.
Exactly ... but the speed the alternator is turning does .. and if it has the wrong pulley size then it will indicate the incorrect RPM for a given road speed.
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  #32  
Old February 25th, 2015, 03:09 PM
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Quote:
Originally Posted by ini88 View Post
The tac pulley has everything to do with the transferbox.

1.4TCs which were fitted to Defenders came with a 49mm pulley thus raising the RPM reading to go with the TC gearing.

1.2TCs which were fitted to Discos came with a 61mm pulley which corresponded with the higher gearing.

Zack's pulley is smaller than it should be so the tac is reading higher than it should. If he fits a 63mm pulley the reading should be correct with his 1.2TC and stock diffs.

Witchcraft.
Fixed it for some of our less technical members ..
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  #33  
Old February 25th, 2015, 03:13 PM
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Quote:
Originally Posted by ini88 View Post
The tac pulley has everything to do with the transferbox. 1.4TCs which were fitted to Defenders came with a 49mm pulley thus raising the RPM reading to go with the TC gearing. 1.2TCs which were fitted to Discos came with a 61mm pulley which corresponded with the higher gearing. Zack's pulley is smaller than it should be so the tac is reading higher than it should. If he fits a 63mm pulley the reading should be correct with his 1.2TC and stock diffs. Science.
Defenders and discos have different tachs to match the alternator. Look it up. All discos have the same tach.
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  #34  
Old February 25th, 2015, 03:17 PM
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Zack, what alternator do you have fitted? do you have the part number?
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  #35  
Old February 25th, 2015, 03:22 PM
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Should be AMR5425. Appears all 300tdi Disco alternators have the 61mm pulley, but mine didn't measure out to that.
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  #36  
Old February 25th, 2015, 03:31 PM
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Just wondering was there a different diesel tach head unit that had a slightly different scale?


This one says Discovery 1 V8 EFi engine models

STC2969 TACHOMETER HEAD UNIT | shop | www.lrseries.com


Because the alternator is putting out power which moves the pointer on the head unit and corresponds to the printed scale.

Simply put is there a different scale for the diesels?
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  #37  
Old February 25th, 2015, 03:31 PM
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Should be AMR5425. Appears all 300tdi Disco alternators have the 61mm pulley, but mine didn't measure out to that.
I bought a AMR4248 100amp alternator and it came with a 49mm pulley. Measure from the inside where the belt sits in. Should be 49mm. I bought a 63mm and now my speed match my RPM correctly from the Ashcroft calculator.

What size tire do you have on there?
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  #38  
Old February 25th, 2015, 03:32 PM
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I'm pretty sure it's a 24mm or 15/16" for the alternator nut. And a 8mm allen key for the shaft. I bought the bostich pass-thru set off amazon so I could get mine torqued right.
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  #39  
Old February 25th, 2015, 03:39 PM
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Quote:
Originally Posted by BRAB View Post
Just wondering was there a different diesel tach head unit that had a slightly different scale?


This one says Discovery 1 V8 EFi engine models

STC2969 TACHOMETER HEAD UNIT | shop | www.lrseries.com


Because the alternator is putting out power which moves the pointer on the head unit and corresponds to the printed scale.

Simply put is there a different scale for the diesels?

Possibly, the V8 and Tdi disco alternators are definitely different.

Quote:
Originally Posted by ini88 View Post
I bought a AMR4248 100amp alternator and it came with a 49mm pulley. Measure from the inside where the belt sits in. Should be 49mm. I bought a 63mm and now my speed match my RPM correctly from the Ashcroft calculator.

What size tire do you have on there?

My boss took the diesel for a spin to see what he though, I'll measure when it's back.

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  #40  
Old February 25th, 2015, 03:49 PM
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Quote:
Originally Posted by BRAB View Post
Just wondering was there a different diesel tach head unit that had a slightly different scale?


This one says Discovery 1 V8 EFi engine models

STC2969 TACHOMETER HEAD UNIT | shop | www.lrseries.com


Because the alternator is putting out power which moves the pointer on the head unit and corresponds to the printed scale.

Simply put is there a different scale for the diesels?
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well the scale is the same but it could be related to the chip on the back of the head unit
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