Speedo and Tach are wrong

1998 D1 85k gas v-8.
Plugged in the Code reader and my tach and Speedo are both off, I read that the alternator has a lot to do with the tach so I made sure the white wire was plugged correctly into the alternator which is 3 months old from RoverFarm. Any ideas why they would be off? I can’t tell you how much I appreciate any responses. Thank you.

RBS Disco said:

Plugged in the Code reader and my tach and Speedo are both off,....

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Just confirm cos it's a bit unclear... are the speedo and tacho off only when you plug in the code reader or they dont work at all

You haven't mentioned the magnitude of the speedo/tach errors.
Speedo simply works off a signal generator on the rear output shaft from the transfer case between case and handbrake backing plate, my experience it either works or it doesn't .
It's not uncommon for the speedo to be out by 2 or 3 mph at a road speed of 60mph (indicated), actual speed will be say 57 mph if fitted with the std size tyres. This inaccuracy can be the other way if the Disco is fitted with, say, 33 to 35 inch dia tyres, indicated speed less than actual.
Tacho signal taken from the alternator, not sure if it's adjustable for accuracy or not, I know some alternators put out a different frequency signal, was the one you recently fitted a genuine replacement?

I’m sorry I am still getting used to the LR way of anything can affect anything. Stock 16” wheels and tires, I know I need a slight lift and bigger wheels and tires and your right. Speedo is off 8-10 mph, Tach is off 350-500 rpm. I changed the wheel and bearing in the tensioner pulley and broke the wheel that was on the car and took the one off my parts car which was original equipment and smaller in diameter than the one on my car but it didn’t make much difference. No codes on the car but the evap system is inop which I don’t know how to repair. If that helps any. No sir the alternator was not the Italian version but I got the new one from Trevor at Rovahfarm, I will rebuild the original so I didn’t want to drop that kind of money on the temp one. It’s putting out 14.2 V and 6.5 to the pin that the tach wire plugs to. The Speedo and tach always come on and don’t flutter or jump just off, I wonder if the scanner could be off, just because this might sound silly, while running at 2500 rpm it’s actually at 3k and before flushing & changing fluids the first time in transfer case, transmission, both diffs, engine, swivel balls with rebuilds it ran at 3000 at 70 mph and I could hear that and it doesn’t sound like that now. Thank you guys so much for responding. I have to get these things fixed or the wife is going to set it on fire one day.

If you have a parts car, then I suggest simply swapping the instruments over at least temporarily as this all sounds a bit weird to me - Viz:- D1's are electrically simple; as above the speedo will either work, or not .... ( it's a LR, so not is a distinct possibility ) - and the tacho, about the same ....

FYI, my speedo started dancing all over the place a while ago - it was the wiring to the transducer on the TB - it was, er, unwell.... remade the joints and all is well again..

Since your conundrum appears to be affecting both, then I'd suggest some cleaning of earth points and the multiplug connectors on the back of the instruments ... and some vaseline to protect them ....

There are at least two different sizes of alternator pulley, so if a smaller one is fitted, then it'll make the alt spin faster for a given belt speed - and indicate the rpm too high ....

As said above, maybe the pulley on your replacement alternator is a little smaller/larger than the original, that is the only thing that can alter the indicated rpm
How are you checking your engine rpm and road speed? I check my rpm with a hand held instrument, (I've both optical and contact type tachs, leftover from my turbine maintenance days), if you can get your hands on one it is most useful to give a direct comparison to check the engine tach. Using road speed, speedo vs tacho to check accuracy of either is a bit abstract. Your speedometer is best checked using GPS, either the function on a smartphone or a straight GPS instrument, not the ultimate but certainly more accurate than just comparing Speedo to tacho.

Disco1BFG said:

If you have a parts car, then I suggest simply swapping the instruments over at least temporarily as this all sounds a bit weird to me - Viz:- D1's are electrically simple; as above the speedo will either work, or not .... ( it's a LR, so not is a distinct possibility ) - and the tacho, about the same ....

FYI, my speedo started dancing all over the place a while ago - it was the wiring to the transducer on the TB - it was, er, unwell.... remade the joints and all is well again..

Since your conundrum appears to be affecting both, then I'd suggest some cleaning of earth points and the multiplug connectors on the back of the instruments ... and some vaseline to protect them ....

There are at least two different sizes of alternator pulley, so if a smaller one is fitted, then it'll make the alt spin faster for a given belt speed - and indicate the rpm too high ....

Click to expand...

Gazbo said:

As said above, maybe the pulley on your replacement alternator is a little smaller/larger than the original, that is the only thing that can alter the indicated rpm
How are you checking your engine rpm and road speed? I check my rpm with a hand held instrument, (I've both optical and contact type tachs, leftover from my turbine maintenance days), if you can get your hands on one it is most useful to give a direct comparison to check the engine tach. Using road speed, speedo vs tacho to check accuracy of either is a bit abstract. Your speedometer is best checked using GPS, either the function on a smartphone or a straight GPS instrument, not the ultimate but certainly more accurate than just comparing Speedo to tacho.

Click to expand...

Gazbo said:

As said above, maybe the pulley on your replacement alternator is a little smaller/larger than the original, that is the only thing that can alter the indicated rpm
How are you checking your engine rpm and road speed? I check my rpm with a hand held instrument, (I've both optical and contact type tachs, leftover from my turbine maintenance days), if you can get your hands on one it is most useful to give a direct comparison to check the engine tach. Using road speed, speedo vs tacho to check accuracy of either is a bit abstract. Your speedometer is best checked using GPS, either the function on a smartphone or a straight GPS instrument, not the ultimate but certainly more accurate than just comparing Speedo to tacho.

Click to expand...

I’m using a Code reader deal that shows live data, temps, rpm, mph, and so on. I will check the sizes of the two pulleys on the alternator, I swapped the pulleys on the tensioner from the larger one to the smaller one. It’s a miracle I go that done on my way to work this morning while turning the steering wheel it suddenly got real hard to turn so instead of running into another car I made it turn and the two stalks and the body behind the wheel turned also. Amazing I have time to go to work. I really appreciate y’all taking the time to help me.