Sometimes a tacho can be of invaluable trouble shooting help.
While I have limited experience with TD5, and zero experience with defenders(check my sig!) .. I had a strange issue a few weeks back.
Brother and I went for a drive for a few hours looking for some interestign places to camp(recon trip) and I wanted to take him into desert country to see what sand driving was all about, he's never done it. 
I let him drive most of the way up, but almost all of the way back. 
I'm the type that usually checks gauges, more than not. maybe every 30 sec or so .. maximum 1 min. 
I have my sensor gauge setup up in the D1/D2 type roof console. 
From the passengers seat, it's the same point of view as from the driver seat. 
I kept my eye on the gauges as he drove, as I knew he wouldn't.
All good. 
What I couldn't see tho was the tacho.
Anyhow, after 4-5hrs of him driving, I drop him off and now it's my turn to drive myself home. 
Strange thing, after a fair bit of experience, tacho is sitting nearer 2K rpm at 80k/h instead of it's usual 1750RPM ????
I'm thinking torque converter may not be locking up properly .. if that could even exist as a situation .. it locked up fine, felt fine through the accelerator pedal . felt locked.. but about 250RPM more than it should be.
Surely I haven't worn 10% of the tyre treads in 1000Klms! 
Next day. At idle I could hear a knock in the engine. First thought was harmonic balancer coming loose AGAIN!.
Next day up early, remove balancer, key worn. 
If I wasn't already in 'alert mode' due to the RPM, I may have even dismissed the harmonic balancer knock, as it was faint. 
I probably wouldn't have got up early to check stuff to get keep car mobile.. it's my daily driver and my only one at the moment too.
So start pulling bit and pieces off, and while undoing stuff I noticed my hand hit the alternator pulley, and moved it weirdly. 
Bloody thing was loose, as in worn loose. Pretty new alternator too. I'm thinking harmonic balancer vibration over 5hrs at 100k/h probably took out the alternator pulley with it .. AND caused the water pump to suddenly spring a leak via it's little peep hole. Car never overheats, so the pressure relief surely didn't trip. 
I'm blaming the troubles I've had in keeping the harmonic balancer from coming loose.
So the extra 250RPM was the result of the loose alternator pulley(worn off it's shaft) .. I didn't pull it apart, but I reckon the pulley wore off the shaft tho .. not sure.
But had I not noticed the higher RPM AND the knock, I could have just dismissed it as something to check at some point, rather than as a matter of urgency. 
Moral of the story .. 'ya never know!
In saying that tho, I reckon tacho is my least looked at gauge too. 
Main reason I look at it actually, is when I'm trying to trace/stop/locate those annoying vibrations and rattles that the rough little Tdi causes. 
I have two at the moment, dash rattle came back at 1750RPM, and now the empty passengers seat vibrates at 2050RPM. 
So I keep an eye on the tacho a bit more than normal just for now .. until I fix those two rattles up for good! 
At least I know which two engine speeds to avoid till that gets done
				
			 
			
		 
			
				
			
			
				Arthur.
All these discos are giving me a heart attack!
'99 D1 300Tdi Auto ( now sold :( )
'03 D2 Td5 Auto
'03 D2a Td5 Auto
			
			
		 
	
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