Disco 1 tachometer "jumpy"

There is also the issue of the diode pack in the alternator.
The 'W' output on the alternator is an AC signal (or pulsed DC depending on the make) that is generated by the rotation of the Rotor inside the Static Magnetic field created by applying battery voltage to the Stator. The number of pulses that are generated depend on how many poles the rotor has. This is always an even number and is most often 12 poles which generate 6 alternating positive and negative pulses of electricity per revolution of the alternator. If a Diode in the rectifier is beginning to break down, you will experience an indication on the tachometer of a drop in revs as the pulse is then not present for a short period of time. The Discos prior to sometime in 1995 or 96 had a 60mm diameter pulley that meant the alternator spun at a slower rate across the rev range of the engine. This was changed in the mid 90's to a 49mm pulley so I assume that the number of poles was reduced or the Tachometer itself was calibrated differently. Reasoning behind this was to improve idle charge voltage (apparently).

A comment re the Taco being erratic. My Taco on Diesel 1997 Discovery began to flick UP in revs - usually when an electrical load was applied - indicators, windows etc. Checked all Taco connections and their continuity. Alternator was 14.9 volts. Fault was (as previously stated on this site) Diodes were on the way out. I ran a scoping meter across the Taco and trace was terrible. Pulled alternator and I ran my diode tester over the diodes and found all of them beginning to leak. Just replaced the diode pack and now Taco is as steady as a rock.
Two points - Taco erratic - jumping UP by some one hundred percent (can't see this behaviour listed in any posts) and (learnt my lesson) don't take a quick 14 + volt check as a definitive measure of alternator health.

Where is diode pack located, sounds like I have the same problem, checked the cable many times and still jumpy and replaced the alternator. (it was on the way out anyways).

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Originally Posted by DaveKelly

A comment re the Taco being erratic. My Taco on Diesel 1997 Discovery began to flick UP in revs - usually when an electrical load was applied - indicators, windows etc. Checked all Taco connections and their continuity. Alternator was 14.9 volts. Fault was (as previously stated on this site) Diodes were on the way out. I ran a scoping meter across the Taco and trace was terrible. Pulled alternator and I ran my diode tester over the diodes and found all of them beginning to leak. Just replaced the diode pack and now Taco is as steady as a rock.
Two points - Taco erratic - jumping UP by some one hundred percent (can't see this behaviour listed in any posts) and (learnt my lesson) don't take a quick 14 + volt check as a definitive measure of alternator health.

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To check the diodes for reverse leakage, do the following.

Disassemble the diodes to get at the individual diodes ( there are six in the alternator ).

Set up a direct current power supply to give a voltage from 24 to 30 volts ( two 12 volt battery chargers in series will do fine ).

Place a 24 volt 100 watt head light bulb in series with the power supply, this is your test cicuit.

Then hook up the diode in series with your test circuit, try the diode to be tested in both polarity ways, the diodes should only light up the headlight bulb in one direction only, any faint glow in the opposite direction is leakage, then that diode is to be discarded and to be replaced with a new one.
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Originally Posted by mogindo

Where is diode pack located, sounds like I have the same problem, checked the cable many times and still jumpy and replaced the alternator. (it was on the way out anyways).

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Someone please correct me if I'm wrong, but the diodes are part of the alternator itself. This being the case, if you have swapped out the alternator completely then this can't be your issue (unless your replacement is buggered as well).

Maybe try crimping a new end onto the cable? It's possible the cable itself is buggered or maybe is lying on something that is causing some kind of interference?

Just a few thoughts, no expert though.

Thanks

Chris

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Originally Posted by libertyts

Someone please correct me if I'm wrong, but the diodes are part of the alternator itself. This being the case, if you have swapped out the alternator completely then this can't be your issue (unless your replacement is buggered as well).

Maybe try crimping a new end onto the cable? It's possible the cable itself is buggered or maybe is lying on something that is causing some kind of interference?

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Yeah, you are right, I don't think its an alternator since its new.

I checked the crimp on the cable and it seems ok. Unfortunately the cable disappears into the sleeve so there is no way of telling if it is kinked somewhere or deficient in some way. What is on the other end of the cable, because I am about to dismantle the whole console to get to behind the clock to check the plug and crimp there.

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Originally Posted by mogindo

Yeah, you are right, I don't think its an alternator since its new.

I checked the crimp on the cable and it seems ok. Unfortunately the cable disappears into the sleeve so there is no way of telling if it is kinked somewhere or deficient in some way. What is on the other end of the cable, because I am about to dismantle the whole console to get to behind the clock to check the plug and crimp there.

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While you have the dash apart, check the printed circuit board that fits behind behind the speedo and tacho for dry joints, then make sure that the earth wire lug's are clean and tight.
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Hi all, my Tacho reads Completly wrong, goes was off the scale at cruising speed, jumps up and down when indicators are on, and at idle it sits on 2000rpm, of course its not really idling that fast, just recently when I started the car from cold the charge light stayed on till engine got warm then battery started charging again, I was advised in a post I submitted here to change the regulator so I bought a new one but before I could fit it the problem stopped (go figure) I have been waiting for the problem to present itself again before I change out the part to see if that was the problem, as for the Tacho, I have no idea and until this thread noone could tell me the Tacho wire color, ill go and check all the wires now and recrimp the lot see if that fixes the Tacho......oh its a 93 D1 3.5lt with 270, 000k apart from that, issues with the central locking and strange temp guage behaviuor the beast is flawless, it doesnt even play colander when parked in the Driveway :)

Kevin

Kevin B, your issues sound suspiciously like a really bad earth. I had earthing issues for a while that I ended sorting by running another cable from the neg on the starter battery to the body.

Its kind of a lazy man's fix, but its really cheap to do and trying to figure out where the actual earth issue was ended up being seriously time consuming.

I could be wrong, but was the first thing that popped into my head. It may be best to start a new thread listing all of your issues and symptoms so people can go through and give you some ideas.

Chris

Kevin B, as your alternator drives the tacho, and you've already had alternator issues, I'd get the alternator and all of its connections in best order before moving on.