Electronic ign. tacho problems

[doffo25]

Hi all

I recently sent my tacho to Richfield Speedograph for repair and it was returned "upgraded"
In the meantime (and whilst it was disconnected) I noticed that whilst the car would start immediately it would then stop immediately. I wrongly misdiagnosed this as an ignition problem and rushed into installing a SNG electronic ignition conversion. I then found that I had no feed to the coil.......
Having found a previous thread on this subject I now understand that the two white wires in 4xconnector block (now disconnected as per pic) behind tacho are instrumental in the supply to the coil.

Speedograph Richfield say I must connect the green feed to the green in the taco and the blue/white in the tacho to the coil -ve. In the previous thread Marek said the white/diagonal blue wire can be repurposed and use it to connect to the coil -ve. Does he mean the white/diagonal blue wire (as shown in second pic) that goes into the pale plastic socket that is plugged into the (old) ballast resistor? ie I cut the wire and connect it to the
coil -ve?

Secondly, in order to restore supply to the coil what do I do with the two white wires in the connector block...simply connect them together?

OMG that was boring and long winded!

Has anyone followed that who can help me? I really don't want to ruin my tacho or electronic ignition!!!

David

[MarekH]

The s3 was originally going to have either a 6cylinder or 12cylinder engine, so the loom reflects a crossover in requirements and there are a few quirky twists and turns if you look at where all of the components are mounted on a late 6cylinder car and an early v12 car.

To get one of the white wire 12v feeds into the engine bay, the s3 loom has a 4block connector with two white wires on it. One brings 12v from a splice under the right hand of the dash and the other goes out into the engine bay to feed the coil/ballast. That's why these two need to be joined up for a v12. (Earlier 6cylinder cars ran the tacho by feeding the white wire into the tacho and back out to the coil - the v12 tacho doesn't use a white wire.)

SR have told you to connect a green 12v to the tacho to give it power. They will also have told you to earth the tacho casing. They also tell you where to take the tacho signal from and where to feed it into the tacho.

Repurposing wires:- often people want to connect up something that needs a wire to go between the cabin and thew engine bay. To the extent there is a free (i.e. unused) wire in the loom, then providing you know where it starts and where it ends, there is nothing to stop you using it for a purpose different to that of the Jaguar wiring diagram.

The only reason this topic attracts posts is because the original loom connector on the tacho is changed by SR when people receive back their tachos from SR and SR ask you to connect the coil negative to the tacho. The original wiring has the tacho feed taken from the ballast resistor. If you look at the wiring Jaguar diagram, you should be able to identify the old top left connection into the ballast (which comes out at the tacho). You should also be able to identify the coil negative.

kind regards
Marek

[doffo25]

Ahhh...that explains the strange routing!!

Thank you Marek so much for that detailed and very clear explanation....very much appreciated.

Ill go straight out to my shed and wire up!

kind regards

David

[thermojac]

I have just sent my Tacho off to SR and am now wondering if I am going to have the same problem as David when it is returned.

My question though is, why do SR mess about with the wiring in the first place?

[MarekH]

Because they probably replace the electronics inside the unit rather than repair them. If you do a bit of research, you'll see a custom chip which is no longer available.

kind regards
Marek

[lowact]

Replacing the electronics is no reason to not reinstate the plug. The tacho, with new electronics, would and should work perfectly and properly when wired as per the oem arrangement. because the pulse can be sensed at the ballast or the coil, same. There is no reason to be accepting of SR butchering the wiring. If they have to splice, suggest that you require the splice to be inside the casing.

Even if you want to get the signal from the coil -ve you would still want SR to reinstate the plug. Because the coil and the ballast are together in the engine bay, there is only disadvantage to owners who allow SR to get away with not reinstating the oem plug.

[thermojac]

I agree with your comments on SR Colin and have now sent them an email asking them to ensure that the plug is fitted to the unit so that I can just plug it in and get on my way.

[thermojac]

An update from Tim at SR
"If the board has gone in your Rev-Counter then a replacement board has to be fitted, the only thing Is this means the instrument has to be wired slightly different, the plug will be returned anyway."

[lowact]

Does this mean they are trying to leave it up to you to splice the plug back on? They would need to have left enough wire on the plug for splicing to be possible, otherwise any unattached returned OEM plug would be unusable.

[thermojac]

Don't know Colin but will update you when I get my unit back.

[jfmassa]

Recently, I had my early Series III Tach "upgraded" with a new circuit board. It was an US-based company. The tach was returned with the original plug. I connected it and it worked without any wiring changes. They used the green wire for power and the white/blue wire for signal. I can only imagine the white wires are are briged within the unit (probably as original unit). As noted they must be connected for power to the coil.

I reviewed some documentation and it appears that the black wire from the coil connects through the ballast resister to the white/blue tach wire. The SNG ignition system documentation shows both wires at the top position on each connector. (figure 7) The Opus Ballast Resistor schematic shows a straight through connection.

I wonder if your service provider is just providing generic installation instructions and has not taken the step to understand the specific application. It might be worth checking to see if the white/blue works before taking any steps to run a new wire to the negative coil terminal.

Best wishes,

Joe

[lowact]

Just to be clear, the white wires have nothing to do with the tacho. The white wires share a plug with the tacho (green, white/blue) wires but have nothing to do with the tacho. So, when SR cut this plug off it’s not just the tacho wiring they are messing up. Solution is fairly straight forward; wrt the white wires, all the oem plug does is connect them together, a separate plug could be used to do this instead.

The upgraded tacho would work better if connected via a resistor (per the original/oem arrangement) rather than directly to the coil negative. Either way you would never need to run a new wire, the same (white/blue) wire could be used.

Ballasted coils are supplied 12 V when cranking and ~9 V normally. With modern tacho electronics this variation should have negligible consequence (unless the new electronics are sub-standard). Older (original) tacho electronics were not as robust and preferred the pulses to be more consistent and cleaner, which is why the (oem) tacho signal wire was connected via a resistor.
There have been different ballast resistor packs. Some, like mine, did not have the top (100 ohm) resistor, it was just a straight wire. The tacho connected in the same place however there was a separate, in-line resistor.

[thermojac]

OK, so I have just received my refurbed tacho back from SR, having replaced pcb, with the plug cut off from the tacho and the same connecting instructions as everybody else . Having read all the posts several times and scrutinised the wiring diagram, it is clear that 12V can still be picked up on the Green wire and that SR need the WU (Blue/White) wire to be connected to the -ve terminal of the coil rather than coming from the ballast resistor via the Opus amplifier.
My question is (and maybe this is what Marek was referring to in post #2), if I reconnect the G & WU wires from the plug to the tacho, connect the plug, disconnect WU from the ballast resistor and reconnect to the -ve coil terminal, will this work without damaging the amplifier in any way?

[lowact]

Won't damage anything and will work perfectly if you wire it up as per the original arrangement including WU to the ballast resistor.
Will also work, maybe not as well, if WU is connected instead to the coil -ve.
Either way you won't damage anything.

[MarekH]

The coil has a low voltage side and a high voltage side, totally insulated from one another.

The purpose of the coil is to charge up the low voltage side at up to 12v* (using a high current) and convert the discharge pulse into an ultra high voltage (but very low current) pulse for the spark plugs. It does that by connecting one side (the coil positive) of its low voltage side to 12v* and this charges up the coil so that the coil negative also ultimately goes up to 12v* to fully charge the coil. The OPUS discharges the coil negative and this creates the high voltage spark.

*as altered by the ballast - its full battery voltage (nominally 12v) during cranking and a lower ballasted voltage during normal running.

If SR say "connect the tacho input to the coil negative", then their tacho input must be happy seeing full battery 12v (which it will during cranking) and it'll work by looking for the drops to 0v on that line, so Colin is perfectly right that this isn't going to harm the tacho. The SR tacho input will always see the full charging voltage (ballasted or not) and they are happy with that.

The old tacho circuit input doesn't work by looking for 0v pulses from a 12v baseline. The resistor in the input line will have limited the input voltage, so it probably works by sensing 0v pulses from something like a much lower 5v baseline.

kind regards
Marek

[thermojac]

Thanks guys for your responses which gives me the confidence to try it in its original set up first and then the -ve connection if it doesn't. Won't be able to test it for another month or so as I have some other engine work going on but will feed back when I do.

John

[MarekH]

SR's instructions weren't complicated. All they said was instead of connecting that top left wire (as in the diagram in post 11) onto the ballast, connect that wire to the coil negative instead. Storm in a teacup I'd say.

kind regards
Marek

[lowact]

Do they really say that? If so, why?

[MarekH]

Speedograph Richfield say I must connect the green feed to the green in the taco and the blue/white in the tacho to the coil -ve.

I am losing the will to live. Can someone please send me some razor blades? As for "why?", it is because the SR modified tacho works on 12v to 0v pulses to count rpm.

kind regards
Marek

[lowact]

RS requirement is ok in electrical sense, not necessarily in literal sense. All voltage sensing tacho’s, including Smiths RVC type, sense pulses in wiring from the negative side of the coil (closest to the distributor). The ballast resistor is seemingly on the coil positive side, except it isn’t entirely, due to the amplifier and ballast resistor internal wiring and connectivity the tacho is actually getting a signal from the amplifier on the coil negative side. This means that connecting yr tacho as per the oem arrangement is also as per the RS requirement.
Next question is why you should not connect the tacho directly to the coil negative: If you have a capacitive discharge type of ignition system, which typically charge to around 400 V before dumping on the coil, you would not want to connect a legacy tacho directly to the coil, instead you would want to use the dedicated tacho terminal that is always provided for this purpose. Jaguar ignition systems were not CD type, nevertheless there will be back EMF (high voltage spikes) from the coil, also the Jaguar system uses an amplifier (power transistor) to boost coil current, whatever the reason a dedicated/protected tacho signal terminal was considered necessary ….
After SR has upgraded a tacho it is not a legacy Smiths RVC type anymore, as such it may (?) require less external conditioning and therefore be able to be connected directly to coil negative, if that’s what you want to do. Just remember its not necessary, the oem arrangement, tacho terminal, is the same signal from the coil negative, just cleaned up a bit …
A final clarification, back on topic, the plugs that RS cut off … reportedly, when digital (voltage sensing) tachos were first introduced, Jaguar devised that 4 pin plug arrangement to enable plug in replacement for older analog (current sensing) tachos.