S3 Tacho / Rev Counter Ground

[Redman]

I'm still deep in behind the dash sorting out wiring. All guage lights work (if relatively poorly though they do visibly work in Dim and Full modes) with the Dash Light Switch on plus sidelights etc - but not the 2 in the rev counter. However even these 2 do work if removed from the instrument and put direct to ground so circuit is operational and bulbs are ok. PO has used one of the fixing posts on the rev ctr to add a ground wire: is this where the ground is supposed to attach? Or should I be looking elsewhere on the back of the rev ctr to make the circuit? Very simple, I'm sure..... Thanks.

[MarekH]

The grounding connection to the chassis at the terminal post obviously isn't good enough....so make a better one directly to the chassis.

kind regards
Marek

[Redman]

Thanks - just checking there is not a dedicated ground anywhere on the instrument itself. Looking at the ground wire the PO used it would seem a issue he was trying to deal with previously. Will have to try and "upgrade" that.

[MarekH]

The grounding is via the case and it isn't a very good arrangement. The previous issue is likely that the tacho stopped working for much the same reason.
kind regards
Marek

[Redman]

So, guage lights seem to be sorted (though still fault finding the Map Light and Interior Light, which for some reason are proving extra difficult ) so now focussing on Tacho. Wiring diagram (S3) shows this as having a ground but presumably only for the bulb circuit and nothing to do with the tacho per se,? But needle is erratic and often seems to get stuck. Is there something else I can do to test it / improve its performance - short of sending it off for professional overhaul? Thanks.

[MarekH]

As I said last time, the tacho circuit is grounded through its case. If you don't want to open it up and improve the grounding connection, then by all means send it off to someone who will.

kind regards
Marek

[Redman]

Thanks Marek - I thought I had read here that the tacho only relied on the 2 wires heading into it and not on a ground. I have certainly improved the latter greatly so that will hopefully fix the trick.

[MarekH]

"Electrical circuit" or "circuit diagram" - the clue is in the title.

For current to flow and so for the electricity to do work, it has to flow in a complete loop from the battery positive all the way back to the battery negative. Pretty much half of all of the car's "circuit" is formed by part of the car's chassis, one way or another. Inside the tacho you'll find a circuit board and it'll have an earth connection to one of the studs or to the case which is either falling apart, corroded or simply not held securely enough. Either that, or one of the components no longer allows the electricity to flow as intended. The external bulbs used the casing and studs as their earthing path to the chassis and thus to the battery.

A much better earth would be to run a serrated tag strip on a black wire from the tacho case to a stud on the chassis directly. That'll sort out the bulbs, no matter how poorly done up the knurled nuts are. Doing a similar thing from the little circuit board would similarly be a good idea.

Without a completed loop, the component simply sits at whatever voltage it is tied to, like a bird on a high tension wire.

Lucas wiring colours are very helpful in this respect.
Brown is direct to the battery
White is a brown but via the ignition switch
Red or green is a fused white
Purple is a fused brown
Black is direct to the chassis (battery negative).

The stripes are often just seen as for component separation, but they can also indicate conditionality, so a black with coloured stripe indicates a conditional earth, e.g. an Otter switch black/red is on the earth side of the component (relay winding) but on the positive side of the Otter switch. A purple/white is on the earthing side of a panel light switch, but on the postive side of the actual light. The cigar lighter was made just plain purple because there is no conditionality as to whether it'll be live or not - it always is.

Looking at your picture and without opening a tacho, I'd guess that the tacho circuit board earthing reaches the casing via that long spring loaded connection on the back of the case going from right to left.

kind regards
Marek

[MarekH]

Looking back at my records, I have a diagram of the circuit board inside the s3 tacho. Pin1 of the chip goes through two 42ohm resistors and that gets earthed onto the casing. I obviously had one of these opened up years ago and have forgotten all about it now.
kind regards
Marek

[Redman]

Thought I'd update, even if it did seem to be a relatively simple circuit/ground issue. I've improved the case ground by taking it direct to the chassis, and this has improved things markedly - while cruising the needle stays rock solid and is indeed relative to road speed and no longer fluctuates. My delight is tempered, however, by the low(er) revs response where the needle appears to "stick" initially before responding as it should once revs rise above about 1500. When revs drop it nevertheless shows what seems to me to be an accurate idle. So my new question is whether this is likely to be instrument related / physical (thus requiring overhaul) or whether it is likely to be something to do with the way the input signal is generated ie I should be fault tracing elsewhere. I appreciate your patience with this!

[Redman]

Anyone?

[MarekH]

It sounds like an issue inside the tacho to me, but whether that is mechanical or electrical isn't clear.
kind regards
Marek