Tacho Not Working

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IFC123
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#1 Tacho Not Working

Post by IFC123 » Wed Mar 31, 2021 8:31 pm

1971 V12 LHD
Hi everyone,
I imported my car from the US a few years ago as a non runner.
Fortunately for me, it was only a detached lead to the starter motor & it started straight away.
Since then I've been refurbishing it but very slowly!
The latest problem is the tacho is not working (no movement of the needle at all) it's never worked under my ownership.
I've removed the tacho from the dash, uncoupled it and reconnected it.
Is there a way of checking whether it is receiving a signal from the ballast resistor or anything obvious to check, before I send the unit away for investigation?
Electrics is not a strong point so as simple an explanation as possible please.
Can anyone recommend a suitable repairer, if possible in South Yorks area?

Many thanks
Ian
Ian
1971 e type V12 LHD, 1991 Merc SL 300, 2001 DB7

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christopher storey
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#2 Re: Tacho Not Working

Post by christopher storey » Wed Mar 31, 2021 9:10 pm

Ian : the electronic tachos require two electrical sources to operate . No 1 is the impulse signal from the distributor/coil to the tacho . No 2 is the continuous - while the ignition is switched on - supply ( usually a green wire ) from one of the fuses to the moving coil which drives the needle . It is also very important that the tacho case is properly earthed and this usually requires a black wire from one of the fastening down studs of the tacho to the binnacle case, and also ensuring that the binnacle case itself is properly earthed to the chassis. You need a voltmeter/continuity tester to carry out checks properly - these are readily available at modest cost

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#3 Re: Tacho Not Working

Post by IFC123 » Wed Mar 31, 2021 9:47 pm

Thanks Christopher,
I do have a multimeter and can use it to check continuity.
I am positive that there was no earth from the binnacle case to the fastening studs.
How is the binnacle case usually earthed to the chassis? the only thing connected to mine was the 4 pin plug connecting the wires you mentioned.
I will have a look again on Friday, armed with this information.
Ian
1971 e type V12 LHD, 1991 Merc SL 300, 2001 DB7

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#4 Re: Tacho Not Working

Post by christopher storey » Thu Apr 01, 2021 9:06 am

The binnacle should be earthed by the fastenings to the bulkhead, but it often happens over the years that the earths break down through dirt/corrosion , and this is why I would suggest establishing a good earth to the tacho case itself as a first step. Similarly, cleaning the plug in connector terminals with e.g. switch cleaner is worth trying

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#5 Re: Tacho Not Working

Post by IFC123 » Fri Apr 02, 2021 6:18 pm

So, as recommended by Christopher I cleaned the 4 pin connector & the connection to the ballast resistor, checked continuity which was fine & cleaned the clamp connections to the bulkhead.
Then ran a new earth from the binnacle case to the bulkhead.
Unfortunately it's still not working, so I guess it's time to have the tacho checked/repaired.
Can anyone recommend a suitable repairer?

Thanks Ian
Ian
1971 e type V12 LHD, 1991 Merc SL 300, 2001 DB7

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#6 Re: Tacho Not Working

Post by christopher storey » Sat Apr 03, 2021 9:49 am

I have used Speedograph Richfield 0115 926 4235 for a number of instruments, with great satisfaction. Whether they are right for an electronic tacho, I don't know, but it is worthwhile ringing them to see, and they are not too far away from you in Nottingham

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#7 Re: Tacho Not Working

Post by IFC123 » Sat Apr 03, 2021 2:41 pm

Thanks Christopher,
I'll give them a go
Ian
1971 e type V12 LHD, 1991 Merc SL 300, 2001 DB7

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#8 Re: Tacho Not Working

Post by Rui » Sun Apr 18, 2021 2:34 pm

It may well be the tachometer. In case it is, the tachometer has a Integrated Circuit from Texas Instruments which is not produced any longer and it may breakdown.
I retrofitted the tachometer with a device you can get from spiyda.com, in the UK,, attached to the back of the case and it has been working since then.
Easy , inexpensive and hassle-free fix.

good luck
Rui

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#9 Re: Tacho Not Working

Post by IFC123 » Sun Apr 18, 2021 6:54 pm

Thanks Rui,
I sent the tacho last week to Speedograph Richfield, should be back next week, so I'll report back.

regards
Ian
Ian
1971 e type V12 LHD, 1991 Merc SL 300, 2001 DB7

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#10 Re: Tacho Not Working

Post by IFC123 » Sun Nov 21, 2021 4:24 pm

Sorry it's taken me a while to come back on this, but I decided to do some other work while I was waiting.
I got the tacho back from Speedograph Richfield & it looks brand new.
Unfortunately they had to change the board inside the rev counter and sent some new wiring instructions:

The plug at the back of the tacho has gone & there is a green wire and a white wire.
The instructions say:
The green wire - Is the 12v+ feed from the ignition switch
The white wire - Is now contacted to the negative(CB) side of the coil.

So I cut the green wire from the plug and joined it to the new wire from the tacho.
Likewise the single white wire from the plug and moved the connection from the ballast resistor to the negative side of the coil.
I rang Speedograph about what to do with the other two (loop) wires still connected to the plug & was told to just tape up the old plug.
The problem is now when I turn the engine over, there is no spark at the plugs.
Can someone please advise please? my knowledge on electrics is basic at best.
Ian
1971 e type V12 LHD, 1991 Merc SL 300, 2001 DB7

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#11 Re: Tacho Not Working

Post by christopher storey » Sun Nov 21, 2021 6:52 pm

Without seeing the latest instructions and/or circuit diagram it is difficult to be definite, but my own view is you have no spark because there is no low tension connection from ignition switch to the CB terminal of coil . I should ask Speedograph about this : on your original setup the white wire came from an ignition switched fuse to the wraparound block on the back of the tacho, and then from there to the CB terminal. What you are missing is the first part of that, so you need to ask Speedograph whether you should reinstate that initial white wire to the tacho, or whether alternatively you should take a new white wire from switched fuse to CB terminal direct

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#12 Re: Tacho Not Working

Post by IFC123 » Sun Nov 21, 2021 8:03 pm

Thanks Christopher, I'll give them a call
Ian
1971 e type V12 LHD, 1991 Merc SL 300, 2001 DB7

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#13 Re: Tacho Not Working

Post by MarekH » Mon Nov 22, 2021 9:15 am

The instructions from SR were wrong (or incomplete).

You need the white wire loop from the original tacho plug to be part of the harness. This is how the white wire gets out from the harness at the back of the dash out into the engine bay.

Reconnect a dummy plug into the old four pin tacho plug harness connection so the white wire loop is reinstated.

kind regards
Marek

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#14 Re: Tacho Not Working

Post by IFC123 » Mon Nov 22, 2021 6:24 pm

Thanks Marek,
So there was no need to cut the two wires going into the plug to connect them to the new wires from the tacho.
I could have used the old plug and connected the new wires to that?
Ian
1971 e type V12 LHD, 1991 Merc SL 300, 2001 DB7

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#15 Re: Tacho Not Working

Post by MarekH » Mon Nov 22, 2021 7:48 pm

No. (EDIT:- or "Yes" if you repurpose the white/blue wire to the coil negative)

You need to follow SR's advice to make their tacho work.

Firstly, put the original loom back together leaving the new tacho unconnected.
Verify that your car starts and runs ok without the new tacho.

Then consult the Jaguar wiring diagram for where the coil negative and an igntion fed 12v can be tapped into and connect up the new tacho white wire as per SR's instructions. The new tacho green wire shouldn't be hard to connect.

kind regards
Marek
Last edited by MarekH on Mon Nov 22, 2021 8:35 pm, edited 2 times in total.

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#16 Re: Tacho Not Working

Post by IFC123 » Mon Nov 22, 2021 8:24 pm

Ok
Thanks for your help Marek, i'll see what they suggest.

regards
Ian
Ian
1971 e type V12 LHD, 1991 Merc SL 300, 2001 DB7

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#17 Re: Tacho Not Working

Post by MarekH » Mon Nov 22, 2021 8:37 pm

Their advice to tape up the old plug overlooked the fact that the white wire in and white wire back out were how the ignition white wire reaches the coil and that is why your car wouldn't start.

kind regards
Marek

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#18 Re: Tacho Not Working

Post by IFC123 » Fri Apr 29, 2022 5:23 pm

Hello again.
Apologies for the length of time it's taken to come back on this, but I've been in for a bit of an overhaul myself!
I went in for a knee replacement in December, so only just got back to the car.
I spoke to the guy who renovated the tacho again and he confirmed that the loop is no longer required, he said this was because putting in the new circuit board made it obsolete.
So I used the existing green wire from the ignition(to plug connector) and connected that to the new green wire from the tacho and then connected the new blue/white wire from the tacho to the white wire (originally going from the plug connector to the ballast resistor) and connected this to the negative side of the coil.
Result is I've now got a spark at the plugs, however now the car fires and tries to start but as soon as I let the key return to run it cuts out.
I'm not sure whether this is a new problem with the ignition switch or related to the new tacho wiring.
Thanks in anticipation
Ian
1971 e type V12 LHD, 1991 Merc SL 300, 2001 DB7

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#19 Re: Tacho Not Working

Post by MarekH » Fri Apr 29, 2022 5:39 pm

Did you read post #17?

During cranking, the ignition is fed through the ballast resistor by the white/yellow wire. After you release the key, it is fed through the ballast but by the white wire.

kind regards
Marek

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#20 Re: Tacho Not Working

Post by IFC123 » Fri Apr 29, 2022 7:22 pm

Hi Marek,
Yes I did read the post, it was just when I spoke to the guy again at SR he said I shouldn't reinstate the loop.
I'll reinstate it in the morning & report back.
Thanks
Ian
1971 e type V12 LHD, 1991 Merc SL 300, 2001 DB7

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