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MG MGB GT V8 Factory Originals Technical - It lives ! but I have a tachometer issue.

Hello All,

Well! after 18 months I finally got the engine running! after rectifying a bunch of silly mistakes from me, requiring me to variously remove much of what I had already fitted - it kicked into life first turn of the key. Once I got everything joined up right.

Sounds good, I'll be putting some pics and words on my web site soon.

But..the orginal 4 cylinder tach is behaving oddly (more likely I have wired it up wrongly and it is behaving normally given what I have done to it).

It is variously doing nothing for prolonged periods, then yo-yoing around like crazy, now its doing nothing again.

I have the lucas distrubutor from the 1985 SD1, with the two wires coming from the distributor body itself to the coil, and no ballast resistor. I am using the coil from the same car.

I have a 12v supply wired from the white ignition circuit to coil+, and the white wire which goes to the tach also connected to coil+.

When I try the white tach wire on the coil neg (black/white from dizzy) it arcs, as it does when I try it on the W+ on the alternator (which is what the chap in shop said I culd do - yah boo to that). I cant see how the tach could be connected anywhere else.

The diagram that is closest to mine is Bentleys diag 11.

Its a 72 GT mish mash.

Can anyone help?

Thanks all,

Liam
Liam H

Could I also clarify - the wire from the tach is definitely white - not black/white.

Liam
Liam H

You have bypassed the tacho by connecting the supply direct from the ignition circuit to the coil+, the supply is supposed to go from the ignition switch to the tacho then to the coil+. However, you now have electronic ignition and 8 rather than 4 cylinders and I have no idea how the tacho is going to react to that - but I’m sure one of the electronic experts will know.
Geoff King

hello Geoff

You are right, the answer came to me in the bath last night, and I tested it this afternoon and it seems fine.

I had temporarily rigged a 12v supply direct to get the engine going and had forgotten about it.

As with most ptroblems on my car, they are self inflicted.

It is counting duble now, so I need to modify it, time to get back into the archives.

Best regards,

Liam
Liam H

Liam,

Please contact me off line. I have an article on converting a 4-cylinder tach to 8-cylinder operation. Even I could make it work and have done so!

Regardsa

Peter
P L Hills

Hi Peter

I'd be fascinated to see your article.

Is there a link ? or my direct is liamjmh at aol dot com

I was going to post back my progress on the group - today I found a small yellow screw at the back of my tacho which seems to adjust it. Got it from reading 1500 rpm to 750 rpm in seconds.

I had assumed I would need to wire in a potentiometer as every article I found in the archive said so.

Can it be as simple as that, mines an early 72ish tacho.

Regards,

Liam
Liam H

This thread was discussed between 13/04/2007 and 21/04/2007

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