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MG MGB GT V8 Factory Originals Technical - Tacho calibration/conversion

Hi again all,
I have been searching the internet for conversion information for my 4 cylinder tacho to 8 cylinder.
I found the site by "Zak" but when i got my tacho open it is totally different. My tacho is out of a 1969 car, it has a yellow pot with 50ohms written on it if that helps identify it.
If the tacho is convertable i would also like to remove the wording 4cyl on the face, does anyone have any ideas how to do this without ruining the face
cheers
Steve
Steve Danson

Steve,

i exchanged the internals of my tach for those of the early SD1 V8, as it also is dialed up to 7000 rpm. I fitted the electronics and the meter to the can of the 4 cyl. tach and the old dial and ponter fitted to the old/new internals.
The '4-Cyl' print can be removed with a black colly marker.

Happy new year

Ralph
Ralph

Ralph,
I like the idea of the SD1 tacho I will have to look out for one
Just a quick question what is a "colly marker"?
happy new year
Steve
Steve Danson

Steve,

a 'colly marker' is a large fibre pen with water tight ink. We have a brand here called EDDING and they call it a 'colly marker'.

BTW, the exchange to my old tach was a simple drop in. All holes for the screws were matching existing ones of the original tach and the electrical connection was the same as that of the 4-cyl.version.

Ralph
Ralph

If you have problems give JDO Instruments a call. John will put you right.tel., 01535 662701
Allan

Or Speedy Cables - Google them.
Mike Howlett

Steve,

I have a Hot Rod Friend who has used a Universal Tach Signal Interface from http://www.dakotadigital.com/index.cfm

It's $80

Bob
Bob Fisher

Thanks for all the options. After looking on the net at your ideas I will problably go with Speedy cables for tacho and speedo as a need a SD1 gearbox to 1969 speedo cable as well
cheers
Steve
Steve Danson

Steve,

when i did my conversion in the early 80's, i used the existing cable of my 1973 B and it still works fine. I made use of an LT77 and a MGC 3.3 axle and the speedo of the car the axle came from. The reading is more acurate than the normal B speedo with larger section tires on it as the ratios of the LT 77 are extremely clost to that of a normal MGB OD box.

Just try it and you will se how easy it can be done.

Ralph
Ralph

I have also had a look at Zak's web site, but my Tacho seems to be REALLY different. The car is a 1979 rubber bumper model assembled in South Africa (yes, they did assemble MGB's in South Africa!).
The problem is that the tacho is of the type where the sensor wire is in series with the positive feed from the ignition to the spark coil, and this wire is coiled once around what seems to be a small inductive pick-up on the back of the Tacho.
I have had a look at all available MGB wiring diagrams, but none show this type of configuration.
The instrument is a Smiths, and it matches the Speedo perfectly, so I can only assume that it is original equipment.
Has anyone else encountered this type of tacho, and would Zak's circuit work on it?
Chris Barrow

That type of tach (current pulse triggered, RVI on the face plate) was used up to 72, from 73 on it should have been a voltage pulse tach (RVC on the face plate) with a white/black wire coming from the coil -ve instead. Apart from the lettering the outward appearance is just the same. Either your 79 has been retrofitted with wiring and instruments from a 72 or earlier car, or it is a 72 or earlier that someone has fitted rubber bumpers to!

Dunno what Zaks circuit does (URL?) but as the two types use a very different signal I doubt that one circuit would do both. If Steve has a 69 then I'd expect it to have a current pulse tach, and if that was completely different to what Zaks site showed than I'd expect Zaks circuit to deal with the later voltage pulse tach and not the earlier current pulse, and hence not yours either, Chris.
Paul Hunt

Paul,

this is the link to zaks website http://www.zaks.com/mgb/leonsmgb/better_tach.htm
my tacho does have RVI on the face
Any thoughts on the calibration before i shell out.
I was thinking of changing the pot to a different size? but what size. I am surprised no one has done this which makes me think it is not possible with my tacho
cheers
Steve
Steve Danson

Looks like Zak's does have RVC on the face i.e. the later tach to yours.

This http://www.classictiger.com/techtips/motach.html is a diagram and description of the RVI tach and includes a note on adjusting for use on a V8 by tweaking the calibration pot R4 (not R5 as the text says). If you find the pot gets to the end of its travel before you get the correct readings on a V8, then if is at the zero ohms end you can use a smaller value for R5, and the easiest way to do that would be to solder another resistor across it, say another 3k, which will bring the effective value down to 1.5k. This is less likely to cause problems on the printed circuit than replacing the 3k with a 1.5k. If the pot is at the high-resistance end then you would need to insert additional resistance, which does mean cutting a wire or unsoldering and resoldering.

I reckon the pulse width of that circuit has to be halved so that double the rate of pulses from a V8 results in the same average current through the meter i.e. the same reading as for a 4-cylinder, which means the time-constant will have to halved, which means that the resistance will have to be reduced ... but I could be wrong. I'f I'm right then changing the pot won't be any help, it is the fixed resistor R4 that has to be reduced in value.
Paul Hunt

My tacho is of the new smaller type with white numerals on a black background. I have been sniffing around some more, and I have found that the MGC used a current pulse triggered type of tacho. I have checked the web site http://www.classictiger.com/techtips/motach.html and I doubt that the newer tacho that I have would share the circuitry of the older tachos (especially Germanium transistors!) Does anyone know anything about the MGC tacho?
Chris Barrow

It's basically the same as the MGB from inception to 72, but calibrated for 6 cylinders instead of 4. they may have been able to do that within the range of the calibration pot, but it may have had different timing components. As one is reputed to be able to adjust a 4 cylinder to a 6 cylinder with the pot, then I'd have thought you would be able to get it to a 6 cylinder as well. It would have a different part number if for no other reason than the face-plate printing.
Paul Hunt

I will have to strip it down and have a look. I guess that this will have to wait for a couple of weeks, as I have now completed the conversion and I am going through all of the legal requirements of getting it licensed for road use. I have tried it out in the dead of night on some back roads, and I can report that with the V8 motor the car is instant laxative!
Chris Barrow

Ralph in Germany:

Have you ever done an article on converting a SD1 tach to a MG can?
I acquired a SD1 tach but am not very competent in converting electronics and would appreciate some instructions.
One thing I am also having trouble with is removing the tach needle. Does it just pull off or is their a trigger catch?
Any help would be appreciated and I feel others could also benefit from some instructions.
Thanks,
Richard
Richard Woodley

Richard,

it is quite easy to do the concersion. You should have an early SD1 tach that is scaled to 7000 rpm with a movement of the pointer of 270. Later SD1 tachs were designed to cover only 180 and will not work in this setup.
You do not need to know anythig about electronics for this conversion. All connetions are the same as on your normal tach and must be connected as it was before. You even do not need to change a single connector under the dash!
Concerning the pointer, just make a mark with a soft pencil on the alloy cup underneath the dail and on the cars alloy body. Then you can carefully hold this cup into half way position and move the pointer a bit in either direction, grapping it at it's center and pull it off with gentle force. It is only pressed on and can be drawn off this way.
Next point is to remove the screws that hold the dail into place to the meter body and exchange it for that of your MGB tach. Refit the pointer and adjust the pencil mark to it's former position. Then realign the pointer. The reading will be as acurate as before.
Now you can drop the whole assy back to the can and put in the screws that will locate the meter and the electronics as before with the old assy you just removed.
If you do not like the '4 CYL' print on the dail, a black watertight pen can be used to cover it before reinstallation of the glas, O-ring and chrome bezel.
After connecting the wires at the rear of the can in the same way they came off, you can use the converted tach without any problems.
I did this conversion more than 25 years ago and it still works fine and when checked with an external electronic tach in the workshop there were differences in a spread of less than 50rpm between both of them.

(As i learned this language from reading my MG Books and Workshop manuals, i hope this information upon changing a 4-Cyl toch into a V8 tach is understandable. Otherwise do not hesitate, please, just ask for more information.)

A friend tried to 'upgrade' his tach with a spindle potentiometre in one line between the electronic board and the meter but the results were insufficient and he changed to a VDO tach from a Volvo Marine V8 engine, bought at a shipyard outlet, but had to modify the wiring for this instrument (SD1's have become extremely rear birds in here meanwhile :-).

Ralph

GHD 337796 V8 conv.
Ralph

Thanks Ralph, that seems very straight forward.
Richard
Richard Woodley

This thread was discussed between 31/12/2008 and 15/01/2009

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