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MG MGB GT V8 Factory Originals Technical - Early MGB Speedometer conversion

I've checked the archives and everyone seems to change the '68 and newer speedos when doing the V8 conversion. I'm trying to stay with the original look for my MGA (215/T5) and the 65, 66, 67 B electric tachs are really close. Has anyone out there changed one of these over and what was done? You late B guys really don't how easy you have it.
Scott

Scott, I think I have the information on the conversion for the early B style tach at home, I'll have to check tonight. I'm using the B tach in place of the A unit with my conversion but haven't modified the electronics yet.
Bill Young

Scott,

on Pail Hunt's page, there is a link within the chapter Electics about modifying an old tach with modern elecronics.

A new circuit is explaned there and the only things that remain are the gauge instrunent and scale itself and the body of the instrument. If you are experienced with electronics or know someone to do you this favour, you will keep everything visuable of your old tach but with nowerdays electronics inside.

An other oportunity is to replace the internals with those of a V8 (scaled up to 7000 rpm on 270 and fix your scale and pointer to the tach. I did so on my conversion and the tach electronics of the old SD1 just drop into the can of the MG instrument without any modifications. The old scale can be screwed on again and the old pointer set to the axle.

Hope this helps

Ralph
Ralph

Sorry, the title was meant to say 'Tachometer' conversion. Bill, are you building an MGA?
Ralph, I'll check the link. SD1 parts are rather rare in the US. Most were crushed decades ago.
Scott

Scott,

I used Theo Smit's replacement circuit board in my B tach with excellent results. You can find infomation about it here:

http://members.shaw.ca/tsmit/tachmod/tachmod0.html

He's a great guy to deal with. He even modified the circuit board so it would work with my GM DIS system for me.

-- Scott
Scott Costanzo

Scott,
Thanks for the link to the site, it's great and exactly what I am looking for.
Scott
Scott

Scott, checked tonight and the article I had saved was for modifying a TR7 tach for a V8. I don't think it's quite the same as the B unit. I know that this information was covered a few years ago, I had some of it saved before my computer crashed, must have lost the files then. Looks like the link has what we need, but if not check the archives, the information was either covered or linked on this BB. Yes, I'm building a MGA, inline 6 powered. Still in the early stages of construction.
Bill Young

Scott, the tach in my '67 B was super easy to convert. I followed the directions for converting to Neg ground (earth) and then simply tuned the internal pot to calibrate to the V8 rpms as I measured from another tach. I think that perhaps the '67 model RVI2401/00B may be the only one with this capability. The result seems to be fairly accurate. When I added an Mallory electronic ignition, I had to take the extra turn out of the impulse pick-up on the back of the tach, The white wire now simply passed across the pick-up. I'm not sure what the theory is here, but it worked.
C Jones

C Jones,
With some luck, I actually have a decent 00B and scrap 00. Upon disassembly I see what I think is the pot and rotates freely. How did you determine to take the loop out? I have the pertronix which is about the same as the points in operation to the coil. I hope it really is this easy.
Scott

Scott, I drilled a 1/4" hole in the back of the case (which is now covered with a small piece of vinyl tape) to align with the small pot or variable resistor. This allowed me to adjust the tach while it was hooked up but pulled out from dash. I used my timing light to read the RPM, and set the MGB tach with a small screwdriver. I found the tip on the loop on a website dealing with Mallory distributor installation troubleshooting. I was originally running a stock Buick distributor and had the tach working fine, but when I went to the electronic Mallory, the tach would not register above about 3000 RPM. I pulled out the loop, running just the single white ignition wire past the impulse pick-up and the calibration returned to normal. I talked to Theo about the problem first and he mentioned that some have had luck with the solution I used. I don't think the MGB tach is all that accurate (may have never been) so I set my tach to read accurately at about 4500 RPM, the top of my useful torque band. It's off a little at idle and about 150 RPM at 2500 RPM, so close enough I guess. I wonder if any other Smiths tachs use the same circuit? Good luck on yours, How did the driveshaft work for you? cj
C Jones

For what it's worth most electronic tachs work on what I call "pulse averaging" where the number and duration of ignition pulses charge a capicator. On the old points type ignition the dwell can alter the duration of the ignition pulse and cause the tach reading to change, same goes for longer dwells of electronic ignitions. That would explain the reduction of inductive coupling in the loop causing the range to return to near normal. Modern tachs are much more digital and actually count the pulses and then either display them digitally or convert the digital information to an analog voltage to drive the meter movement and aren't influenced by pulse duration, just the frequency.
Bill Young

MGB tachs use a monostable circuit which gives a fixed width pulse signal to the meter every time they get an input pulse from the ignition. The current from this drives the meter, and if it were long enough would cause the meter to peg at maximum rpms. But it is only a very brief pulse so a single pulse barely causes the needle to flick. You can often see this as you turn the ignition on and off.

When the engine is running the pulses come fast enough for the movement to get its next pulse before it has got back to the end stop, so the needle moves higher. The faster the input pulses come the more often the monostable fires. But because the 'on' signal to the meter is the same width every time, the effect is that the 'off' duration gets shorter and shorter as the revs rise. So the ratio of on time to off time gets more and more, which causes the needle to flicker further and further round the dial. But because the movement is damped you can't actually see this flickering, even at very low rpms, and the needle gives a steady reading. It's the physical limitations of the meter movement and its damping (i.e. not being able to move up and down fast enough) that averages the on/off pulses from the electronics, it is not reading a varying averaged voltage off a capacitor. See http://www.classictiger.com/techtips/motach.html

By using this monostable circuit the output i.e. the meter is decoupled from the input i.e. the ignition, and so voltage and pulse width variations in the ignition have no effect (up to a point) on the tach, only the rate at which the leading edge of an off (points open)/on pulse occurs. With the very crude circuits that may have been used with some tachs using pulses to directly charge a capacitor and a meter then reads the average voltage across the capacitor, not only did different dwell cause different readings at the same rpm, but so did different throttle openings at the same rpm! The reason for this latter effect is that with a bigger throttle opening you get a bigger charge in the cylinder, a higher compression pressure, and that makes it more difficult for the spark to jump the gap. So the HT voltage goes higher before the plug fires, and this is reflected back into the LT circuit and gives a bigger spark across the points which gives a bigger voltage pulse into the capacitor, and hence a higher reading on the meter. I can remember making such a circuit in the late 60s and seeing these effects. This type was never used in the MGB.

Early current pulse MGB tachs used up to the 73 model year often don't work properly with after-market electronic systems, but that is more to do with the very different current characteristics of some of these systems being outside the spec of the pickup and electronics. The later voltage pulse tachs used from 73 on tend to work better with electronic ignition precisely because it is only monitoring the oltage on the coil and not the curent going through the trigger. With the current pulse type sometimes changing the pickup so that the ignition feed only takes one pass through instead of two helps.
Paul Hunt 2

This thread was discussed between 18/12/2007 and 22/12/2007

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