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MG MGB Technical - tach and electronic ignition

I just converted my '70 B to electronic ignition, and now the tach doesn't work. I don't think I did anything else to cause the problem, and seem to remember hearing this can be an issue. Is this an easy fix, or have I misdiagnosed the problem?
thanks,
mk
mk

I haven't heard of this particular issue, so let's let the experts chime in. In the meantime, have you checked out www.advanceautowire.com? Dan has some very comprehensive wiring diagrams in an easily accessible pdf format free for the downloading. These may give you someplace to start looking while waiting for a better response.

R.
Rick Stevens

Do you have an electric oil pressure guage? Did you tap into the green circuit for a power supply to the electronic ignition? How is the white wire now connected at the coil?
Kimberly

Whose electronic ignition did you use? What are your wiring connections as described in the unit's installation directions?

Bob Muenchausen

The 70 B uses a current sensing Tach.What I did was to go to Pick-a-Part where a found a 77 B. That Tach works like a normal white wire (-) trigger off (-) side of coil unit and there was a spot on my MSD 6a where it plugged right in. Though MSD did have some schematics to try, my tach had always been jumpy anyway. So I switched.
D.A. Abbott

Go to http://www.advanceautowire.com and click on stock schematics. You will notice the tach in a '77 is wired differently than a tach in a '70
Kimberly

I put a Crane on my 69 B about 12 years ago. Instructions said to take the tach apart and remove one of the two loops going through the sensor inside the tach. If if you don't do this the tach will read way to high. I have a pertronix in my TF but it doesn't bother it because it's mechanical. I suspect that Dave Duboius will have your answer.

LaVerne
LaVerne

Yes, tachs used before 73 were current sensing and these often don't like electronic ignition. When I bought electronic ignition in the 70s they came with a tach driver module to overcome this. I've heard changing the tach so that the white only makes one pass through the pickup instead of two (one complete loop) can solve over reading. But if this tach isn't working at all maybe you took the 12v supply to the tach from somewhere else and have consequently bypassed the tach. Or maybe the elctronic ignition is of a type that draws a relatively constant current and not the clear pulses that are required for the tach. In the latter case you may have to install a tach from a 73 or later.
Paul Hunt 2

Yes, Kimburly, The Tach from a 77 is wired differently. Its a more contemporary design wich operates just like most contemporary US auto tachs. In my MSD documentation, they have a fix for currant sensing tachs that uses a duel ballast resistor from '73 thru '76 chrysler's. MK, perhaps the company that manufatured your Elec ign has a similar remedy listed on their website (if they have one).
D.A. Abbott

The ignition is pertronix, and I will look at their tech info. No other changes were made, adn though the tach needle occaisionally spikes up, normally it does not indicate anything. I do have an electric oil pressure guage.

mk
mk

a pertronix ignitor? Thats not an "electronic ignition" by definition. It just replaces the points with a magnetic trigger ignitor. I really like the pertronix. Does your tach fluxuate when you turn the wipers on? If so your problem could be your oem wireing. A quick fix is to run a 10 guage multi-stranded wire directly from the battery to the brown wire (constant +) into your ignition switch.
D.A. Abbott

"a pertronix ignitor? Thats not an "electronic ignition" by definition"

That's definitely arguable. The Pertronix contains electronics to detect rotor movement and uses electronics to fire a voltage pulse at the coil. There are a wide range of electronic systems in use, not all available for aftermarket fitment. These use a wide variety of electronic methods to produce an HT pulse, ranging from the simple 'electronic trigger' type in the 45DE4 distributor which replicates the signal of the points, through the 45DM4, Pertronix and others which alter the length of the pulse, capacitive discharge systems, inductive discharge systems, and various others.

It is well known that fitting one of the aftermarket systems can upset the tach, and if it was fine before it almost certainly *is* the fitting of the unit. Even if the voltage to the supply *is* iffy there are better ways of fixing it, like finding the real source of the bad conenction, like at the solenoid, and certainly not running an additional unfused wire all the way back to the battery. IMO.
Paul Hunt 2

I talked to a local british mechanic here and he said to replace with a later tach is the best option. I don't mind doing that if it fixes the problem and if it looks similar to the original. If there is a cheaper and easier solution that is good, but not sure if that is the case. I don't know of any loose connections since there were not problems before. Paul, are talking about the starter solenoid?
MK
mk

Paul, you got me there. I think of elec ign as something that does more then replace points, but you are absolutly right. You will note I said "for a quick fix" when I refered to running a new power lead. The "not" quik fix is cleaning all connectors and terminals to see if that fix's it, if not you'll be replacing the harness. I also never said to run an unfused line to the battery. I would suggest a fusable link connected at the (+) term of the starter solonoid. I would also suggest adding a fusable link to all the OEM (+)unfused lines attached to the solonoid and replacing the old OEM battery to solonoid cable with 2 guage and replaceing and re-routing the (-) on the battery to a more robust ground.
D.A. Abbott

Ah, you mean for diagnostics, fair enough. But for me a 'quick fix' is something people do all to often as a 'permanent cure' and is something I hate. Much better to fix the real problem properly.

Fusible links in the brown wires at the solenoid (you would need two - one for the one or two browns feeding the cars electrics and one for the one or two wires feeding the alternator) will prevent harness destruction in the event of a short but are nothing to do with these tach problems. And even iffy connections at the solenoid are highly unlikely to affect the tach only, but cause all electrical components to flicker or fail.

A 73 or later tach is almost certainly the easiest option.
Paul Hunt 2

This thread was discussed between 04/10/2006 and 13/10/2006

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