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MG MGB GT V8 Factory Originals Technical - jumping tacho needle

Hi there,

I have an American specification 79 MGB with a 1984 Rover 3.5 Vitesse engine equipped with the standard electronic ignition from the Rover. Its the one with the ignition module mounted remotedly behind the coil. The twin plenum efi is gone, replaced with the Edelbrock 500 carb.
Upon acceleration in lets say the fifth gear, the tacho reacts instantly and climbs very hight on the scale totally out of relation to roadspeed. Almost as if it was under vacuum influence. When I let go of the accelerator, the tacho goes back to normal engine speed in relation to roadspeed.
I disconnected the vacuum advance module out of curiosity with no change on the tacho behavior at all.

The car is also fitted with the following devise,

http://www.zaks.com/mgb/leonsmgb/this_is_the_circuit_i_use_on_my_.htm

,adapting the the v8 ignition signal to work with the four cylinder tacho.

Does anyone have a clue what has gone wrong?

Kind regards and thank you in advance.

Erik
Erik

Does your clutch slip??

Wayne
Wayne Pearson

Wayne,
No, the clutch is in excellent condition. Just changed it aswell as the flywheel. The car is capable of burning rubber without any hints of any clutch slipping.

Thanks anyway

Erik
Erik

OK, I'm not an electronic whizbang, but if I read your first post correctly, you are using the "4 to 8 cylinder" converter for your tach. The last paragraph in the write up in the above link makes me wonder if your 1 uf capacitor is large enough (you say "if the value is too small, you'll get double triggering").

Or, is there high voltage leakage at the coil secondary tower to the + and/or - terminals? As you know, when you try to accelerate in a high gear, it really stresses the secondary circuit and any "weak points" tend to show themselves.

Wayne
Wayne Pearson

Wayne,

You might be on the right track, since I have observed charring in a crack of the insulator plastic at the HT lead coil connector. I might just order a new ignition coil??

Thank you and Cheers
Erik

Erik

The standard tacho circuitry ignores voltage level changes of the tigger signal (once they exceed a certain value) as the signal triggers a pulse of a known voltage and length to the gauge, it is how frequently these pulses occur that drives the needle round the gauge as the damping in the gauge causes it to register the *average* voltage of these pulses over time. In my early days we tried building our own tachs using a capacitor/resistor time delay circuit which worked after a fashion, but exhibited exactly the same characteristics as you describe in that it showed throttle opening as much as engine speed! This is because the input signal wasn't decoupled from the gauge. Under large throttle openings and higher compression pressures the HT voltage needed to fire the plugs rises, and this voltage is fed back into the primary, and hence to the tach trigger circuit. It seems to me your pulse halver is responding to voltage changes and not just pulse frequency, maybe because it does use a CR time delay circuit and not a pulse generator.
Paul Hunt

I would like to use a GM 10 bolt posi in my 69 mgb. I dont have any with the narrowing of the rear end but I have wire wheels and would like to keep them. Any suggesions.
gb goodenough

Paul,

Thanks for a comprehensive answer.
There is still something that puzzles me. The pulse halver unit has worked fine for three years up until now and suddenly it went corrupt. Do you have a clue what I could do in terms of mending/upgrading a component in the pulse halver or elsewhere to make it function again? Do I read you right that the capacitor might the culpret an that it needs to be changed/upgraded?

Cheers
Erik
Erik

I suppose like anything it can gradually wear out, or suddenly fail totally, although the latter is more likely with electyronic components. From the description on that link the capacitor causes either double triggering or no triggering, I got the impression from your original post that your tach seemed more to be responding to throttle opening rather than engine revs. What you could do is bypass the pulse halver, your tach would then read double, but if it stopped responding to throttle opening and only responded to engine revs that would confirm it is the pulse halver that is the culprit. If so, again the text on that link indicates that the resistor, capacitor and zener diode on the input circuit act as a voltage limiter, and the CD4011 acts as a buffer and cleaner, before feeding pulses to the 2nd chip which is the one that does the pulse halving. Any of those components could be faulty of have dry joints, but you would really need an oscilloscope to go much further with diagnosis. It could also be that something has changed in your ignition system that while it isn't affecting the running of the engine, and maybe won't affect the original tach if used on its own, could be outside the limits of what the filtering circuit of the halver was designed to remove. That is going to be even harder to determine.
Paul Hunt

Paul,
Ill bypass the halver as a first step, as a way of determining possible cause and effect.

Ill be back.

Cheers Erik
Erik

This thread was discussed between 25/06/2008 and 01/07/2008

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