Ballast resistor

[mickjo]

Need to fit a ballast resistor to my 1972 stag, this has a RV83.5 fitted. With fuel injection and electronic ignition. (Ok don't boo and hiss its what I want, really it is), now then my problems are these:

The rev counter reads immediately over the top and drags the voltage down and stalls the car. So I've disconnected the rev counter supply to the coil. So now no direct 12V to the coil and no rev counter. I found a separate switched supply to supply the coil once the starter is released. To protect the coil and plugs I fitted a ballast resistor as per schematic drawings point 60 to point 13. Then went to start the car. The amount of heat that came off the resistor was huge it was glowing red hot. Almost a dead short. The resistance is 1.6 ohms so then it would be wouldn't it. What am I doing wrong here?

I was half expecting the electronic ignition to affect the rev counter. Is there anything i can do here ie will a MK2 rev counter work?

Mick

[V Mad]

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The rev counter in the Mk1 does not have any electrical connection to the ignition circuit; the white ignition wire is just looped through atransformer. Because of this I dont see how it can short outyour supply to the coil.

When you connect the ballast, I suspect there is a short somewhere that is cooking it. Measure the volts each side of the ballast; one side should be 12 to 13 volts.The other side should read about 6 to 7 volts; if its much less than that something else is dragging it down. I suspect you have conncted the dissy points or elec ignitionto the same part of the circuit.

[mickjo]

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Doesn't drop any voltage, 12.8v both sides. should drop by 6 volts it shouldn't it. Ballast resistance is only 1.5 Ohms. I admit when I saw it thought it wasn't big enough.

Before the resistor was fitted, I was jumping direct from the battery+, to the + of the coil so I could start it. (Which is not a problem in the short time) The car starts runs etc etc. so know there is little or no problem with the associated wiring.

Mick

[norstag]

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sounds like you electrics are well bodged, as V Mad said there is no direct connection to a mk1 rev counter, it's done by induction and it's unlikely the rev counter would work with SD1 electronic ignition and a ballast resister certainly would not help or be required.

Has the set up ever worked properly, because it sounds like it needs rewiring to suit the installation.

[mickjo]

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Ok things have moved on,

There was what looked like a capacitor from the +ve side of the coil this was tied down to earth. I removed this and reconnected the rev counter without Ballast resistor, it started and I now have a working rev counter. I haven't had time to fit the resistor, so will try tomorrow.

thanks just for your words V Mad, just what you wrote was enough to get the gray matter going, you were right there was something shorting it out, thanks a billion.

mick

[mickjo]

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sorry norstag.

Missed your post whilst answering V Mad. The wiring requires a lot of rejigging but it all making a bit of sense now.

mick

[V Mad]

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mickjo wrote:

Doesn't drop any voltage, 12.8v both sides.
Mick

Something wrong with your test; If theres no volts drop it cant get hot.

[Ian Durrant]

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You can't measure the potential difference (understood as voltage to most of us) across the ballast resistor with no current flowing, therefore you can only measure it with the coil earthed (on it -ve terminal). With points that's easy, measure it when they are (both if you have twin points) closed. Electronic ignition complicates this as its harder to tell when the coil is earthed and those with an amplifier of certain types I don't think put 12v to the coil anyway (though I admit I am guessing here as I haven't really studied electronic ingition in that sort of depth.

[mickjo]

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I understand what both ian and vmad are saying, so will try again tomorrow with the resistor.

but there isdefinately a link between what looks like a capacitor on the coil +ve and the supply from the rev counter, as the rev now works. Hopefully will crack it, again thanks for your replys.



mick

[Ian Durrant]

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Capacitors on the coil are usually fitted for radio interference suppression, whether its normal for this to affect the older tacho I have no idea, but it could well do.

[mickjo]

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Ok, Finally got time to fit the ballast resistor, and all is well and working fine. Not quite sure how or why the current was so high with the capacitor fitted, maybe there was a short in it. The capacitor is removed and binned, I wasnt going to fit a stereo anyway, Ive got the biggest v8 stereo I'll ever need. Infact octophonic music if you please.

Anyhow, thanks for your assistance, another goodresult from the forum.

mick