That Ballast Resistor Again.

codekiddie

codekiddie

Active Member
I'm changing to a Simon BBC electronic ignition and need to get rid of the ballast resistor. Doing electrics doesn't phase me (pun intended), but I still have a question :?

I have read on here various threads about bypassing the ballast resistor for new coils requiring the full +12V, and most suggest simply obtaining a feed off of one or other fuses that is only live when the ignition is on, for example the windscreen wipers fuse.

So far so good, and I am sure it will work just fine, (and I'm also sure that some of you will tell me that it does), but I can't for the life of me see how my tacho is then going to operate :?

In the following drawings I have shown the +12V routes in RED.

The following is the current setup:

ballast1.jpg


The +12V feed from the ignition switch (35) goes to the tacho (30), via the white wires, then on to the ballast resistor (31), via a white/yellow wire, and finally to the coil (32), via another white/yellow wire. The pulses generated by the points in the distributor, (33), opening and closing is what registers on the tacho as they break and make the circuit.

The following is the modified wiring to give a permanent +12V to the coil (albeit from the ignition switch directly and not via a fuse, but electrically the same).

ballast2.jpg


Now the tacho effectively has +12V on both sides of it, and certainly wont be seeing any break/make pulses from the points/electronic ignition, so surely wont work at all.

This version simply shows the ballast resistor shorted out.

ballast3.jpg


The coil will get close to a +12V feed, via the tacho, and the tacho should also work, now being able to 'see' the points opening and closing again. My question, (eventually), is has anybody else done this without any problems with the tacho, since the lack of a ballast resistor would increase the current flowing through it.
 
Hi Phil . I wont pretend to be the electrics expert . However , when i first got mine it had recently had a simonbbc kit fitted . It had been wired from the original ballasted feed , and it didn't do more than 100 miles in my ownership before giving trouble . I wired it up to the washer bottle feed and left it that way for over a year without problems ( the tacho was fine ) But i was never happy with this set up so eventually i took the dash out and traced the ballast wire back , replacing it with a plain wire . the ballast wire is still in place if ever i want to revert back , can't see why i would though . Tach has always been fine :D
 
Thanks Stina.

I have already installed a new wire to replace the ballasted one, but just didn't get the Simon BBC kit until my birthday yesterday :D and thought I would ask the question before installing it. I just can't get my head around how the tacho works when a direct +12V feed is connected to the coil :? :? :?

It'll drive me mad :roll:

I have done the same as you and left the ballast wire in place, in case I wish to revert back. The answer to your question as to why you would want to revert back, is if the electronic one fails and you are forced to re-install the old coil and points at the side of the road, (which you kept in the boot just in case), rather than wait for the nice AA or RAC man to arrive :( (other recovery services are available).
 
Spookily a post by WarrenL on another thread today may have the answer I'm looking for :D

Other thread

I think the tacho wont work as is, but I need to change it from an RVI to an RVC type, and adjust the wiring accordingly.

Apparently this is quite a common thing to have to do when changing to electronic ignition. Can anybody out there confirm this is the case, and that there is nothing I can do with the current RVI tacho I have :(

Wins sell second hand units for £25.00, (2 terminal type which I believe is the RVC type), so not too much of a problem :D
 
1) I think the primary (LT) resistance of 12v coils is generally higher, so I wouldn't think any extra current is going to phase (pun intended too!) the tacho too much. That said, it's a good question to ask and if anybody has actually ruled out my theory (below) please do feel free to correct me!

My theory for RVI's (when wired in the coil supply side, as opposed to the contact breaker side) playing up with electronic ignition is because the supply to the amplifier box is taken from the coil +ve terminal (which is often advocated); that therefore means that in addition to the wanted coil current, the supply current for the amplifier box also flows through the 'current sniffing' terminals of the tacho. i.e. in the third diagram of your original post (ballast effectively shorted out):-

ballast3.jpg


If the supply to the amplifier was taken at points 30, 31, or 32, then that's clearly after the tacho and therefore the supply current for the amplifier is going to be flowing through the tacho "sniffing" terminals. Should the amplifier supply be taken before the tacho (or somewhere else other than 30, 31, or 32), I'd think the RVI problem would not occur.

That's only a theory I have though.

But you're sure your car is wired that way (as in the diagrams you've given)? I mean you're sure it's not with the tacho sniff terminals in the contact breaker/points side (between the coil -ve terminal and the points)?

2) There's merit in changing to RVC, basically it's connect the male and female bullets that go/went to the RVI together, connect the spade/lucar terminal to the tacho, and then run a wire from the coil - terminal into the car behind the dash, and connect that to the sniff terminal (there's only 1 sniff terminal on RVC's - a male bullet - so the wire will need to be terminated in the car with a female bullet connector)
 
darth sidious said:
1) I think the primary (LT) resistance of 12v coils is generally higher, so I wouldn't think any extra current is going to phase (pun intended too!) the tacho too much. That said, it's a good question to ask and if anybody has actually ruled out my theory (below) please do feel free to correct me!

My theory for RVI's (when wired in the coil supply side, as opposed to the contact breaker side) playing up with electronic ignition is because the supply to the amplifier box is taken from the coil +ve terminal (which is often advocated); that therefore means that in addition to the wanted coil current, the supply current for the amplifier box also flows through the 'current sniffing' terminals of the tacho. i.e. in the third diagram of your original post (ballast effectively shorted out):-

ballast3.jpg


If the supply to the amplifier was taken at points 30, 31, or 32, then that's clearly after the tacho and therefore the supply current for the amplifier is going to be flowing through the tacho "sniffing" terminals. Should the amplifier supply be taken before the tacho (or somewhere else other than 30, 31, or 32), I'd think the RVI problem would not occur.

That's only a theory I have though.

But you're sure your car is wired that way (as in the diagrams you've given)? I mean you're sure it's not with the tacho sniff terminals in the contact breaker/points side (between the coil -ve terminal and the points)?

2) There's merit in changing to RVC, basically it's connect the male and female bullets that go/went to the RVI together, connect the spade/lucar terminal to the tacho, and then run a wire from the coil - terminal into the car behind the dash, and connect that to the sniff terminal (there's only 1 sniff terminal on RVC's - a male bullet - so the wire will need to be terminated in the car with a female bullet connector)
Click to expand...
Thanks for the reply Darth, and I understand exactly where you are coming from with your theory, and tend to agree with you, but I am more interested in the new coil wanting a direct +12V feed to work at its best, i.e. without a ballast resistor, and whether or not supplying it through the tacho would be enough for it. If not then I think my only option would be to supply it direct, and change to an RVC tacho and a little rewire :shock:

I definitely have the tacho wired that way....

tacho1.jpg


...with all three connections present and correct, and it is an RVI type...

tacho2.jpg
 
I see the "pigtail" off the lucar connector to the female bullet: RVI wired in the supply to the coil +ve terminal alright!

You've got me wondering now. :LOL: I know Brian Northampton uses his car's RVI tacho with Petronix ignition. The difference there is his car has the tacho wired between the coil - terminal and the 'points' connection, and I think he's kept the original coil... but he seems to think that the coil is a non-ballasted 12v one i.e. that his car wasn't never fitted with a ballast.

Assuming the same tacho is used on your car (how it's wired in your car is different, but it still sniffs the current fluctuations to compute the rpm), that would suggest you could use the tacho in the supply feed to a 12v coil (by removing or bypassing the ballast)

If you're using a sports coil which may actually pull more current, then that may pose another issue.
 
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