Ignition System/Electric Tachometer

[Flatlander]

I am still trying to trouble shoot my bad tachometer. At some point in time the electronic ignition system in my 77 Spitfire was removed and points added. The ignition coil also looks like it has been replaced. My questions are:

1. Would going to a "points" ignition system adversely impact the operation of an electronic tach?

2. If the original coil was replaced with a 12 volt coil would this have a detrimental effect on the tach? (All the wiring diagrams I have seen show a 6 volt coil. The car seems to run fine but have not had it running long enough to know if there would be any long term problrms. Points burning, coil overheating, etc. The auto parts guy tried to sell me a 12 volt based on his parts book but it didn't seem right. Since the wiring diagram showed a 6 volt, I passed until I had more info.)

Any ideas?

Thanks,

Gary

 

[BRSLimited]

Spitfires were all 12 volt cars. AFAIK the points cars had a cable driven tach but I could be wrong.

 

[dklawson]

Later Spits did have ballast ignitions, electronic ignitions, and electronic tachometers (depending on the year).

If you have a multimeter, it is easy to check what you have and what you need as far as coils go.

Set the meter to measure volts.
Connect a jumper wire between the car's chassis and coil (-) (the dizzy side of the coil).
Turn the ignition switch on and read the voltage between coil (+) and ground.

If you measure 6V to 9V, you have a ballast ignition system. Other clues would be either a ballast resistor or "pink" and white/yellow wires on coil (+). If you measure 12V, you have a "standard" ignition system. Choose the coil type to match your measurements. Standard coils = 3 Ohms across the primary windings, ballast coils measure between 1-2 Ohms across their primary windings.

Running a standard 3 Ohm (12V) coil on a ballast ignition system will not cause any problems with the tach but the engine may be hard to start. Running a ballast (6V) coil on a standard ignition system will cause burned points and failed ignition components... but the tach will work fine until the ignition components die.

So what is the tach doing?

 

[Flatlander]

The tach idles properly at 1000-1300 rpm but when increasing engine rpm the tachometer falls to 0 then after a number of times just ceases to work. I have tried a couple of other of tachs but they don't work at all. Thought maybe the signal they were receiving varied between a "points" and "electronic" ignition system.

 

[Flatlander]

I don't see a ballast resistor or the pink/white/yellow wires. I am reading 5.5 volts across the coil.

 

[DNK]

Doesn't the tach get its current from the coil?

 

[dklawson]

Doesn't the tach get its current from the coil?

None of the tachs really get current from the coil as such. The early electronic tachs were designated as type "RVI". Coil current passes through an induction loop inside or on the back of the tach (depending on vintage). However, this current does not interact directly with the gauge. The later electronic tachs were type "RVC". Those are voltage pulse sensing like a modern tach. They don't draw current from the coil but they are electrically connected to the coil (-) terminal. Type RVI has a reputation for NOT working correctly/accurately with electronic ignitions (even when wired properly). Many people hook them up like they were a later type tack and they don't work at all.

Flatlander, you measured 5.5 Volts on the coil? That is indicative of a ballast ignition system. Can you post a picture of the wiring on your coil? Does it have a "ceramic brick" connected somehow either directly to the coil or off somewhere in the wiring to the coil? Keep in mind that if some of the other tachs you were trying were the RVI type, they connect differently than your car's original RVC type tach.

 

[Flatlander]

Doug,

Can't figure how to attach a photo but thanks to you I think I have found the problem. In editing the picture showing the wiring of the coil I noticed that the tach was connected to the "+" side of the coil rather than the "-" side. ( Can't believe I didn't catch that before. Sometimes it seems I can't see the forest through the trees.) I'll rewire, try it and let you know what happens.

Thanks for your help!

Gary

 

[dklawson]

Hopefully the connection is the only problem. Be sure to post back your results.

 

[Flatlander]

The connection appears to be the problem. My old tach works but the reading does not appear quite accurate. I borrowed a friend's tach and his reads more in line with what the engine is revving. Will hook up a dwell meter tomorrow and check them both for accuracy. At least it is acting like you would expect it to.

Thanks for your help Doug,

Gary