Coil Swap

[Ralf the RR]

I've been chasing a slight electrical misfire for ages.
Checked all the common things:
Points
Timing
Plug Gap
New Dizzy Cap

Swapped the leads for some spares I had lying around.
No improvement here.

Also looked for stray sparks at night - none found.

Finally decided it must be the coil.
mmmm - three terminals. WTF?

Tried to work out what each wire did.
Black/Blue through a small box (maybe a ballast resistor - or maybe a diode) to +ve of the coil.
Red/Black to +ve of coil
Black from points to -ve of coil (well that's normal).
Black/White to terminal B ????

I couldn't be bothered tracing the wiring, and the on-line workshop manual didn't help. Multimeter didn't help either.

"Normal" coils require +12V with IGN to "on" (dropped to +9v through a ballast resistor) and also +12V when IGN to "Start". Using my multimeter, I couldn't determine this configuration with the current wiring.

So, +12V with IGN to "ON" was best achieved with the Back/White wire. None of the other wires served any purpose.

So, this is how I wired up a "normal" coil with a ballast resistor:
- Black/White to the input to the ballast resistor.
- Wire from output of ballast resistor to +ve of the coil
- Black from points to -ve of the coil

All other wires were left disconnected.

This is a fairly standard configuration - but lacked the extra start circuit.
I didn't think this would be an issue.

Numerous attempts to start the engine failed.
Tried different point gaps, and different timing. - No success.

OK, I need a start circuit to apply +12V to the coil (bypassing the ballast resistor).
I wired a standard 4 pin horn relay that gets a signal from the starter motor as follows:
30 - +12V from battery
85 - earth
86 - jumpered wire from starter motor terminal
87 - +ve of coil (bypass ballast resistor)

Instant success
The engine fires into life.
Point gap (dwell) and timing set according to the manual.
Stopped and started the engine several times easily - woo hoo.

A test drive was successful - with the slight misfire gone.
Driving to and from work showed the slight misfire had completely gone.
The MB drove smoother - but there was no power increase .
Fuel economy seems to have improved.

I can provide photos and a wiring diagram to those who are interested, but this is not rocket science, and I now have a "normal" system.

The only problem I see with this setup is if the relay goes short circuit, then +12V will be applied to the starter motor.

[Brayden]

My god you certainly made a simple job difficult! Just connect the B wire to the +ive side of the coil - job done!

[Ralf the RR]

My god you certainly made a simple job difficult! Just connect the B wire to the +ive side of the coil - job done!

That's a guts call considering I had no information on how a "3 terminal" coil works.
Connecting +12V to a coil that requires a ballast resistor can easily burn out the coil.

I had a suspected faulty coil, so a conversion was the easiest (for me) as I had a spare coil.

Determining the wiring 1/2 hour.
Coil swap took 5mins.
Not starting - and subsequent thinking up a solution 1/2 hour.
Installing a relay 15 mins.

So yes, it was a simple job. Maybe my explanation of the steps to get to the end result sounded worse than it was.

I referred to this thread
http://www.tamon.org/forum/viewtopic.php?t=4048
but I was reluctant to "run everything to the "+" post, except the wire from the coil to the dissy, and the tacho wire (if you have one)" without knowing what all the wires did. I normally regard your comments as valid, but this comment seemed vague and without any technical confirmation. I did my own research, and found the other wires were superfluous.

I just thought I might share how to convert to a "standard coil" with justification of my findings by going back to the basics.

I still have no idea of the function of the Black/Blue, Black/Red wires, small black box (resistor/diode), but they serve no purpose at all with with my conversion.
I couldn't find how a 3 terminal coil worked. Is it a centre tapped primary?
What's the function of the small "black box"? My guess was a blocking diode for a start circuit - but couldn't prove it in the very little reverse engineering I performed.

So, as I said. A simple conversion going back to basics, with the inclusion of a simple (but necessary) extra starting circuit.

[Brayden]

Okay, so I probably should have been a bit more detailed in my reply to the other thread.

Black/White wire is the 12V feed that is reduced to 9V by the ballast resistor.
Black/Red wire is the 12V bypass wire that runs from the starter solenoid.
The little black box is a noise suppressor. It connects to the Black/Blue wire and reduces negative loop feedback through the electrical system.

[me217]

so brayden what do i have to do to hook up a 2 pin coil instead of the 3 pin one. i've also got a starter motor that only has 1 small wire coming off it instead of 2 like the old one cause i burnt it out. i'm having a lot of trouble trying to get it the car to run. it hasnt run propperly in about 6 months and in that time its drive under its own power for about 10 meters b4 it died and wouldnt start again.

[Brayden]

My suggestion is that you connect all wires (except the dissy earth) to the +ive post. Your coil needs to be the right one for a 9V ignition system though, or it will never run properly.

If your starter solenoid is the wrong one then swap the one off your dud starter (assuming the solenoid is okay)

[shavenyak]

OK, I need a start circuit to apply +12V to the coil (bypassing the ballast resistor).
I wired a standard 4 pin horn relay that gets a signal from the starter motor as follows:
30 - +12V from battery
85 - earth
86 - jumpered wire from starter motor terminal
87 - +ve of coil (bypass ballast resistor)

That's what I was thinking of doing as well - how many amps does the relay need to be to handle the starter current? I always thought starters drew a lot, and that you'd need a really grunty relay.
So I take it you have a starter motor that doesn't have the bypass circut (is that the solenoid brayden mentioned?)

On the three pin coil the "B" terminal is the same as the + terminal - it's just there for ease of connection, rather than piggybacking off the +. (At least that's what I got from the manual - mine's two pin so I don't have a three pin one to inspect.)

[Ralf the RR]

My suggestion is that you connect all wires (except the dissy earth) to the +ive post. Your coil needs to be the right one for a 9V ignition system though, or it will never run properly.

NOOOOOOO.

If you connect the Black/White (+12V) to the +ve of the coil, then you will burn it out.
If you do this, then you need a coil that runs on 12V.

The coil I used was a 9V coil, and requires a ballast resistor. You connect the Black/White to one side of the the resistor, and the other side of the resistor to the +ve of the coil.

Mine wouldn't start with this combination - so I had to introduce a start circuit, ie the relay.
My little bit of multimetering could not prove the Black/Red was from the starter, so I didn't use it.

[Ralf the RR]

so brayden what do i have to do to hook up a 2 pin coil instead of the 3 pin one.

That is exactly what I did.
That's what the post is all about.

[me217]

and i cant swap the solenoids as the whole starter is dead

[Brayden]

My suggestion is that you connect all wires (except the dissy earth) to the +ive post. Your coil needs to be the right one for a 9V ignition system though, or it will never run properly.

NOOOOOOO.

If you connect the Black/White (+12V) to the +ve of the coil, then you will burn it out.
If you do this, then you need a coil that runs on 12V.

The coil I used was a 9V coil, and requires a ballast resistor. You connect the Black/White to one side of the the resistor, and the other side of the resistor to the +ve of the coil.

Mine wouldn't start with this combination - so I had to introduce a start circuit, ie the relay.
My little bit of multimetering could not prove the Black/Red was from the starter, so I didn't use it.

I've been running a Bosch GT40T coil for 5 years now hooked up this way and it hasn't burned out.
It only sees 12V through the starter solenoid on cranking.

I'll check the wiring diagram to confirm.

[ike849]

I might be stating the obvious here, but isn't the GT40T capable of handling 12V all the time???

Its the GT40R & GT40RT that need a ballast resistor otherwise the 12V at cranking will blow it....

[Brayden]

Sorry, typo. I have a GT40R

[Psi_Wagon]

sorry, old thread.

just confirming with the GT40R. you do not need the ballast resister on the mighty boy?

[gadj]

My spare parts experience says the 'R' in GT40R means that coil is designed to be operated with a resistor. Non resistor or 12V coils have no suffix eg. GT40. Memory sugests the GT40RT is the solid state version.
There are other part numbers less simple that require consulting the parts listings - just tell the parts bloke what the coil is for & he will look it up