vmb

ive been having problems with my 89 turbo
this is whats happening at this time
car will run great ,no codes etc , but after driving it for 15 minutes the tach will stop working and it will start to buck above 3000 rpm .

where does the tach get its signal from crank sensor or trailing coil ?
does the coil ground on the body or the spark plug?
can i check the coil without the mazda special tool per fsm?

thanks

1FastT2

iTrader: (1)

Im not a "guru" but maybe I can help ya. The tach does get its signal from the trailing coil and the coil does ground to the body. I would say you probally have a prolbem with the coil. It doesnt take any special tools to remove just a 10mm to remove 3 bolts.

Icemark

The tach on the dash gets its signal from the trailing coil.

The coil grounds to the car body. It must be grounded for proper operation. There have been several people suggesting that the coil ground through the sparkplug, but that is really only the secondary whindings

DaleClark

iTrader: (55)

Bzzt. The coil does ground itself to the spark plug. I've had a car running with the coil sitting on a rubber pad, completely not grounded to the car in any way.

Here's how it works -

The leading coil is the simplest - there's a power wire (switched 12v) and a signal wire from the ECU. When you turn on the car, the coil charges from the 12v power supply. When the coil gets the firing signal pulse from the ECU, the circuit completes - the coil arcs to ground at the spark plug. The block is actually the ground.

Trailing coil is similar, but uses more wires. There's a 12v switched power source, a timing signal, a select signal, and a tach output signal. The timing signal fires every time the trailing coil fires. If the select signal doesn't fire when the timing signal does, T1 sparks. If the select signal and timing signal fire at the same time, T2 sparks. It's more complicated since you can't fire T1 and T2 at the same time or you'll get detonation - one will be firing at the right time, one will be firing at the wrong time in the rotor's path.

To help answer the original question, I would check the main engine ground - it attaches to one of the tranny to engine bolts high on the driver's side of the motor. It's a big thick wire. If that connection is dirty or bad, you can lose the tach and the timing signal. There is a "check" wire on the trailing coil that feeds back to the ECU - if the ECU doesn't get the right signal from the trailing coil, it will discontinue firing the trailing coil, leading to no tach.

If that fails, might be worth adding a good ground strap from the motor to the chassis. Might help.

I've also seen an odd case where one of the trailing coil's wires to or from the ECU was corroded in the pin at the ECU plug. After fixing the corrosion and contact problem, the problem was solved. That was a frickin' pain to finally figure out!

Dale

vmb

is the signal wire a yellow and blue wire ?
what color are the ground and power wire to the coil ?

DaleClark

iTrader: (55)

*sigh* Did you just read the lengthy post I just wrote? There is NO GROUND WIRE going to the coil.

Off the top of my head, the tach signal is yellow and blue. Download the shop manual to double-check.

Start with your engine ground, and go from there.

Dale

vmb

which is power wire going into coil?
the coil themselves have +AND-marking
why do i have power on both of them ?

Aaron Cake

iTrader: (1)

Dale, what you are saying doesn't make sense electronically. There must be two grounds on the coil: primary ground and secondary ground.

We have a transformer with two windings:

----- || ------
) || (
) || (
) || (
----- ||------
Pri Sec

Each coil in the trailing pack needs a primary ground and a secondary ground. There's no other way for it to work as it will violate several laws of physics. I'm at work right now, but if I was at home I'd prove that with resistance measurements. So there must be a ground connection somewhere.

vmb

what color is it?

Aaron Cake

iTrader: (1)

Don't know off hand...I'm at work right now, so I don't have my books handy and I can't very well go to the parking lot and mess with Tina's trailing coils...There is a factory service manual at http://www.fc3s.org which will provide the circuit.

vmb

i tried looking in it but it doesnt really show anything, i looked under electrical and fuel and emission control.
they show how to check the coils but with a special tool from mazda .
i know the trailing coils have 2 connectors going to it ,im just wondering what wire does what

vmb

i think im gonna try to see that all input and output from ecm are ok .

DaleClark

iTrader: (55)

Dude, I'm telling you, CHECK YOUR ENGINE GROUND. Don't mess with anything else until this is done. This will most likely solve your problem.

Dale

DaleClark

iTrader: (55)

There isn't - look at the wiring diagram if you're not sure. I've been through all the ignition circuits a jillion times with Haltech installs and troubleshooting the dead tach problem I spoke of above.

If nothing else, this is known -

- There is no ground wire going to the coil packs.
- The coil packs will fire and work perfectly not bolted to the car and not even touching the car for a ground.

I'll research the wiring diagrams in my shop manual tonight (hey, a real paper shop manual!) and post up the wire colors and functions.

Dale

vmb

AIGHT im gonna strap 200 wires from the eng to the firewall!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

HAILERS

Wait a minute. I've taken the lead coil assy off the car and layed it on a towel before. The coil would not fire. I'll go out to the car in a while and pull the trail coil connectors off, pull the lead coil assy and lay it on rags. I'm betting the car WILL NOT start because the coil WILL NOT fire. Lets see if I have to eat my words. I doubt it very much.

HAILERS

So I went out to the 87turboII. Started just fine with all connected up. Shut it down. Removed the two little white connectors off the trail coil assy. Started the car. Ran fine. Shut down. Unbolted the LEAD coil assy. Placed it on a rubber mat. TRIED to start the car. Guess what. NO START. Put LEAD coil assy back on its studs. STARTED the car. Idled the car. Got two pieces of wood and gently lifted the LEAD coil assy off its mounts. Guess what. The car died.

Sorry, been there and done this routine last December. NEITHER coil assy if not making contact with the cars chassis, WILL NOT today, tomorrow or next month start or run off the coil assy that is not making ground with its chassis.

VMB.....if you ground that blue/yellow or green/yellow you WILL fry your ECU and you'll burn in the hot place down below. part humor

HAILERS

Translation is that the igniter needs a......... ground.


http://www.teamfc3s.org/info/articles/demystifying.html

FJ

Can't have it both ways.

The fact that there is no ground wire going to the coil means that the coil case must be grounded. Otherwise, as HAILERS said, how could the igniter within the coil function. As seen in my little diagram (leading coil for simplicity, but same basic operation), it's the igniter that grounds the primary and without a ground, it won't work.
(diagram in next post.)

-John.

FJ

DaleClark

iTrader: (55)

I stand corrected. You learn something every day!

To get back to what I was fixin' to do - the color and function of the input wires to the two coils.

Leading coil -

Black/yellow stripe - 12v switched power
Green/yellow stripe - firing signal from ECU

Trailing coil, two-wire connector

Both are black/yellow stripe - 12v switched power

Trailing coil, four-wire connector
Blue/yellow stripe - ECU timing signal
Brown/yellow stripe - ECU select signal
Blue/red stripe - Feedback signal to ECU
Yellow/blue stripe - output to tach

The feedback signal is basically a check signal that the ECU gets to make sure the signal gets to the coil and is interpreted correctly .

Again, I apologize for the confusion I may have caused - you guys pulled a new one on me! I'm always happy to learn something about the car that I didn't know, and I'm hoping to give back some of what I know .

Dale

scathcart

Not only am I impressed that the legendary Dale Clark was corrected about an FC, I'm impressed with his ability to admit he was wrong.

Welcome to the Forum Dale. You and Ted are the two I look up to most when it comes to car info on 2nd gens.

vmb

lets stop the pissing contest and get to the matter at hand MY CAR!
i actually put a extra ground from one of the trans bolts to the fire wall my tach worked all the way to work ,before it would not work once the car got hot .
but i have a code 18 .....will the ode 18 indicate that my tps needs adjustment or failed

vmb

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DaleClark

iTrader: (55)

Can't remember what Code 18 is off the top of my head - if it's TPS, it's either the TPS is out of adjustment or is bad. Time to test the TPS, of which I'm sure there are plenty of writeups somewhere on the forum of how to do that.

Dale