RustX7

I am smoking crack, I just don't know why. Please someone tell me in what way I'm being an idiot, because I can't figure it out..

Pin 2C on my 87 TII's ecu is supposed to be a ground, according to the FSM. I checked resistances on all the ECU ground pins (2C, 2R, 3A and 3G) and 2C was the only one that didn't show an essentially 0 resistance to ground. Instead, the DMM acted like I wasn't even touching the lead to anything.

So I figure, bad ground.

So to see if fixing that ground would fix my idle jumping problem, I clip the wire and route it directly to a known good ground. Start up the car and now it starts and dies immediately.

Not only that, but the wire coming out of that line on my ECU leads to a different place than the same wire coming out of my friend's 88 N/A ECU. Mine goes up into the harness on its own, my friend's goes and meets up with the other grounds.

WTF? Is my 2C pin special in that something else hooks up to it other than ground?

1987RX7guy

iTrader: (3)

Jumping idle means your TPS is not adjusted properly or has gone bad.

Are you reading the 87 TII FSM? You sure you were testing 2C?

Icemark

2C is ground, what color is in your "2C"

I suspect that you didn't find the correct 2C

RustX7

Looking at the plug with the wires coming out towards you and the pins facing out away from you, and the flexible clip on the top, is 2C on the top row, 2nd one in?

I believe mine is brown with a black stripe. On my friend's, the same wire is black. On both, the wire directly beneath comes from the O2 sensor, confirming my interpretation of the FSM diagram.

As for the TPS, I tested the voltage on the green TPS check connector -- if I describe it as looking like this:

A
B C

Then my voltage AB was ~ 1V and my voltage AC was 10V (it could be the other way around, but either way one was hot and the other wasn't), which I believe would make for a single lamp being on and therefore a somewhat properly adjusted TPS. Correct me if I'm wrong on that...

Also, I checked the resistance of the TPS itself. At WOT it was ~6.4 kOhm, closed throttle it was ~1.4 kOhm. Not perfect but close.. FSM says 1 kOhm and 5 +/- 1.

ALSO, I keep seeing on here that the TPS has a 6-pin connector, but the connector I tested for what I thought had to be the TPS resistance was only three... so was I testing the wrong thing?

adamlewis

S5's have dual TPS's so thats where youre getting 6 pins.
S4's only have a single TPS, and thus, only 3 pins.

RustX7

Aha! Excellent. <removes one small crack rock from pipe>

RustX7

bump. any ideas on my 2C wire anyone? Am I looking at the right one?

BlackRx7

VroomVroomVroom

I only have access to the 88 manual, and this is what I see. 2C is on connector EM31, 2nd from the RHS, top row (confirms what you see).
However, the schematic & connector id says "B" and "EM", that means a black wire on the Emissions Harness. The turbo models go directly to ground 24?.
But the non-turbo models go to a common line that appears to be a return line for the sensors; I would expect the ECU to ground it, but not neccesarily so.

So yes, the turbo and non-turbo ECU's/harness' are wired differently!

I wouldnt go poking around on the ECU terminals with a meter by the way, just in case you thought to try that.

Hope that helps,
Dave

RustX7

What manual are you reading from? Is it available online anywhere? What do you mean by ground 24?

Err, and a return line for the sensors. What exactly do you mean by that? Like I said, my friend's 88 N/A wire 2C hooks together to other wires which are grounds... but yes, on his 88 N/A it is a black wire on the emissions harness. My wire is brown with a black stripe.

And uh.. why wouldn't you go poking around with a meter?

thanks...

BlackRx7

the FSM can be found at www.fc3s.org

HAILERS

1. On a 87 turbo OR non turbo, pin 2C is a ground.

2. Somewhere in 87/88 they changed the color from Brown/Black to pure Black.

3. 2C is located on the center plug on the ECU. With all connectors connected, and looking in from the wire side, the 2C is on the middle plug, and is the second wire from the right, on the top row of that middle plug.

4. From your description, it sounds like you found the right wire.

5. The wire goes to ground. The ECU does not put a ground on that wire. It receives the ground From that wire.

6. Series four and series five wiring are different.

7. Go to http://www.iluvmyrx7.com and look at the 88 manual and the wiring diagrams. The 88 and the 87 are virtualy the same with the exception of the color of the ground wire i.e. brown/black vs pure black.

8. To prove/disprove the tps causing your problem......just disconnect the plug from the tps and see if the problem has resolved itself. The car should work plenty good fine without it connected up....on the whole. Not recommended for long time usage in that configuration.

RustX7

I will most definitely try running it with the TPS unplugged ASAP.

About that 2C ground - isn't it really weird then that the car would die with it connected directly to another ground? It shouldn't matter where the ground comes from, should it? I really don't think I messed my ground up, either...

Well hopefully the TPS thing works. If not I'll include pics soon to show what I mean about that 2C being messed up..

Thanks everyone for your help so far, please keep it coming as I need it!!

And just FYI I have been reading from the FSM from iluvmyrx7.com

RustX7

Unhooked the TPS (after warming it up) and the car wouldn't go below about 2400 RPM. Above 2100 or so, the idle bounce wouldn't happen anyway, so I'm not sure what this means. What's supposed to happen when you unhook the TPS?

RustX7

Oh sweet.. the TPS is the same on all S4's, turbo and n/a?

http://www.mazdatrix.com/c-6.htm

That means I actually have other ones around I can try..nice.. thought it was going to be a PIA to get one.

Unless mine sounds like it works to you guys... which brings us back to my question above

RustX7

bump. anyone know what should happen when you unplug your TPS?

HAILERS

I've no idea why the car would die when 2C was connected to another ground. That should not have happened. Normally there are about three grounds all spliced together about a foot from the connector at the ECU.

I'm also clueless why your engine idles soooo high when the tps is disconnected. I've disconnected mine before and it made no difference in rpms, at idle at all. It almost sounds like a large air leak has been created when you took the intercooler off to disconnect the tps plug.

RustX7

Actually, you know what's weird? When I went to take off the intercooler to disconnect it, I discovered that on a previous occasion of taking it off (sometime during this very troubleshooting session) I put it back on and didn't seat the rear intercooler tube properly - part of it was folded over at the intercooler. So I'm like "Oh," and I disconnect the TPS and put the IC back on right. Then it does its high idling. Then I take IC off again and reconnect TPS and put IC back on correctly and I'm back at square 1.

Also, as an update, I've since tried a separate TPS altogether (from an 88 n/a) with absolutely no change in the problem.

I need to read more.

RustX7

OK checked all my ECU voltages. Three were out of range.

1) BAC valve, pin 2Q - read 0.12 V, supposed to be 8-12. Replaced the connector at the valve and this fixed it - brought the voltage to 9.9-11.3 fluctuating.

2) Intake air temperature sensor, pin 2L. FSM says it should read 1-2V at 176 degrees F. It read 3.87 one time I checked it then 3.17 another. I highly doubt my intake air was 176 F, so those may be good readings for the colder temp, but it raised my eyebrows anyway.

3) 5th gear switch, pin 1I. Car was in neutral and this read 13.73 V, which is what it should read if the car was in 5th. Shifting around did not change this voltage. So the car thinks it's in 5th all the time. I don't know if that would ever affect the idle, but there's an issue anyway.

Just an update, any thoughts appreciated. Still have to read more and figure out all the throttle body adjustments.

HAILERS

About 5th gear......on a 86-88 n/a, when the shifter is put into 5th gear, a ground path is put on the SPLIT AIR SOLENOID. When you put the ground on the solenoid, you should see a drop from 12v down to below 2.5v at pin 1I

So....I bet you don't have your harness connected to the Split Air Solenoid.

And by the way, putting a ground on the Split Air Solenoid dramatically increases the air thru the Split Air pipe to the catalytic converter. A little birdy named SixRotors put me on to this as a way a person might increase his chances of passing emissions if said person was on the verge of passing/not passing emissions. To clarify...a person might jumper a ground to the Split Air Solenoid just prior to arriving at the emission check out station.

RustX7

Maybe I'm reading the FSM wrong. Here's what it says - for pin 1I it says "below 1.5V; (Others), approx. 12V (5th gear)"

Now I think I just read in another thread that the 5th switch actually shunts the 12V from the coil on the split air solenoid away from pin 2I, which would mean that I should see 12V at 2I in every gear EXCEPT 5th, which is not the way I was reading it.

Which is correct? I'm going to check that split air solenoid.

Good advice about the emissions test, BTW. Will do that.

RustX7

Both electrical connectors coming off of the ACV were indeed connected.

HAILERS

Your right, I'm wrong. What I stated above is for a Turbo II car, not a N/A. If you read the 87FSM, and compare the Control Unit pages for a N/A and then a Turbo, you'll see what I mean. The reading for pin 1I is assbackwards b/t the two.

The fifth gear switch makes a ground circuit when in fifth which pulls the voltage down at pin 1I from 12v to less than 1.5 and also on the turbo puts a ground (same ground) on the Split Air solenoid.

The N/A car works just the opposite.....except on your car and my wifes 86. On those cars it seems we both have a constant 12volts. As in not working right.

I looked at a spare n/a transmission. It seems the switch has two green and two red wires. The Red go to the reverse. The green to the fifth switch. Someday I'll jack the 86 up and see just why the fifth ain't a workin.

HAILERS

A revisit to the fifth gear switch........

You have a turboII. If you have the key to ON or running, pin 1I should read battery voltage.

If you select fifth gear, the voltage should drop to less than two volts. That's because you put a ground on the circuit to Pin 1I when you selected fifth gear.

IF you had a non turbo I believe it should work just the same as described above.

I think the fsm is wrong/confusing in the CONTROL UNIT description of the various pins on the ECU, when it comes to talking about the fifth gear switch.

Both versions, when in fifth should show battery voltage when in any gear except fifth When in fifth there should be a voltage drop to less than two volts.

IF anyone disagrees.....let me know where I'm going wrong.