TPS Differences (Wiring)...?!

[Sh0ck3d]

Hello all. If any of you remember I am swapping a 99 2.2l Impreza motor into my 90 Legacy wagon and I have run into a couple of snags with the wiring...

The TPS on my old motor was a 4 pole (white, red, black, brown) and all the diagrams I can find on the new motor show it as ALSO being a 4 pole... The new motor I have is a 3 pole which means it was either JDM or a California car...

My question is, what wire do I NOT need for my old 90 ECU to understand this properly... How can I make this work? I seem to have lost a wire somewhere... ;P

PS: I am also having some difficulty with finding which wires go where on 2 sensors (idle air and purge control I think) on top of the new throttle body... Both are on the right side and are 3 pole.

[Matt Monson]

Maybe I forgot to reply to your last PM. Switch your old throttle body onto the new manifold. That will resolve most of the problem. You may need to bring over the coolant lines to make it work, or you can do the coolant by pass mod.

[Legacy777]

The fourth pin on the TPS is for the idle switch. The newer ECU's determine idle position based upon initial startup resistance.

So if you use the older ECU, you will need to swap the TPS, or as Matt mentioned, just switch the TB's.

[Sh0ck3d]

Well this sucks... The old TPS has a different type of connection to the throttle body then the new one (tried swapping TPS already).

I can swap the old throttle body onto the newer intake manifold, but then my idle air control valve will disappear (I believe this is what it is; directly on top of the throttle body with a 3 pole connection going to the right)... On the newer motor this is on the throttle body, on the old motor this is actually connected to the intake manifold (which I would no longer be using). The intake manifold itself CANNOT be swapped onto the newer motor just due to the size...

Is there any way to 'fool" my old ECU into always thinking that the missing wire is getting a "good" sensor response (IE: soldering in a resistor or something?)

PS: Thank you for the replies! I cannot believe how long this is taking me! Im soo close I can taste it but these dang wiring issues are giving me fits!

[Legacy777]

I don't know of any way to trick the ECU regarding the TPS's idle switch. It's an on/off signal.

Options, use the newer ECU, or swap the whole intake manifold....assuming that will work.....

[Matt Monson]

Shock3d,
You you have a '99 wiring diagram? If you get me a copy of that, I can sort this out for you...

[vrg3]

You could try to rig up a little comparator circuit that pretends to be the idle switch and triggers based on TPS voltage. It might work.

Or you could mount a microswitch under the throttle cam somehow.

I'm curious to see whether the 1st-gen ECU can work with the later-style IAC valve.

[free5ty1e]

I've kinda already got that circuit in the LegacEBC. I can easily burn a microcontroller set up to just do that TPS threshold / IAC thing if you need it... it'd need 5v, ground, TPS signal, and IAC output. If you're handy with circuits, I'd be glad to burn the chip for you.

[vrg3]

You mean idle switch, not IAC, right?

A pot and an op amp could do it too...

[bmxpunk]

I am doing the exact oposite. Putting EJ22t into an impreza. I need to use the Three wire impeza Connector on the 4 pin TPS from the 2.2t.

Cant swat TPS- different
Cant swap Throttlebodies- different sizes
cant swap intake manifolds- different sizes

[vrg3]

Just don't use the idle switch, and switch the TPS's power and ground around.

[free5ty1e]

oops, yes I mean idle switch. Yeah a potentiometer and an op-amp could be an adjustable threshold too. Too bad Radio Shack isn't as cool and useful as it was in the 80's (Short Circuit, anybody?), or I'd recommend just going there and picking up what you need. Now they're pretty useless, so never mind.

[vrg3]

You could probably use a random pot and an LM741 from Radio Shack. It'd probably be better to use a big resistor or two and a small pot, though, so you can adjust it more precisely. It'd be really cool to get one of those 20-turn trimpots, but I don't think Radio Shack carries those.

Hmm, actually, you'd want to base it on the TPS power supply... and you might need to avoid loading the TPS power supply, so you may need another 741 to buffer it. Or maybe use the pressure sensor's power supply, which is lower impedance but should be at the same voltage.

Another catch is that the ECU may not accept the output from an op amp directly; it seems to be picky about what happens on the idle switch line. You may need to put a diode (maybe a Schottky diode) in series with it so the circuit can only pull to ground. Or use the LM339, which has open-collector outputs. If those don't work, you could try the brute force solution -- just have the op amp drive a reed relay, exactly simulating a physical switch. Just be sure to socket the relay because it probably won't last as long as the engine.

[Sh0ck3d]

So if I have this right... I can just leave off the idle wire and switch the power and ground wires and I am good? I will try to upload a picture of the 99 wiring diagram today if I get a chance...

THANK YOU FOR ALL THE REPLIES! I have to make this damned thing work, and I do n ot currently have a 99 ECU. If anyone can confirm that by leaving off the idle switch and swapping the power/ground wires this setup will fly I'll do it tonight!

The other 2 connections I was worried about were the iac and purge control solenoid. Not sure what to do with the purge, used to be a 2 pole (blue connector) on the old harness, appears to be a 3 pole on the newer throttle body. Anyone have thoughts on this? THANKS AGAIN GUYS!

[vrg3]

No, you could only do that if you were using the 1999 ECU.

For now you can try just switching the power and ground wires... at least then your TPS will work correctly even if you're missing an idle switch.

To get the idle switch functionality you'll need to do what we're describing with a comparator or a microswitch.

Use the stock 1990 purge control solenoid.

I don't know if the 1990 ECU will be able to drive the 1999 IAC valve. Is your 1990 a 5-speed?

[Sh0ck3d]

My 1990 is an auto. The newer 99 Impreza motor is also from an auto. I wish it was a 5 speed.... I might be able to get my hands on a 99 ECU, but I would rather stick with the 90 and not bother. What type of microswitch do I need to purchase and where does it need to go?

THANKS!

[vrg3]

I don't think anybody's done the microswitch thing yet, so you'll kind of have to figure it out for yourself (and for us), but basically you need a switch that will withstand the elements and heat present in the engine bay, and will close when the throttle is nearly fully closed.

Use the specs which Josh has posted for the factory idle switch and try to simulate them with the microswitch:

http://main.experiencetherave.com:8080/ ... sting1.jpg
http://main.experiencetherave.com:8080/ ... sting2.jpg
http://main.experiencetherave.com:8080/ ... sting3.jpg

(The last link is the most relevant)

[Sh0ck3d]

So basically I am still stuck in the same position as I was at the last reply. I cannot seem to find a microswitch that is resistant that has the proper specs.

I am considering swapping a newer ECU into my 90 Legacy. Which ECU would I need to use for it to work properly with this motor? The biggest hurdle that I am STILLLLL stuck on is the problem with not having a wire for idle switch...!

[vrg3]

I don't know exactly what ECU would work best... You'd need one from a model that didn't use an idle switch but also didn't require any signals from the TCU.

Have you tried mounting a microswitch somewhat removed from the throttle cam and attaching a rigid piece of metal to the switch for the cam to press on maybe?

Do you want to try the comparator thing? I can draw up a circuit for you to try.