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IAC and TPS connection help !!!

cheekychimp

Well-known member
Joined
Apr 19, 2004
Messages
7,333
Location
East Sussex, U.K.
Car is back in the shop to address the idling issues and a few other things. One thing we are looking at now is the throttle body. I have a Forrester intake manifold that I purchased from Jackson Auto Machine,modified to take a Wilson Throttle Body.

First issue is I purchased a TPS (and only the TPS) from them to go with the throttle body. I asked for a wiring diagram but JAM are being a bunch of idiots about sending me one because they say they have no records of my sale. There is a possibility that the manifold was purchased by Alex on my behalf but it was so long ago I cannot remember. Anyway is there an easy way to ascertain which wires are which and how to wire this in place of the OEM TPS?

Second issue is that whilst I don't feel overly inclined to drop anymore money on JAM after their stunning customer support after spending well over $1000-USD with them, they do actually now sell a machined bracket which bolts directly onto the throttle body into which an IAC unit can be fitted. This looks to be a clean install option. This is the unit itself :-

JAM IAC Housing TPS Kit

I've sent an email to JAM asking about the IAC. But what do you guys think the chances are that the OEM ECU can control an aftermarket IAC?

I'm going down to the shop tomorrow to take another look at the throttle body and OEM TPS IAC setup. I'm wondering if it might be better, easier or even possible to get the Wilson unit machined to accept either the Mitsu TPS/IAC or even both.
 

toybreaker

iconoclast
Joined
Apr 30, 2006
Messages
3,581
Paul, the important thing with the tps is that it's a 5 volt unit.

Most of the "aftermarket" units are actually an o.e. application that'$ been reboxed, and so you're probably good to go as far as that goes.

The only complication is the progression from your particular throttle linkage, and even there, you're probably in the ballpark. From idle, (throttle plate closed) to wide open is the same sweep, so unless you've got an anomoly on the bottom of the scale, you shuld be good to go.

If you can get a pic of the tps you have, or pull a part number off the unit, and we can identify it, I can get you a diagram from alldata that will allow you to verify the wiring.

You can also just test what you have fairly easily. Just have a helper sweep the throttle and log what you got, and I'm betting it'll be close (+/- 5%) to what the o.e. range was. (unless somehtings not kosher, and then you should fix that /ubbthreads/images/graemlins/wink.gif )

Are you simulating a closed idle switch, or is there one installed?

The ecu uses that as a primary imput for making idle decisions, so if the ecu isn't seeing that imput, all bets are off as to what the ecu is thinking when you're expecting the thing to idle.

When you're running a non standard set-up, things get to the edge of the o.e. ranges/envelope fairly quickly, and the train can really fall off the tracks tune wise if you don't hit the center of the target when you set things back up. This is all about injector drivetimes, latency and all that fun sh*t... combined with fuel pressure and timing decisions, this is the meat of the tune. (and where I defer to the more knowledgable members here)

But, unless I'm missing something else, to be honest with you, troubleshooting too many things at once will drive you nucking futs. It might be a good idea to just dial in a thousand or elleven hundred rpm with the mechanical adjustment available to you, (idle air bypass/throttle stop) and then troubleshoot anything else you've got going on, and then come back to this issue.

If you could give us a full run down on your set-up and a log or two, I'm pretty sure Keydiver or one of the other saavy cats here can give you some excellent imput on what to address next.

To put it another way, adding an iac into the mix of a fluctuating/unstable tune, will drive you absolutely batshit crazy, so you might want to solidify everything else first.

Just my humble opinion. /ubbthreads/images/graemlins/smile.gif
 

cheekychimp

Well-known member
Joined
Apr 19, 2004
Messages
7,333
Location
East Sussex, U.K.
No it's not a humble opinion John it's clearly a very well balanced one drawn from years of experience and I value it.

The issue of whether the IAC is even necessary has been debated at great length here already to the point that I have more than once thought it might be easier to leave the damn thing off altogether. Living in a tropical climate it pretty much never gets below freezing. Cold starts might be an issue although I have discussed a theoretical solution to that issue and even the rpm drop issue when using the A/C can most likely be reconciled by setting idle at around 950 rpms.

I don't want to 'change' too much at the moment but we suspect there has been some hacking of my ECU to engine harness. We only came to that conclusion after we tried to get some logs for diagnostic purposes and were unable to get any logger or even DSMLink to connect through it. So another harness is going in.

I'll see how much space we have to work with on the side of the throttle body and see if we can setup the TPS first and leave ourselves the option of installing an IAC at a later date. I think the lack of a BISS, no FIAV and IAC might as you say give us a much simpler baseline.

Thanks however for reminding me about the Closed Idle Switch. I knew there was something else on the OEM assembly that needed to be fitted onto the new throttle body. We do have the option to simulate that through DSMLink but unless we have a significant breakthrough I'm trying to stay away from DSMLink until I am sure the car is operating properly on one of Jeff's chips so I have a solid baseline to work from.

If I was already competent with DSMLink and had tuned with it before I might look at this differently. I realize it is fairly simple but as you have pointed out throwing too many variables in at once just fucks things up. I am just not confident enough in my ability to setup a tune on DSMLink yet to be able to discount the possibility of my settings causing the symptoms problems I'm seeing. As soon as I know the car runs good on Jeff's chip I can look at using the 'Link' to fine tune, safe in the knowledge that any problems occurring at that point are the fault of my tune (which just needs more work) and not a base timing, compression or boost leak, ground issue etc etc etc

There is a picture of the TPS in the link in my first post but if I remember I'll try to grab a part no. from it tomorrow and post that up.
 

Paul, how many wires does the TPS have? If its just a simple 3-wire setup, it should be easy to figure out which wire is which with an ohmmeter. Two of the pins should show the same resistance no matter where the throttle is set, those are your 2 ends of the potentiometer. The pin that varies resistance is the wiper, which goes to the TPS pin of the ECU. The pin that shows very low resistance to the wiper when the throttle is closed is the sensors ground, and the end that is high resistance to the wiper at idle is the +5 volts from the ECU. If possible, you also want to calibrate the TPS so that at idle it is .45 to .55 volts on the wiper.

 

cheekychimp

Well-known member
Joined
Apr 19, 2004
Messages
7,333
Location
East Sussex, U.K.
It's a three wire sensor. JAM got back to me and stated they only have wiring diagrams for aftermarket ECUs so I just asked for any diagram showing the sensor wire colours and inputs.
 

Yes, those wire colors exactly match the ones in the GVR-4 diagram I posted above.
 
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