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Old 11-14-2010, 08:05 AM   #1
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Question Suddenly lost idle control- can't figure out why

Hey Chaps,

MS1 V3. '92 Mx5.

I got the MS1 second hand, so I did not build it.

Right- I had fully working idle until about 6 months ago, I came off the motorway and idle shot upto 2.5k rpm. Changing the dc values to the ICV don't give any effect. Unplugging it makes the idle fall back down a bit. Bunging it makes the problem go away- but that's not a solution.


Nothing had been changed in the car so I tried to isolate the problem.

I've replace the ICV- no change.

I've opened up the MS1 and we've got the TIP122 transistor connected up to the Q4 connections, a diode connected to S12 and R39 has been jumped.

All well?

Did some continuity checks and the left and middle pins have continuity- is this correct? I suspected transistor failure, but removed it and there was no more continuity.

I assume it's correct for the middle and bottom connectors on Q4 to have continuity?

what checks can I do to check the idle circuit is working correctly?


I've noticed in the MS3 build, the right pin on TIP122 now goes to R43- should i do that? The right pin does go to ground as it is.

Any suggestions welcome. I'm thinking about replacing TIP122, but that's probably not the answer.

Last edited by Paulusuk; 11-14-2010 at 04:52 PM.
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Old 11-14-2010, 10:30 AM   #2
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do you still have d8 in place?
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Old 11-14-2010, 12:50 PM   #3
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No, D8 has been removed and R39 has been shorted.

What would cause it to suddenly fail? could TIP122 be 'stuck'?

As far as I can tell, the idle valve just sits open.

it may be worth mentioning, i've had endless noise problems with low-z injectors. I've wired q9 and q12 to batt and put a cap in the boot header. but this is probably independant. i've since gone hi-z

Last edited by Paulusuk; 11-14-2010 at 01:06 PM.
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Old 11-14-2010, 03:49 PM   #4
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Disconnected the idle valve and can't get the idle down, I've closed the idle screw completely and air is still sucking through it. I'm fairly sure it's not a vac leak.

Hooking it back up and altering the min and max dcs up and down makes no difference.

Still suspect the ms..
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Old 11-14-2010, 04:20 PM   #5
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Quote:
Originally Posted by Paulusuk View Post
Disconnected the idle valve and can't get the idle down, I've closed the idle screw completely and air is still sucking through it. I'm fairly sure it's not a vac leak.
Hooking it back up and altering the min and max dcs up and down makes no difference.

Still suspect the ms..
It's a vac leak.
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Old 11-14-2010, 04:30 PM   #6
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But air is leaking through the icv- A vac leak in the ICV?

When I remove the mini filter, the flow rate massively increases and it idles @ 2.5krpm!

If i bung the ICV then i can finally settle my idle by slightly openeing the tb.

What i'm saying is that the ms settings do not alter the amount of air entering through the idle valve. and I have replace the TB and air valve, thinking the air valve was defunkt

However, I shouldn't be removing vac leak from the equation just yet... as that's not scientific

Yet, i'm seeing boost of well over 160kpa

Last edited by Paulusuk; 11-14-2010 at 04:51 PM.
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Old 11-15-2010, 10:02 AM   #7
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if you have continuity between pin 1 and pin 2 you could have a problem.

that transistors sends out a rapid ground signal to open/close the idle solenoid. pin 3 is the ground source and the output goes out pin 2. Pin 1 is the signal pin from the CPU that triggers the transistor to jump pin 2 and 3.
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Old 11-15-2010, 12:08 PM   #8
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Thanks for clarifying the way idle control works- Iím guessing idle frequency, is the frequency with which the transistor switches to gnd.

If I was to multimeter the ICV output when itís on, Iíd see fluctuating continuity to gnd?Ö but Iím probably simplifying things too much.

Key question:
On the MS circuit board, I currently have continuity on Q4 middle and bottom pins (which goto left and middle of TIP122). I noticed that I also have the same continuity on Q20; middle and bottom. Is this right?

My second question is the ICV behaviour when itís disconnected- should it still let air through when the screw is closed and itís disconnected?

Help is much appreciated.
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