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Vanagon Nut
Samba Member
Joined: February 08, 2008
Posts: 10379
Location: Sunshine Coast B.C.
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 Posted: Sun Apr 21, 2019 2:47 pm Post subject: OBD1 ABA Conversion: Idle air control valve. |
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Hi folks.
On the OBD1 ABA engines, the IAC valve only buzzes (operates) when the engine is running.
While starting a hot ABA engine, does the ECU supply a ground to the IAC valve and does this cause it to open or close further than it would when no power was applied to it?
I have an intermittent hot no start issue on my ABA (Jetta 2.0 engine) swap. The IAC valve Ohms out, fully opens when connected to a car battery but rests at partially open when power disconnected (A spare IAC does same). It's been cleaned several times with carb cleaner then with WD40. I have not checked current draw when engine is at idle. Idle is almost always fine but may dip or surge up/down after disengaging clutch while coming to a stop. Particularly in stop and go traffic. I now wonder if heat soak into that part is affecting it. Lightly rapping on the part when the issue happens does nothing so I assume the valve isn't sticking. I have not tried same while measure its resistance.
AFAIK, this is quite different from the WBX Digifant IAC valve but from Bentley re: the ABA IAC valve:
"The IAC function is completely adaptive. The ECM remembers how far the valve was open the last time the engine was at idle and reverts to this as a base setting. In addition, the valve has a limp -home feature that maintains a fixed idle speed in the event of an IAC valve failure."
Unlike the WBX, on the ABA, at idle, all air is controlled by this valve.
Since the valve does not "buzz" when ignition is on, engine off, I can only assume that the opening at air valve should normally be the same as opening used for limp-home mode or when part is on the bench. Leaving the IAC disconnected raises idle slightly.
Disconnecting the wire connector to the IAC always allows the engine to start when no start happens. When issue happened recently, I closed off the air inlet to the valve. The engine started briefly. Opening that hose part way allowed engine to start normally. This leads me to believe that the fuel to air mix while trying to start engine is air rich.
I experimented starting the engine with IAC valve removed and one air inlet hose entirely open, partially closed, or with both air inlets closed and just the EVAP inlet nipple at TB open. The amount of air allowed in by that EVAP nipple seemed best for allowing the engine to start; the engine management seems very sensitive to the amount of air allowed in for starting and idling purposes.
Neil.
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1981 Westy DIY 15º ABA
1988 West DIY 50º ABA
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skills@eurocarsplus
Samba Peckerhead
Joined: January 01, 2007
Posts: 16879
Location: sticksville, ct.
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| Posted: Sun Apr 21, 2019 2:51 pm Post subject: Re: OBD1 ABA Conversion: Idle air control valve. |
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it's been a looooong time since I have been under the hood of a obd1 mk3, but as I recall, the iac should hum koeo. it will be pwm via the ecu and generally speaking the 2 wire was + all the time and the - was ecu controlled.
but...verify that
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a1steaksauce
Samba Member
Joined: September 23, 2010
Posts: 612
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| Posted: Sun Apr 21, 2019 6:28 pm Post subject: Re: OBD1 ABA Conversion: Idle air control valve. |
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| skills@eurocarsplus wrote: |
| it's been a looooong time since I have been under the hood of a obd1 mk3, but as I recall, the iac should hum koeo. it will be pwm via the ecu and generally speaking the 2 wire was + all the time and the - was ecu controlled. |
He’s right. |
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Vanagon Nut
Samba Member
Joined: February 08, 2008
Posts: 10379
Location: Sunshine Coast B.C.
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| Posted: Mon Apr 22, 2019 10:20 am Post subject: Re: OBD1 ABA Conversion: Idle air control valve. |
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| skills@eurocarsplus wrote: |
it's been a looooong time since I have been under the hood of a obd1 mk3, but as I recall, the iac should hum koeo. it will be pwm via the ecu and generally speaking the 2 wire was + all the time and the - was ecu controlled.
but...verify that |
Thanks.
not sure if koeo meant KeyOnEngineOff but Bentley indicates that the engine has to be running for IAC to buzz. As you say, and reading online, most rotor type IAC valves appear to be negative switched by ECU.
I think measuring current draw of IAC when the no start happens would be useful. I see an interesting diagnostic tool and useful info here:
https://www.picoauto.com/library/automotive-guided-tests/rotary/
if I'm understanding things, it seems the valve doesn't pulse open closed less or more to meter air but actually opens further as required by added loads etc. :
"The ECM controls the valve opening position by varying the duty cycle of the switched earth signal. Therefore, the greater the on time, the further the valve opens allowing more air into the engine resulting in higher idle speed."
I suspect the valve is sticking further open than needed when hot. Assuming the clearance between valve and body is small and that the bearings wear most at idle or off idle position, I could see the valve sticking as the issue vs a fault at coil winding or whatever. [edit: but then rapping on the valve when no start happened made no difference so....  ]
After swapping in a spare IAC a while ago, the engine created a giant back fire while decelerating down hill. I assumed that IAC was faulty but it shows the same specs on the bench as currently installed IAC so I may try it again.
A comment from a VW tech regarding the ABA PCV system relative to the 50º mount position was something like 'it's not ideal, there can be oil drain issues'. But the PCV system is clear, PCV valve is new low miles and IAC is clean.
Anyhow..... trust me to dive down "rabbit holes" of details. 
Neil.
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1981 Westy DIY 15º ABA
1988 West DIY 50º ABA
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Vanagon Nut
Samba Member
Joined: February 08, 2008
Posts: 10379
Location: Sunshine Coast B.C.
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| Posted: Tue Apr 23, 2019 10:37 am Post subject: Re: OBD1 ABA Conversion: Idle air control valve. |
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| Vanagon Nut wrote: |
I think measuring current draw of IAC when the no start happens would be useful.
Neil. |
So like duh. I have Ross-Tech. I'm not that familiar with it and not sure if it will measure data as the engine is restarted but I used Advanced and Vscope graphing to monitor IAC duty cycle. Cold engine, it's ~ 50% which makes sense, engine at temp at idle no load its much lower. 3% ?? Unfortunately, I'm not well versed in how to save a real time graph of various inputs but will find out how.
My other ABA OBD1 swap has a VSS installed at the speedometer. It does not have a hot start issue nor does it surge at idle. The ABA in question does not have a VSS installed.
I've read online that some engine management systems, even OBD1, require the VSS as an input to the idle stabilization system. e.g. the OBD1 Subaru 2.2 swap. A quote from this web page shed some light
https://www.gt40s.com/threads/vss-ecm-stalling.41687/
" The biggest problem I've seen on swaps with no VSS is idle
instability and/or stalling after coasting to a stop. Picture a car
coasting down hill with no VSS. The PCM sees 0% TPS but high RPM.
Since there is no VSS the PCM doesn't know the car is moving; it just
thinks the idle speed is high and needs to be brought down, so it runs
the IAC all the way in. Then when you finally stop, the IAC can't
always recover quick enough causing a crazy speed fluctuation and
sometimes a stall. "
On the ABA in question, though the MIL wasn't turned on, I harvested a DTC for VSS and a "31-10 open or short to ground - intermittent" for ISV. The latter may be due to me unplugging, plugging it back in to facilitate starting the engine when no start happens.
Neil.
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1981 Westy DIY 15º ABA
1988 West DIY 50º ABA
VE7TBN |
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Vanagon Nut
Samba Member
Joined: February 08, 2008
Posts: 10379
Location: Sunshine Coast B.C.
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| Posted: Thu Apr 25, 2019 2:00 pm Post subject: Re: OBD1 ABA Conversion: Idle air control valve. |
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I measured mA draw of 3 different IAC valves.
I had to use the 10 Amp scale on my digital VOM. Bentley spec is 0.620 mA +/- 0.20 mA.
On all 3 IAC valves, increasing RPM increased mA draw by roughly the same amount.
IAC valve that was mounted on 50º ABA: 0.60-61 which settled to 0.58-59 mostly at 0.58
Spare IAC valve which previously caused large engine backfire: 0.59 then kept dropping. At 0.44 I stopped the engine. "Rapping" on that valve almost stalled the engine but that wasn't repeatable.
IAC valve from 15º ABA swap that idles fine: 0.58 settling to 0.55
I'll run the 15º ABA valve and see what happens. Pure speculation as to possible causes of intermittent idle issue:
at idle position, subtle difference between an "automatic" transmission TPS and "manual" TPS as they relate to a given ECU. i.e. an ECU designed for the automatic or manual tranny Mk3. Quality of "Engine speed signal" from G28. The after market one I installed on the 50º ABA looks and feels cheap. e.g. it appears to allow a little oil to leak out where it fits to block. It's not a great fit IMO. Of course disconnecting it while engine was running stalled the engine so the signal is obviously present.
Neil.
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1981 Westy DIY 15º ABA
1988 West DIY 50º ABA
VE7TBN |
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Vanagon Nut
Samba Member
Joined: February 08, 2008
Posts: 10379
Location: Sunshine Coast B.C.
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| Posted: Fri Apr 26, 2019 3:03 pm Post subject: Re: OBD1 ABA Conversion: Idle air control valve. |
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| Vanagon Nut wrote: |
I'll run the 15º ABA valve and see what happens. Pure speculation as to possible causes of intermittent idle issue:
.... at idle position, subtle difference between an "automatic" transmission TPS and "manual" TPS as they relate to a given ECU. i.e. an ECU designed for the automatic or manual tranny Mk3. Quality of "Engine speed signal" from G28. The after market one I installed on the 50º ABA looks and feels cheap. |
Both the low mile "cheap" RPM sensor with plastic housing and spare OE looking metal housing sensor Ohm out close to the low end of spec. Waving a heat gun set to low at each sensor for 2 minutes increased temp to ~ 112º F. This increased resistance value but neither went past ~ 620 Ohms.
Parts installed to a spare TB, at idle position, I measured a used spare "automatic transmission" TPS and used "manual transmission" TPS that I'd taken apart and cleaned. I saw about a 700 Ohm difference:
manual: 3116 Ohms
automatic: 2400 Ohms
Difference could be due to several things but when installing the automatic type to the TB, it looked and felt like it preloaded the wiper more than when installing the manual type.
Of note, the "manual transmission" TB incorporates a dashpot to soften throttle movement as it returns to idle.
I've used both types of TPS on my 15º swap currently using the manual type. It shows 2800 Ohms at idle position and the same type of TPS currently on the van in question shows 2900 Ohms. I don't know if the ECU on the van in question was for an auto or manual tranny mk3.
Neil.
The "automatic transmission" TPS is on the left.
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1981 Westy DIY 15º ABA
1988 West DIY 50º ABA
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Vanagon Nut
Samba Member
Joined: February 08, 2008
Posts: 10379
Location: Sunshine Coast B.C.
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| Posted: Fri Sep 25, 2020 1:49 pm Post subject: Re: OBD1 ABA Conversion: Idle air control valve. |
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Is it normal for a 2 pin PWM type idle valve to increase current draw as engine RPM is raised via throttle? Does that happen on the WBX 2.1 Digifant idle control valve?
To me this makes no sense as most or all of the intake air is going through the throttle; the idle valve serves no purpose at that point.
My impression is that increase in current draw directly correlates to increase in duty cycle %
Thanks,
Neil.
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1981 Westy DIY 15º ABA
1988 West DIY 50º ABA
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RawUmber
Samba Member
Joined: June 23, 2019
Posts: 223
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| Posted: Fri Sep 25, 2020 4:02 pm Post subject: Re: OBD1 ABA Conversion: Idle air control valve. |
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On a "B" unit I tested:
| Code: |
150-840 RPM = 0.61A
1000-2750 = 0.54A
3000-5000 = 0.50A |
higher amp == higher duty cycle == higher valve open time == higher air flow
My amp values might be out of spec, but I think the trend is right.
Digifant ISV always hums, so it never actually gets turned 'off', rather its duty cycle is varied based on the parameters.
I also doubt that the ISV assist when >1000 RPM is helping very much, but I haven't experimented to see for sure. |
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Vanagon Nut
Samba Member
Joined: February 08, 2008
Posts: 10379
Location: Sunshine Coast B.C.
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| Posted: Fri Sep 25, 2020 7:56 pm Post subject: Re: OBD1 ABA Conversion: Idle air control valve. |
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| RawUmber wrote: |
On a "B" unit I tested:
| Code: |
150-840 RPM = 0.61A
1000-2750 = 0.54A
3000-5000 = 0.50A |
higher amp == higher duty cycle == higher valve open time == higher air flow
My amp values might be out of spec, but I think the trend is right.
Digifant ISV always hums, so it never actually gets turned 'off', rather its duty cycle is varied based on the parameters.
I also doubt that the ISV assist when >1000 RPM is helping very much, but I haven't experimented to see for sure. |
Thanks RawUmber. Ok. That logic is what I'd thought. The ABA valve is "always on" and controlled by ECU on a dedicated circuit.
So far, with two different clean used valves that Ohm out ok.....
Van stationary, no added loads (e.g. AC) on engine, valve current is within spec but, valve current use rises as RPM is increased which is also reflected in % duty cycle figures in VCDS logs. But it seems to me that above idle, there really is no need for the valve to be doing anything. And, most or all the air is going through the intake boot, not the bypass.
For various reasons, I wonder if another sensor(s) is outputting bad values thus causing ECU to operate the idle control valve even when not required.
Neil.
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1981 Westy DIY 15º ABA
1988 West DIY 50º ABA
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Vanagon Nut
Samba Member
Joined: February 08, 2008
Posts: 10379
Location: Sunshine Coast B.C.
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| Posted: Fri Oct 02, 2020 3:39 pm Post subject: Re: OBD1 ABA Conversion: Idle air control valve. |
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Video of valve connected to ECU while another same idle valve installed on engine meters idle air. Video shows why current draw increases as rpm raised. Two different valves showed same current draw pattern. At rest, engine off, valve opening is at other side, if that makes sense.
https://photos.app.goo.gl/jfP7vQfv1TWEyekHA
Image of same valve, engine, ignition, off
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1981 Westy DIY 15º ABA
1988 West DIY 50º ABA
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