VW 1.8T or 2.0 Conversions: G28 crank sensor affect idle?

[Vanagon Nut]

Hi folks.

Edited:

If a G28 crank sensor is failing intermittently and - or generating a consistent but weak signal, can this affect idle?

Can engine block heat soaking into sensor exacerbate an intermittent fault in the sensor?

On my OBD1 ABA 2.0 typically after engine up to temp then driven ~ 15 + minutes (higher ambient temps likely a contributing factor):

- when disengaging clutch coming to a stop, about 2/3 of the time, idle dips to ~ 400 RPM or lower sometimes causing engine to stall.

- engine hot, shut down ~ 15 minutes, at restart idle sometimes goes up/down several times then settles at ~ 840 RPM.

I:

- cleaned ISV, measured Ohms (ok), swapped in known good ISV
- moved ISV away from exhaust manifold heat
- Ohm'd out harness: ok. (didn't check for cross wired mistakes by me though but MIL doesn't come on, no DTC's)
- TPS checks out ok
- all connectors seated

No improvement seen.

I logged with Ross Tech and from what little I know of using VCDS, Lambda, throttle angle, timing, coolant and IAT temps, voltage, all seem ok. IAC duty cycle is 40 - 60 % but will double check that.

I'm still a newb to Ross Tech (VCDS).

Thanks,

Neil.
_________________
1981 Westy DIY 15º ABA
(VW Gas I4)

1988 Westy DIY 50º ABA

VE7TBN

[dkoesyncro]

My experience has been hard starts to eventually a no start. Although the IAC is robust and probably one of the better designed they will still coke up/dirty. I guess if you are experiencing a drifting idle I'd look here or even a vacuum leak??

[Vanagon Nut]

Thanks. About a year ago, while miles away from home, I swapped out a then new G28 sensor (it allowed oil to leak out) for my spare which has electrical tape wrapped around wires where they enter sensor. (outer layer had fallen off). I'll re inspect that sensor.

What would be ideal would be to measure the wave forms from the G28 sensor while driving but I don't have an oscilloscope or logging meter.

AFAIK, the ISV meters all "idle" air bypassing throttle so if there's a leak, it'll be post ISV I think.

Logged a VCDS session on the way home and the dippy idle happened but for some reason, I can't view the whole file on my free .csv editor. Maybe I'll find the event in V-scope.

Neil.
_________________
1981 Westy DIY 15º ABA
(VW Gas I4)

1988 Westy DIY 50º ABA

VE7TBN

[dkoesyncro]

I've been toying around with the stock systems. Things just never work like the donor they came from. My problem may be bad solder joints that looked good and ohm good and over time may fail, possibly heat, vibrations, or simply old ECM.

I know you're flirting with the diesel and I don't blame you, my ABA is the only gasser in our fleet. At a time before thinning the shop floor I had an AHU, ALH and a BHW on the floor ready as potential swap candidates. Then my trans rebuild came sooner than expected...after much talk and lengthy discussions and junk pile examples. My heart broke and the dream died. Which brings me to the moment, I do love my ABA and am moving towards the Megasquirt. I've ordered the kit and will be swapping out soon enough. You will have full tunability and control over the system via lap top or wireless devices. Cost is cheaper than going through any swap or on par. Maybe this would be a good choice for you?

[valvecovergasket]

[Vanagon Nut]

[dkoesyncro]

VCG has been super helpful and willing, I'm glad he chimed in when he did. He's south of you Neil, look at his link in the sig.

[hans j]

It could definitely cause those symptoms, but honestly it's best looked at with an oscilloscope. That will tell you exactly what the sensor is reporting.
_________________
1986 Canadian Syncro Westy TDI - 1989 Syncro Single Cab - 2001 Audi S4 - 1981 VW Caddy ABA - 1980 VW Caddy EV - 1973 VW T-181

[Vanagon Nut]

Thanks hansj. Yes, this is what I was thinking.

The issue really seems to be heat related. Right now, weather is colder, issue isn't happening. So, I may try heating up the G28 sensor with a heat gun, on low setting, while engine is running. Maybe do same with ignition coil and ISV.

Neil.
_________________
1981 Westy DIY 15º ABA
(VW Gas I4)

1988 Westy DIY 50º ABA

VE7TBN

[Vanagon Nut]

Does ABA OBD1 ECU use G28 and distributor hall for engine speed or just one or the other?

Throttle angle engine off ignition on: 10º At idle or coasting in gear: 9.2 - 10º

Ross Tech (VCDS) logs show what *look like* incorrect sporadic low ignition angle BTDC values. Maybe due to ignition adjustment "catching up" after a sudden release of throttle to idle position? IF those values are not normal......

IF an intermittent incorrect engine speed input is causing those low ignition angle values thus causing sporadic idle or stall issues, I'd like to know if I should focus on G28 or distributor hall (wiring, sensors etc.)

Ross Tech sometimes goes off line when logging so i have yet to catch an idle surge or stall event, but I will.

Thanks

Neil.

Data transferred from VCDS (Ross Tech) logs.

similar multiple low ignition angle BTDC readings. They appear to happen as I coast in gear, foot off gas. The idle or stall issue happens after clutch disengaged while coasting to a stop. You can see the dip in idle to 640 which is below normal.

This includes MAF values. To me, it appears I'm coasting foot off gas in gear when low ignition angle values occur.

_________________
1981 Westy DIY 15º ABA
(VW Gas I4)

1988 Westy DIY 50º ABA

VE7TBN

[Vanagon Nut]

So.....

resistance of old G28, newer G28 and G28 in other ABA Vanagon: ~ 515 Ohms. Bentley spec: 500 - 700 Ohms. Moving wires on old G28 where they enter sensor did not change resistance.

With VOM leads securely connected to old sensor, rapping *lightly* with metal of small side cutter tool at end of sensor that reads toothed wheel, caused resistance to fluctuate up/down and went down to ~ 450 Ohms. Using a 3/9" extension to lightly rap on other end of installed newer plastic housing sensor, and doing same at hold down bolt, and near sensor at block, did not alter resistance value.

I have no idea if that means my old metal G28 was failing. I don't know how to explain why I think this but the sensor is magnetic so I doubt it has any solid state components. It's probably just a permanent magnet with coil wind.

Pics:

- slightly smaller OD of new part (~ 1mm, a non issue IMO). Rubber O ring is temporary.

- worn wiring on old part

A plastic ring is required to seal this sensor to the block.

I theorized that maybe the air cleaner wasn't getting enough air in so I opened up the other hole (originally the intake hole on a Ford). But, I did a VCDS log before that, with old sensor. Will log with new sensor.

Neil.

_________________
1981 Westy DIY 15º ABA
(VW Gas I4)

1988 Westy DIY 50º ABA

VE7TBN

[hans j]

It's a shielded sensor, that's why those wires are there.

In the ABA, it uses the G28 for crank sensor and the distributor as the G40 cam position sensor. That's why you get a cam fault key on engine off, because the distributor isn't spinning.

Crank sensors are notorious for failing in that engine, as is distributors and ecm's. Typically a crank sensor will fail when they get hot, creating a stalling issue or a no start until they cool off. You will also see the tach is not responding when the crank sensor fails.

Distributors and ecm's fail randomly.

I don't use resistance testing for much anymore, pretty rare actually. I do however use a scope almost daily. You can pick up pretty cheap single channel units off ebay. My favorite retailer though is AES Wave. I have one of their U-scopes and use it all the time. I also have a pico 4425 and 4823 and will pull out the u-scope if I only need to check one signal and not save a waveform. A scope is the best way to check distributor or crank sensor operation.
_________________
1986 Canadian Syncro Westy TDI - 1989 Syncro Single Cab - 2001 Audi S4 - 1981 VW Caddy ABA - 1980 VW Caddy EV - 1973 VW T-181

[Vanagon Nut]

Thanks a lot Hans J. Yes, for sure. I know that's the shield. The 3 wires appear intact. And thanks re info on inputs to ECU; I'll re read my A3 Motronic sections agin and will check out your gear references.

I did a bunch of varied driving with the new G28 installed. This still happened randomly but I could "make" the engine stall or idle dip really low by coasting in gear, foot off the gas for 10+ seconds then dumping the clutch at ~ 1300 RPM's. Especially if engine driven for a while. So, nothing has changed, issue still present. I have to wonder if it's a culmination of things causing this e.g.

- leaks between valves and guides (why, I don't know. Just a gut feeling and it shouldn't factor in as guide seals were replaced)
- manual tranny coded ECU and missing throttle dash pot (ECU expects idle to stay higher for short period when foot taken off accelerator)
- weak spark at lower RPM
- heat soak adversely affecting a sensor(s) or ignition coil.

Yeah, I'm "fishing" here.

It's all good though. I'm getting more familiar with what typical "good" values might be, and how they might correlate, in VCDS logs.

I think it's time I invested in some more electrical test equipment.


Neil.
_________________
1981 Westy DIY 15º ABA
(VW Gas I4)

1988 Westy DIY 50º ABA

VE7TBN