Use G28 RPM Sensor w/o ECM?

Posted elsewhere but was recommended to crosspost here.
Newb to the TDI world. Have not entered the honeymoon phase...still in courtship phase.
1.6L TD 'JX' being swapped to 1.9L mTDI AFN in a '91 Syncro Doka.
No plan to upgrade with an ECM (none was in the stock vehicle).
Is it possible / feasible to use the G28 Engine Speed / RPM Sensor to provide a signal to a tach ('cause the AFN has one)? No W signal off the shiny new alternator (as yet); have considered a VSS but want to run this option down 1st.
Does the G28 take a simple 12V power supply?
Most importantly, what function would each lead 1/2/3 have? I have difficulty understanding the Bentley...
That is, which lead is source voltage, ground, output signal?
Thanks for any help.


http://forums.tdiclub.com/showthread.php?t=507188

These type of sensors create their own voltage. Generally they put out an ac voltage at different voltages and frequencies. What voltages I'm not sure. The frequency should be equal to the RPM, though I may be wrong.

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you can use it to supply a tach signal but the four slots on the reluctor will give you double the pulses needed for a 4 cylinder tach.

I've sourced a black box that will do the math on the number of pulses according to the vendor (not Dakota Digital). But how to wire up the sensor?

I've sourced a black box that will do the math on the number of pulses according to the vendor (not Dakota Digital). But how to wire up the sensor?

Might take a little work studying. I would get pins for the plug and create a harness that can be repinned.

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eddieleephd said:

These type of sensors create their own voltage. Generally they put out an ac voltage at different voltages and frequencies. What voltages I'm not sure. The frequency should be equal to the RPM, though I may be wrong.

Click to expand...

I've learned there are two types of sensors, inductive and Hall Effect. Inductive sensors do indeed create their own voltage. Hall Effect typically require a source voltage / ground. At least, to my understanding. I'm operating under the assumption that the device on my AFN block is an HES, but could be wrong.


Placed leads across terminals 1 and 2, get the spec resistance 1310 ohms +/-.

What happens when you shake the sensor up and down on ac current.

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Seriously?

I have a B3 Passat PD converted using the analogue original dash, all i did was to take the cover off the back of the alternator and solder a wire onto 1 of the AC windings, feed the wire through the cover and connect to the original tacho wire in the Passat loom, and it works as it should.

Achilles3588 said:

I've learned there are two types of sensors, inductive and Hall Effect. Inductive sensors do indeed create their own voltage. Hall Effect typically require a source voltage / ground.

Click to expand...

Yup, so inductive sensors are generally two-lead devices whereas Hall Effect sensors are three.

My guess that the crank sensor is a Hall is based partly on it having three wires. Could be two wires plus shield, of course, but that would show up as two of the pins being a dead short.

One check would be to see if you can read a 12V supply line as one of the terminals on the harness. If so, it's probably not an inductive sensor, and once you confirm the ground terminal with an ohmmeter the 3rd one will be the output signal, which may well be able to drive your math box directly. The manufacturer should be able to tell you?

Well, my hunch may be wrong, at least if your engine shares the same sensor tech as the PD.

Tracked down the PD Study Guide (one of the better guides out there)... it says the cam sensor is a Hall Effect but the crank sensor (G28) is inductive:

https://www.myarchive.us/richc/VW_TDI_with_PumpeDuse.pdf

Page 33.

It's scanning a toothed wheel with 56 teeth, so the frequency is going to be very high compared to a 4-banger gas tach.

May need to track down an oscilliscope to figure out actual voltages, or maybe modifying the alternator for the W terminal is an easier fix.

Again, all of this applies only if the PD uses the same style of sensor... the PD Study Guide is the only model I've ever seen that manual for.

Is your sensor round and cylindrical, like the version shown in the PD manual?

hey Vince, he's running an AFN mechanically...it has a four slot reluctor.

Good point, was thinking more if the sensors look alike they probably are... VW using parts as Lego like they do.

4 slot reluctor makes the math way easier, for sure.

Vince Waldon said:

Good point, was thinking more if the sensors look alike they probably are... VW using parts as Lego like they do.
4 slot reluctor makes the math way easier, for sure.

Click to expand...

very similar but different part number ...same type of three wire sensor though.

Vince & Jim - almost certainly an HES.
The black box vendor provided everything just short of actual help to determine the function of each specific lead on the G28. More like, I got scolded for trying to do use the G28. So I've kind of punted and started thinking about adding a W signal from the alternator. Got just to the point of breaking out the soldering iron when...I looked at the wiring diagram for models from Sept. 1997 on page 97-407 of the Bentley, covering Golf & Jetta '98-'99 w/AHU's which is the closest engine configuration to my AFN. This is where can be seen wiring corresponding to my AL0723X re-manufactured Bosch alternator, which has B+, D+ and DF terminals. To me signal DF signal looks electrically equivalent to a W signal. The only difference would seem to be the output of this signal is being sent to an ECM (in the stock vehicles) vs. an instrument cluster (like the W signal on other vehicles of that era), or to a Oil Pressure Warning Control Unit (which is my Doka's case). To my way of thinking it is an equivalent W analog signal, it doesn't seem likely that it is a digital input to the ECM from the HES. Conversely I think the ECM can accept / requires various analog signals, that is doesn't require digital only inputs, and then do whatever it needs to with them internally, electronically and/or digitally. So this suggests DF is the same as W. It's important to have that W signal for this Oil Pressure Unit, guessing it is like an 'enable' or 'run' signal to whatever is doing the 'math' there on startup or while running. I'm intending to treat this connection as equiv to a 'W' and attempt to use it to drive my tach, I can see no reason not to do so. As described earlier, other options are not working out as envisioned. But I hold out hope on the HES as a backup option, should I be able to finger out the wiring on the sensor lead. Perhaps that pseudo-W signal will have to pass thru a convertor box (such as Dakota Digital) or else the gasser tach I have mounted will have to be modified, I'll just have to figure that all out.

The W terminal's pre-diode bridge actually...non-rectified AC from any one of the three field windings, so easy enough to add. I've a schematic somewhere, from my IDI days, plus a scope picture of the waveform/frequency.

Will track 'em down sometime today and re-post.

You will need a math box or a re-cal of the tach you're using for sure, as the frequency is not an even number due to the ratio of the pulleys.

Here's the schematic... any field winding will do:



Fuzzy scope picture of the output waveform:



17V (ish) peak to peak. Can't trust the timebase on that scope but IIRC the ratio is somewhere in the 7-8 pulses per revolution.


Here's a good thread from over on vwdiesel.net about re-calibrating VW gasser tachs to work with the W-terminal:

http://vwdiesel.net/forum/index.php/topic,3741.0.html


A thread on adding the W-terminal itself:

http://vwdiesel.net/forum/index.php/topic,34707.0.html

CasaEd said:

I have a B3 Passat PD converted using the analogue original dash, all i did was to take the cover off the back of the alternator and solder a wire onto 1 of the AC windings, feed the wire through the cover and connect to the original tacho wire in the Passat loom, and it works as it should.

Click to expand...

Already mentioned the option of using a wire off the AC windings, very easy to do, no fancy boxes needed

Just to confirm, you were driving a cluster with a gas engine (aka 7000 RPM) tach directly from a W terminal added to the alternator?

The OP is buliding a mTDi I believe, if you use a gas Tacho it will read wrong, My Passat is a diesel engine driving a diesel instrument cluster.

Yuppers.

Was sure I had read he was planning to use a gasser Eurovan tach for all of his... and after a bit of re-looking did just find it again... it's referenced in his *other* thread as opposed to this one. Hence the comments from several on a math box being part of the equation....which only make sense if one has also seen the other thread.

CasaEd said:

The OP is buliding a mTDi I believe, if you use a gas Tacho it will read wrong, My Passat is a diesel engine driving a diesel instrument cluster.

Click to expand...

Indeed yes, mTDI. Apologies for any confusion. Understand that using a gas tach will read wrong. Have a convertor box on order.

using a clutched alternator pulley? "W" will not give an accurate signal when lifting the throttle...mightnot be a big deal but i would prefer to read the flywheel teeth or use the cps.

That's a good catch.... I retro-fitted a clutched pulley on my AAZ back in the day and it is in fact a little annoying to have the tach hang for a bit exactly when watching it during a shift.

jimbote said:

using a clutched alternator pulley? "W" will not give an accurate signal when lifting the throttle...mightnot be a big deal but i would prefer to read the flywheel teeth or use the cps.

Click to expand...

Sure am using a clutched alt pulley and gosh darn you for being smart enough to bring this up!

I'll be happy when I can get this thing running again, then if the tach performance is truly annoying, go back to my original plan of using the Engine RPM Sensor G28.

If any one is interested there is a good thread here by regcheeseman, for those who would prefer to modify there exsisting gas tacho to diesel :-


http://vwdiesel.net/forum/index.php/topic,3741.0.html


As regards to not having an accurate rpm signal when you lift off the throttle, not so important to me, hardly notice it to be honest, more concerned it's correct under power conditions