Engine RPM Sensor Connector

[Achilles3588]

1.6 TD 'JX' mTDI swap to 1.9 TDI 'AFN' in a '91 Syncro Doka.
Note: mTDI, no ECU on this rig. I suspect that's why I'm having trouble finding a connector.

Planning to use the Engine RPM Sensor output (conditioned) to drive a tach for this truck.
Have scoured DigiKey, eBay and other online VW sources for the (free hanging, in-line) mating connector to no avail. 3 position per the wiring diagram.

Any help is appreciated.

[Achilles3588]

Did some more digging on ETKA.
P/N 357 972 753 is the one.

[libbydiesel]

If it was a JX and had a tach, then the tach is a diesel tach.  You can simply connect it to the W-terminal of the AFN alt and adjust it for proper rpm reading at the tach.

[Achilles3588]

If it was a JX and had a tach, then the tach is a diesel tach.  You can simply connect it to the W-terminal of the AFN alt and adjust it for proper rpm reading at the tach.

The JX had no tach but did have a W signal coming off the stock alternator.
I have secured a gasser tach and a Dakota Digital box.
The AFN's alternator (remaf'd Bosch 90 Amp unit) has a DF signal which, if understood correctly, is potentially electrically equivalent to the W signal. I am running now, but the DF signal only yields 0.2V output, so I don't think it is delivering what the DD box needs.

[Achilles3588]

Swapped out the AFN's alternator (actually 120A vs. 90A as stated previously) for a 90A Bosch unit AL0181X which has a 'W' signal.
Confirmed an input and output from the DD box and then 19V +/- on the signal wire at the back of the tach.
Contacted DD and they confirmed yet again that VDO gauges require something on the order of 150V so I think I've confirmed this setup (W signal, DD Box, VDO Vanagon gasser tach) will not work.

However:

Wonderful thread. I have nearly finished my mTDI AFN-based build. For monthts my brain has worked on the problem: how to use the nicely built in vw sensor. Biased more to mechanics than electronics I knew that filling two gaps on the sensorwheel with steel would do the job. Not nice work wih an assembled engine! I knew that a divider by two would do the jobb. This and a device to protect it from spikes is all I knew on the electronicall side of it, so now I look forward to Smokey Eddys solution. The vw sensor (G28) has three terminals.1 and 2 is the coil and I guess 3 is grounding the capsule. I connected 1 and 2 to a volvo/yazaki gasser tacho and tested it on my lathe and it showed correct and stabel rpm.


Bo Gunnar

[libbydiesel]

I've converted a couple gasser tachs to diesel.  Here's the how-to:  https://www.thesamba.com/vw/forum/viewtopic.php?t=610274

[Achilles3588]

Before moving into the PCB mod / gas to diesel conversion I explored another option.
I had purchased a black box which was designed and built to take the G28 CKP signal do the necessary math for the gas tach to use. I decided to try it and at least rule it out as a option before moving on.

Wired this all up as follows; G28 terminal 1, provided input reference voltage (key on power, circuit 15, which also provides power to the black box) from the relay box in the Doka's engine bay; terminal 2, is the G28 output a.k.a. input signal to the black box; terminal 3, common ground for the box and the G28. In this configuration, the W signal from the tach is unused.

The tach did indeed register a signal, albeit low. 0 RPM showing at idle initially, but was responsive to the throttle. Adjusted the potentiometer at the back of the tach from 0 to 100% of full rotation but could get the tach to show only 250~300 RPM at idle ( which is ~960 RPM per handheld laser tach) with the pot tuned at one of the extremes. Other engine tweaks are preventing me from taking the rig out on the road to see performance over a roadspeed range, but I'll report back real soon now.

[libbydiesel]

It doesn't matter to me which way you go on this but in the interest of helping you make an educated decision I would mention that the components for converting the tach are very inexpensive and it is easy to do.  It is also fairly easy (and virtually free) to add a W-terminal connection to an alt that does not already have one.  That results in the fewest components to go wrong, lowest overall expense, and that wheel has already been invented and documented.