Diesel engine in gas car tach question

[Henchman]

So I have a 91 Passat wagon that started life as a gas, but now has an AAZ in it.  The gas tach isn't working.  Is there any way of making the gas tach work with the w term on the alt. or am I just better off to extract the diesel tach from my other 93 passat diesel and go that route?  How does the W term work?

Ian

[Vincent Waldon]

The clusters are proprietary to VDO and Motometer so there's not much actual information available other than what folks have reverse-engineered.

Having said that, there are three really good threads in the "Tach" section of the FAQ that cover a ton of ground on this one... I'd recommend starting there and then see if you have any outstanding questions.

[Henchman]

Ya, I've been reviewing those FAQ's, nothing really on point.  Do you know if there is even a pinout for  the cluster out there?  Cluster connectors are the same for both gas and diesel, but circuit boards are different.  Gauges are interchangeable.

Ian

[Henchman]

So her is what I've done.  Used the cluster shell from the 93 passat diesel (all the lights had been replaced), circuit board and tach.  From the Gas cluster I used the speedo/odo and the fuel/temp.  Buttoned it all back up, everything works except RPM.  Tach input is pin 10 on back of cluster, but previous owner had done some custom electrical work to get the diesel running in the gas shell.  He got the engine from a 96 jetta, so he didn't have the benefit of a diesel passat like I have.  I love how these clusters go together like leggo!

One more question.  The W terminal on the back of the alt.  From what I read, the right pin (if looking at back of alt) is the tach reference, and the left pin is for?

Thanks,

Ian

[Vincent Waldon]

Yeah, little is known about A-chassis clusters (Golf/Jetta)...even less is known on the B chassis like the Passat.

In terms of the alternator:



The D+ line is the excitation voltage from the cluster thru the "BAT" LED.

[Henchman]

Tell me more about the B+ .  I've been able to determine on my 93 passat diesel that if I disconnect it, the tach won't work, even if the W term is still connected.  What purpose does it serve and is there a way of jerry rigging it?  It seems to get/put out 5-7 volts when running.  Any help is appreciated.

Ian

[Henchman]

I got it!  The problem was the picture you posted relative to my alternator in the car is upside down, so what I thought was the W term was the exciter.  Previous owner already had exciter hooked up, so I just ran a wire to term 10 on the back of the cluster and voila.  Just to recap, I have used the circuit board and tach from a diesel car into the gas cluster.

Ian

[rodpaslow]

Would you know if it's the same pin on a 96 golf or jetta?  I have a 96 golf I've just changed and I'm trying to get the tach working.  I hope it's the same.

[Vincent Waldon]

Would you know if it's the same pin on a 96 golf or jetta?  I have a 96 golf I've just changed and I'm trying to get the tach working.  I hope it's the same.

Here's the Golf/Jetta MK3 cluster pinout.. the tach is in fact pin 10:

[rodpaslow]

Many thanks!