AHU to Mk2 Jetta questions

Installing an AHU diesel in a 1991 Mk2 Jetta.

I have both Mk2 gas and Mk3 diesel instrument clusters. Would rather use the Mk2 cluster 1) to avoid extensive mods to the cluster bezel & instrument panel; and 2) the Mk2 cluster has the MFA feature.

However, the Mk2 gas cluster lacks a glow plug lamp and needs a signal from the Mk3 alternator to activate the tachometer. (It has a tach from a diesel model.) I could probably install a LED from a diesel Mk2 cluster to get the glow plug lamp.

Does the Mk3 alternator provide a signal compatible with the Mk2 diesel tachometer?

First, there are three types of Mk2 clusters, there's the US production (assuming it's from a Golf or GTI) then there's the early CE2 with two connectors and then finally there's the CE2 cluster with single connector. These last two I think would be from Mexican production cars. Early CE1 cluster from Germany would have had to come on a Jetta but for later electrics these early clusters (German and US) would be more difficult since there's no CEL.

What you really need is the late Mk2 diesel cluster with single connector AND the harness connector for the CE2 fuse box to go with it. Alternatively you might be able to use the middle cluster with dual connectors for diesel but works with CE2 too, but I don't know that for sure. Since the late 90/91 diesel didn't have tach, nor did the ECOdiesel cars, you'd have to add one to that cluster, similarly to how people added a tach to the early Golf diesel clusters. That or get a late CE2 diesel cluster out of Canada from a TD car, since they got them there. I have one of those somewhere.

Even though your gas cluster has a diesel tachometer the circuit (green) board might not be suitable for diesel cars and for certain the dynamic oil pressure circuit in the speedo head is different for gas and diesel. I know there are different cluster circuit boards for gas versus diesel and definitely differences between early and late as well. Also the MFA function requires sensors that are not generally incorporated in the tdi cars, i.e. you have to add sensors (outside air temp, oil temp etc) to get those functions anyway AND the mpg function is notoriously bad on MFA anyway.

Then there's the issue of the GP circuit too.

You really should have the last of the CE2 MK2 clusters, IIRC it also has a check engine light incorporated into it and that would be something else you'd need too, besides GP light.

To your final question, yes, the Mk3 alternator has a W terminal like the older ones but what it actually does I don't know, the engine crank position sensor tells the ECU the RPMs so I don't know why they still have W function with alternator.

Anyway, hope this helps you out.

Steve