ALH to Mk3

IF an ALH engine were installed in a Mk3, and IF it used a waste-gate turbo instead of the VNT turbo, could the Mk3 ECU be used?

You would need to use the ahu wiring harness as well. Change the pump connectors, the 3rd injector plug would probably be different and the coolant sensor is different as well. Not sure if the ALH has the right bosses for the mk3 chassis mounts. I have a ahu running with a alh wiring harness and ecu

Its in a ranger swap project. It gives a little better/finer tuning options from more maps as well as port flash ability instead of chips. Those are the main things, but really the only reason I have that combo is because I fried the original ahu ecu and had the alh stuff lying around. I would have stuck with the ahu ecu if I didn't kill it.
Whats your situation? Killed the ahu in your car?

Neither. Have a Mk3 gas Jetta in good shape and want to put in a tdi. Both AHU and ALH are available. The AHU has a complete engine harness and ECU. The ALH has the ECU and some wiring from ECU.

The AHU conversion would require removing engine and harness and replacing with MK3 parts—the engine and the wiring harness, shifter, etc.

The Mk4 ALH to the Mk3 car would require many changes—wiring, custom motor mounts, etc.

Opinions, please. Worth the time to use Mk4 ALH?

The ALH block still has the holes to mount the rear engine mounting as do the PD 1.9 tdi blocks so no need for a custom engine mounting, the wiring is a matter of preference, it depends whether you want to flash over obd or have the AHU ECU socketed. Yes there are different plugs on the ALH loom, temp sender, Crank sensor and the oil pressure switch works the other. The injector pump if you go for the AHU is a simple plug swap and three wires to solder, the crank sensor can be swapped out for the AHU one as can the temp sensor, and use the AHU oil pressure sensor, so not so difficult imho

Do you mean that an ALH can be used with an ECU for the AHU an a 98 Jetta? If it can, that's a vote in favor of the ALH.

The sensors (temp, and crank position) should not be difficult, especially if the Mk3 units will fit the ALH.

If I understand you, the AHU ECU can be adapted to the ALH pump by "a simple plug swap" and soldering three wires. If that's what you meant, is the plug—connector—to be swapped the pump connector?

If the ALH were put in this Mk3, will the factory turbocharger clear the Mk3 motor mount? In a Mk2 a special mount is needed for some turbos.

Thanks for the info!

I personally changed the 10 pin plug on the wiring harness as I had a spare one, but it would work the other way too. If you are using the waste gate turbo from the AHU it will clear the engine mounting.

Installing an AHU in a Mk3 Jetta would present fewer challenges than putting a Mk4 ALH in that Mk3 1998 gas Jetta. With an AHU engine, use Mk3 TDI engine wiring harness, intercooler, fuel tank, and instruments—those are the main parts. Seems much simpler.

It's using a Mk4 ALH in a Mk3 Jetta that will require more changes.

For example, you said you "changed the 10 pin plug on the wiring harness." If you mean you changed the 10-pin plug on the car harness to fit the 10-pin plug on the ALH injection pump, I understand. If not, please clarify.

If I install an ALH engine in the Mk3 I would use the factory ALH turbocharger. The ALH fits the Mk4 cars that do not have a rear motor mount on the right side, but the Mk3 cars have that rear mount. I wondered about interference of ALH exhaust with Mk3 rear motor mount.

Thanks for the info!

In the case of using an alh turbo, get a bus exhaust manifold from a b5.5 pass at and you'll have to contact Matt Whitbread to make you an adapter. Look at my mk2 build thread in the conversions section for the sky engine harness to ce2 fuse box wiring conversion.

Forgot about the accelerator pedal and its throttle position function. The Mk4 pedal probably won't fit a Mk3.

Can the Mk3 throttle apparatus be adapted to the ALH engine/ECU?

Sorry, bew exhaust manifold. Find a mk3 jetta or b4 passat tdi accelerator pedal and bolt it to the existing pedal cluster. You will need to switch around the pedal wiring to adapt to the mk4 harness. Many good write ups on this forum

You can use a mk3 or mk4 pedal, but the mk3 is the easier one to use, the mk3 pedal is very versatile, can be used externally, removing the short cable attached to the quadrant, and either make a mounting bracket, or buy one, and then use a normal throttle cable with a solderless nipple to operate the quadrant, this is the method I used on my PD B3 passat. ALH wastegate turbos do not hang down as far as the vnt turbo you are thinking about, just look at a B4 1z and you will see what I mean, it's more or less the same turbo.
Yes the plug I changed to the 10 pin was on the original wiring loom with a 028906021xx ECU