Relay Switch: Troubleshooting from 40 km/h

Problem:
Around 40 km/h, a loud humming sound can be heard, which seems to be coming from a relay. The frequency and volume are constant and independent of the engine speed. The relay is numbered 992 and is located in the fuse box.

Question:
Which hidden (additional) function depends on the vehicle's speed? Is it an undocumented (not mentioned in the user manual) safety feature? Where could a defect be located? Could it be related to the all-wheel drive system?

Vehicle:
VW Bus Caravelle Synchro, Diesel, Year of Manufacture = ???
(Unfortunately, there is no one present who can check the registration document for more detailed information.)

Hi, well, I can give you an extremely hot tip, just this once!

In my case, the exact humming sound was produced by the EGR control valve, which is conveniently located on the injection nozzle housing, next to the coolant reservoir.

This valve is a 3/2-way solenoid valve that controls the vacuum air supply to the AGR membrane sensor.

If it's defective, the internal component (the control valve) starts to vibrate, rapidly switching the vacuum air on and off to the diaphragm chamber (creating a harmonica or reed pipe effect?). This only happens when the engine control unit, according to the EGR map, closes the EGR system under higher engine load. (see technical article on exhaust gas recirculation!).

A solution is to carefully seal the vacuum hose connecting the engine to the control valve. The best way to do this is to glue a plastic part (a dowel or something similar) inside the hose using a two-component adhesive. Then, discreetly drill a hole in the hose leading to the diaphragm chamber, ensuring that no vacuum can build up.

If you want to keep the AGR (Abgasrückführung system) to operate the vehicle on public roads, the above tip is only a temporary solution, and you will need to obtain a new valve.

Regards,
Holger.

EDIT: Oops, your car doesn't really have much in common with mine. Maybe it's something else entirely. In any case, I would still follow that tip. Locate the mentioned valve and first disconnect and seal the hose (coming from the engine side), then take a test drive.

Hi,

For a reversible test, you can also disconnect the hose from the EGR valve and seal it airtight with a suitable screw to prevent dirt from being drawn in.

Best regards,

Jan.

Hi Jan, I don't want to come across as a know-it-all, but even then, the solenoid valve will still vibrate. (I've personally experienced all of this ). The valve only stops vibrating when it no longer receives any vacuum air.

Okay, so don't disconnect the hose between the valve and the diaphragm, but disconnect the hose between the motor and the valve, and temporarily seal it with a large screw.

Regards,
(and don't be angry ).
Holger.

Hi,

"Sorry, you're right. I hadn't realized that you wanted to disable the entire valve; otherwise, this wouldn't make any sense here."

If an error entry is not critical, it can also be cleared by unplugging the device and then deleting the error. This can be useful for testing purposes.

Best regards,

Jan.