Horn compressor overhaul/repair (Hella)

The horn on my car (like so much else) was non-operative when I got it. I had fiddled briefly with the wiring but although I got a burble, any sound was short lived and it soon became silent again. I quickly worked out that I had 2 faults:

Firstly the relay operation was erratic.!when it works there is an audible click when the horn switch is operated. In my car this relay is located on the right hand side in the driver's footwell. Changing the relay sometimes helped but even a known good one didnt always restore function. I could detect power at the relay power "in" pin but can't check supply to the switching circuits as I have an insufficient number of hands to probe the relay socket whilst also operating the horn button.

Secondly: Applying power to the relay power-out socket failed to rouse the horn- not a peep from the compressor so this has obviously failed too!

I couldn't find much on servicing a horn compressor- maybe they are usually replaced as they are cheap enough and readily available- however I thought I'd have a go and so stripped the horn out of the right-hand pod. The horn blower is mounted at the back right of the pod cavity bolted onto the plywood bulkhead. The rear bolt is inaccessible but for once this doesn't matter as its only a slip-over mounting. Loosen the front bolt and lift off the compressor. It has two wires on the base. purple/white (power feed from relay to pump positive) and black (earth) from pump neg terminal. The purple/white was continuous back to the relay terminal and the earth was good. Nonetheless, applying  12V across the pump fresh from the battery still failed to get it moving, so clearly a pump fault. Simple jams can apparently often be fixed by adding WD40 in the air spigot- but my pump had plenty of oil and I suspect that route had been tried many times before! I think its time to delve deeper...

I started by unscrewing the 3 self tapping screws at the top. 

Top of compressor- oil hole and air outlet spigot

These secure the cover over the impeller which spins in an oil filled well. Sliding vanes in the impeller maintain contact with the sides an eccentric well, scooping up air and expelling it to the small cavity under the outlet spigot.

Impeller cavity- note vanes in rotor, oil sponge and spacer in side cavity

I found no gasket here but I think there should be something given all this oil- again maybe someone has been here before me? In my pump the impeller rotor and motor shaft wouldn't turn at all so obviously a fault. The rotor is stiff but will pull off the shaft. Its keyed to the shaft with a small ball bearing that fits in a drilled out hollow- so catch this when it falls out and keep it safe.
Turn the motor over and loosen the two slot-head screws beneath.

 The top cap with rotor cavity will now come off but it might need a tap first. As the cap comes off the motor armature is revealed, and in my case was in a shocking state, jammed with rust and muck. The armature simply pulls out although you have to pull against the magnets and any jamming muck so it can be very stiff.

Pump body with armature removed, brushes just visible in base, sides covered in rusty magnetic debris!

Armature removed and cleaned, polished commutator with emery and cleaned  bearing surfaces

Clean the case thoroughly... this very difficult to do as its lined with 2 powerful magnets and the rust is magnetic too! I found it useful to scrape the sides with a screwdriver and then use a magnetic pickup tool to remove the rusty debris. I got it cleaned up as much as I could- much better anyway- and then I smeared corrosion block grease on the inside but not on the magnets.

The motor brushes are awkwardly located at the bottom of the case and hide the lower armature shaft bearing. My brushes were in good order so I carefully pushed them back in their cases and cleaned the bearing. Last job clean up armature and the brush commutator region.

Reassembly is tricky because of the brushes. I made a forked spring from a cable-tie.

Tool made as described. May need to trim the branches to get a good fit

Make a 2 cm loop and cut this to make a "Y" shape. then fold the tail of the Y across itself and clamp the fold in a vice to fix it. This puts a 90 deg bend near the Y and forms a handle that can extend up the side of the case between the magnets. Insert the springy Y between the brushes pushing them back clear of the lower shaft bearing hole.

Tool in use, note how brushes are pushed back into their housings, - these need to go back a little more to clear the bearing entry.

Carefully insert the armature (great care as the magnets try to grab it) until the lower end slips past the nylon Y and into the bearing. You can then pull the cable tie up the side as you slip the armature fully back down. Make sure you have removed all the cable tie (its a bit brittle) and that the armature can turn freely. This is a bit fiddly so I would test it before screwing up the case. I just placed a finger lightly on the end of the armature spindle and briefly applied 12V to the motor terminals to make sure it spins. When the armature is fully down its shoulders will be below the top of the magnets but I found this didn't really happen until I fitted the top and tapped it down. Something clicked and in it went- no idea what it was! Check rotation again and then refit the impeller. I found that if you hold the pump body horizontal with the ball bearing hollow in the shaft uppermost, the ball bearing will stay in place while you slide the impeller over it (lining up the bearing and its slot first). The blades are bound to fall out at this stage so refit them making sure that they are the right way round to scrape the walls closely as they rotate (anticlockwise in this case). The direction is obvious since they have to scoop air towards the outlet point. They are bevelled on one side to improve their fit with the wall- again its obvious. Add some oil, reinstall the oil sponge and its spacer and finally refit the cap with the 3 self tapping screws. I found this final step trapped the impeller and stopped it turning so I suspect there should be a gasket here.

I refitted the pump and connected it up to the air-hose and horns. Although the relay still doesn't work (job for next week) I applied 12V to the relay's outlet contact and was gratified by the ensuing horn blast  which brought  my neighbours running from their back gardens to investigate!
OK so its a cheap component but I reckon it should now last for another few  years!