Harley Davidson M8 Engine ACRs – Testing and Removal

Ah the dreaded ACRs for the Harley Davidson Milwaukee Eight engines…the removal after testing is not an easy proposition.

So the ACRs or Automatic Compression Release valves (HD-18400073B) are merely solenoid valves that are activated on start up to release some pressure in the cylinders making it easier to start…however they can fail or not function properly.

Some tests will be, placing a tissue on top of the ACRs which are located on top of the engine adjacent to the spark plugs. On starting, the ACRs should let out some compressed air from the cylinders and you will see the tissue move. If they don’t, then it could be an ACR failure. But this should be followed with an associated error code…but sometimes that doesn’t happen either. This would be a failure in the closed position.

Now a more common failure is that in the open position. There may or may not be an associated error code. You will notice the tissue moving during start but should also continue to move as the engine is running too. This in itself isn’t a major issue albeit you are losing some compression and there will be an imbalance between the two cylinders.

However you will likely notice a knocking sound any time you start to roll the throttle on, and it can be quite distracting.

Sometimes the ACRs end caps have burnt off which means that air is being released, although the solenoid is still intact, hence no error codes. At other times the ACR has just loosened itself enough in the head that it lets out air, thus producing this knocking sound when riding.

You can further diagnose the ACRs by removing the tank and finding the ACR connectors. Perform continuity tests, ground resistance test but also get a test light with probes to see if there is a signal from the ECM telling the ACRs to open during start.
All of these small test will lead the way for you to try to remove the ACRs to visually inspect them or retorque them in place…and this is where the problems really start.

The service manual would say to remove the petrol tank and also the oil lines to access the ACRs; front and rear. However even with the oil lines removed, the access is very tight indeed. You will also need a 13/16ths or 21mm slotted deep socket to access the ACR thus allowing the wiring to be freed.

But the socket cannot be thick walled or else it will not clear the head casings, nor can it be too long as it will not clear the top frame and associated wiring and plastic housing.

There is a special tool from Harley Davidson (HD-48498-B-1) and other specialist tool suppliers, but you are talking of a princely sum of £150 or so just for the privilege of using it just the once. However even with the oil lines removed and using the specialist socket, it is still a very tight fit, and almost impossible to access from the oil line side.

So the other option is to modify a 13/16th or 21mm thin walled socket, cut it down and grind away until you can just slot it home with just enough clearance to break free the ACR, all of this from the opposite side of the oil lines, i.e. the left side of the engine.

This is probably the best option, but there will be a lot of cursing to get it done and a lot of moving electrical just to access the ACR, to undo it and then inspect or replace. Hopefully you can get it back in position without damaging it, because to replace it is nearly £95, so not cheap at all.

Of all the components on the bike, the ACRs are one of the trickiest to remove. They are also prone to seizing in the heads and breaking off which adds to even more pain and expense.
So if this is a job you are going to tackle yourself, go easy and try not to damage anything along the way. If clearance is an issue, then don’t force anything because it will likely break something.

If and when you have successfully removed and replaced the ACR, your next headache is to retorque it properly. Good luck with that!

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