560SEL EHA replacement

Today’s job was EHA replacement (Electro-Hydraulic Actuator) on my 1987 560SEL.   The EHA is part of the KE Jetronic fuel injection system and makes minute adjustments to the air/fuel mixture.   One of the sources for these adjustment is the oxygen sensor, if present.   The EHA can leak either because the o-rings between the EHA and the fuel distributor have gone hard, or because of internal leaks in the EHA itself.   In the case of my car, it looked like the EHA was leaking, not the o-rings.

The EHA is still available from Bosch, but it is not cheap.   Still, I can only imagine it will get more expensive in the future.   It is easy to tell if your car has a replacement unit as the screws holting it on will be torx.    The original ones are flat head.  If it is not pretty clear the EHA is leaking, it can be worth trying to change the o-rings first.

EHA replacement

The EHA is accessible once the air cleaner assembly is removed.   It attaches to the side of the fuel distributor, near the fuel pressure regulator.    Its only a 10 minute job to change it out.  The most important thing is to make sure the o-rings are seated properly.   I like to put some paper towel underneath the EHA during this process.   Not only will it catch any fuel that leaks out, but it will also catch the o-rings if you drop them.

Obviously, as I was working with fuel I had disconnected the battery.   Since I was doing the battery tray at the same time, I had remove it completely.

EHA replacementEHA replacementThe EHA makes minor adjustments, but it is possible to adjust for baseline settings.   I already covered the adjustment of the EHA in a previous article.   It is worth watching the two videos linked in that article.   I hooked up my gauge set to check the pressures.  My initial readings were 5.9/6.35.   The ideal pressure differential between control pressure and working pressure is 0.4.    I adjusted the EHA 1/4 turn, which changed my pressures to a tad above 5.8 and 6.35.    This mean the differential was very close to 0.4, just a tiny bit over.   This was exactly where I needed it to be.   These pressures are tested with the EHA unplugged as I was trying to set baseline pressure.

After re-checking the pressure then removing the gauge set I carefully checked for any leaks.   After the EHA replacement I took the car for a test drive it felt no problems.   At this point it is normally worth checking the idle mixture.   My injectors are about to be replaced, so that would be premature.

EHA replacement complete


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