My a/c compressor was working perfectly one day and then stopped working overnight with no apparent symptoms or strange noises. It attempts to switch on and then stops. All the fuses, fans etc are working perfectly and the belt is new with good tension. When I "jump" the compressor by supplying 12V to the clutch (blue-green wire I believe) the a/c works perfectly again with no strange noises or symtoms, no bubbles in the sight glass etc. My diagnosis is a bad speed sensor...... Do you agree?? Is it dangerous to run the a/c as i have it rigged with the speed sensor & pressure sensors by-passed??

YES, the compressor will wear out quickly if it doesn't cycle at all! ALSO the system will freeze-up & then you will need a evaporator ($2000).

That system has a computer that checks the speed of the engine VS the RPM of the engine called a BASE MODULE. Most MB scanners can retrieve live data & codes from that computer.

ALSO many times the speed sensor wires right at the compressor are broken on that model!!

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

This is the procedure for checking the RPM Speed Sensor:

Pull Compressor Control Relay (N6) off the 12 pole coupling.

Connect volt meter to poles 9 & 11 of the 12 pole coupling and adjust volt meter to alternating current. Terminal 5 & 7 of the 12 pole coupling remain bridged. Allow engine to run at idle speed approx 750 rpm.

Nominal value: atleast .3V AC

NO

Stop engine and check resistance of the speed sensor (L4) on terminals 9 & 11. Nominal Value 530 to 650 ohms, renew speed sensor if required.

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Zafar
94 E320 58000 Miles

You can bypass the speed sensor by using an "ICE CUBE RELAY". Do a search and you will find the instructions.

This way you will keep the pressure sensors in the loop.

The purpose of the speed sensor is to protect the drive belt incase the compressor malfunctions.

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Zafar
94 E320 58000 Miles

The E420 doesn't have the system that "zaferhayathan" refered to!!

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

The AC speed sensor is one possibility, The associated wiring is however more likely.

The speed sensor wires carry a signal not just a voltage, so they must be perfectly clean, ie; No ground leaks, shorts, etc.. The wiring was the problem in my case. The speed sensor itself was fine.

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]