I have 12V at the leads going to the compressor, but it will only engage if I jump it straight to the battery. I think the clutch is not getting enough current.

Common problem?

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1976 BMW 2002- 100k
1995 BMW 325i 175k
1984 Mercedes Benz 300D Turbo Diesel- 270,000 mi

1981 Mercedes Benz 240D- Estimated 300,000+ mi Returned to the earth

did you check for voltage at the plug? sounds like maybe your low on freon and the low pressure switch isnt letting the clutch engage.

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have no worries.....President Obama swears "If you like your gun, you can keep it

I'm checking at the black female plug, which plugs into the two male prongs on the compressor. Somewhere over 12 volts.

__________________
1976 BMW 2002- 100k
1995 BMW 325i 175k
1984 Mercedes Benz 300D Turbo Diesel- 270,000 mi

1981 Mercedes Benz 240D- Estimated 300,000+ mi Returned to the earth

you might borrow an amp meter and see how much current your drawing straight from the battery... if its more than say 15 or 20 amps then the problem is in the compressor...

an amp meter looks like a C clamp and clamps around the outside of the wire.
and reads the magnetic field.

http://cgi.ebay.com/1000-Amp-Speedway-Digital-Clamp-Meter-Price-is-no-fluke_W0QQitemZ7631619421QQihZ017QQcategoryZ50961QQrdZ1QQcmdZViewItem

You can bet that he doesn't have a problem with "current". It's either a switch or the CCU.

BTW, the clamp on ammeters don't work on DC..........no magnetic field.....

The clutch uses about 3 amps or so.

So I guess automotive ignition coils are AC devices?

and make sure it's not just a lousy ground. measure the coil resistance on the compressor, should be about 4 ohms. if it's close, it should draw 3 or 4 amps when given 12VDC.

Sort of off topic, Brian is right about the clamp on/AC only, however, I have a el cheapo 12VDC current tester that measures current by being held up against the conducting wire. Not Swiss made, but handy for checking alternator and starter lines.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Closer than you think, they do not get a constant DC current, more of a pulse that allows the use of a clamp-on type that measures the change in the inductive field (very similar to how you would measure AC).

..........you guessed wrong........

Coils are DC devices, DC current flow does indeed produce an electromagnetic field. I have an Fluke inductive current probe, it works just fine measuring AC and DC current flow - no "pulsing" is required.

No problem - I'm use to it.

When you use a clamp on amp meter you put it around the wire, don't clamp the wire between the jaws of the clamp. Also you only clamp it around either the hot wire or the ground (neutral). If you clamp it around both they cancel each other out and you will read zero amps.

Most amp meters will measure either AC amps or DC amps but not both.

ok, guys,
the guy said, his ac compressor works fine connected directly to the battery and does not work connected to its source. He also said he reads 12 volts at the source. Well the battery and the source both read 12v. So the only difference is current/amps a car battery can produce 350amps where as the electrical system of a car uses 15 and 20 amp fuses.

So by jumping to the car battery he may be feeding 35 or 40 amps to the compressor and it gives the appearance of working correctly....so a dc amp meter will let you know if you are drawing these high amps...

Now an ac compressor will usually spike the amps on start up then drop to normal during its run... and this spike is pretty quick... so watch the spike and if its to high then you can be assured your compressor is bad or clutch.. These are bought seperately... but I would replace both....

You guys are remembering back to your electrical theory class, remember you can not use a transformer on dc that is what your thinking about.. DC circuits will not transform becasue of the theory of inductance. The reason a coil works on a car is because it is connected to the distributor creates a type of ac. (so to turn dc form 12 vots to 2000v you must turn the dc to ac.)

here is a little something I am pasteing in describing how dc is used in an auto coil.
The next component is the coil's primary winding. Inside the coil are two sets of wound wire, comprising of the primary and secondary windings. The primary windings carry battery voltage through and create a large magnetic field inside the coil (this is discussed thoroughly in the section on secondary windings). Although the coil's primary windings receive voltage from the ignition switch, they are actually turned on and off by the distributor's contact points.

The contact points are opened and closed by a cam on the distributor's main shaft. As it spins the cam's lobes move the actuator outward, disengaging the contacts. When the lobe passes, the contacts close, turning on the coil primary windings. The amount of time the points remain closed is referred to as dwell, and is an important factor in engine tuning.

You're way out on a tangent here.

The clutch takes 3 amps. There is plenty of current from "the source".

I don't have the schematic for that vehicle, but, it's possible that the clutch is engaged by closing a ground at the CCU. So, he'd have 12V at the clutch all the time, no matter whether the clutch is engaged or not.

It's not an issue of current.

hes the guy who says it works when connected to the battery...

all I am saying is to do a little amperage check....there is no harm in that

and it only takes a couple of minutes... if the compressor only pulls 3 amps
then he will know his problem is some where else...then reason I keep saying 15 to 20 amps is becasue that is what the fuses are..

Now if the compressor has leaked out of freon and you have replaced it then you may need to jumper the compressor...