1987 190D 2.5 Non turbo with issues with the AC.
It comes on for a few seconds when I start the car, then it shuts off.
I Charged the AC with an AC machine a few months ago. So I am very confident that it is not the refrigerant, because it held a very good vacuum and when the AC does work, it is COLD.

I can wire the AC compressor directly to 12v, and the compressor will stay on, so I know the AC is working, but it seems that something keeps sending a signal to shut off the compressor.
I also found that I can unplug the relay (behind the battery) and plug it back in, and the AC will run for a few seconds, then shut off again. I replace the Relay with a known good one, but the symptoms are the same.
Also, the PO bypassed the switch on the low-side of the dryer many years ago.

I have searched this forum for similar issues, and found this one- A/C diesel problem

Before I tear into the dash, I was wondering if there are any other things I can check that would be simpler/easier.

Compressor speed sensor signal.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

I don't have an answer for you but you can check this thread, post #9.

W124 - stumped by the Climate Control Unit

It could be the evap temp sensor behind the driver's kick panel. Try to time the cut-off time to see whether it is the same or very consistent. Good luck.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

UPDATE-
I have noticed that the fuse in the fusebox get very hot. Every so often, it will blow, and I have to replace it. I can bypass everything by connecting a wire (with a 25 amp inline fuse) from the battery directly to the AC compressor. The compressor turns on, and gets cold, but the inline fuse will blow. I put a 30 amp fuse inline on the jumper wire, and it did not blow the fuse, but the wire did get very hot. I have got something that is drawing a lot of amps.
I am getting to the point where I dont know what to do next. any ideas?

First and foremost, disconnect that wire and stop increasing fuse AMPs ASAP. You're lucky you didn't cause a fire under the hood. You need to find out what's causing the short and fix it. Disconnect the a/c compressor until you figure out what's causing that fuse to blow otherwise anticipate an engine fire and a lot of melted wires needing to be replaced

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1990 190E 3.0L

I had already unplugged the AC compressor prior to posing the question-to me that is a given.
Any ideas on how to trouble-shoot further?

Your compressor clutch is failing and when compressor rpm doesn't match engine rpm the relay shuts it down.

__________________

90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

The high amp draw for the clutch also suggests slipping.

Later MBs with a single serpentine belt have a speed sensor in the back of the compressor. If the engine and compressor rpms synch (not match), then the compressor stays on. If out of spec, the power to the clutch is removed. The initial start up comparison is bypassed when engaging the compressor because the will be a discrepancy between the rpms until the clutch stops slipping. That's why it engages initially, then cuts out.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

Thanks all!!

It turns out that it is slippage of the clutch, due to a front seal leak on the compressor.