The elusive AC and Front SAM issue

760 Views 47 Replies 6 Participants Last post by efair, 4 h ago

Trisha Edwards

Discussion Starter · 11 d ago

Good day, I did the capacitor replacement repair but it only worked for a month. There must be something else in the SAM contributing to this problem…
Usually I’d turn the fan speed dial to 0 to turn it off but I started to fiddle with the A/C button on and off this week. Maybe that has helped to cause the problem idk.
Based on what I’ve read here, my front SAM should not even have been affected by the issue since it has the most recent part #.

Any ideas?

21 - 40 of 48 Posts

mchild

· #21 · 7 d ago

Trisha Edwards said:
well said.
But what about known issues?

As I understand it, at this point you have limited knowns: (1) compresor will operate if provied a seperate 12v supply and (2) there is no voltage at the compressor connector when it is believed there should be.

This is the perfect situation for the diagnostic expert to determine what is causing the issue with the power not getting to the compressor. They would likely be looking at the in-dash controller, the SAM, the ECM or any/all of the other items that interact with the climate control system. They will also likley look at the wiring/circuit integrity to make sure nothing is keeping the signal from getting through. A problem with integrity can be damaged wiring, corroded connections, or issues with either power and grounds.

Sneaky Pete

· #22 · 7 d ago

mchild said:
If I remember correctly, the wire connection at the compressor (two wires) is acutally coming from the engine control module (ECM). I also seem to remember connector M18 on the top of the front SAM is a PWM for the compressor. Maybe that connection goes to and through the ECM.

Afaik (but would have to dig up documentation to confirm) compressor is activated by the front SAM. With,as you say, a PWM signal that controls the duty cycle. The engine cooler fan, that is also needed to cool the A/C, is indeed controlled by the ECM.

mchild

· #23 · 7 d ago

Sneaky Pete said:
Afaik (but would have to dig up documentation to confirm) compressor is activated by the front SAM. With,as you say, a PWM signal that controls the duty cycle. The engine cooler fan, that is also needed to cool the A/C, is indeed controlled by the ECM.

This is what I was thinking about. Compressor A9 and ECM N3/10. Does not the schematic show A9 connected to N3/10? I'm not a fan of MB schematics.

Sneaky Pete

· #24 · 7 d ago

mchild said:
This is what I was thinking about. Compressor A9 and ECM N3/10. Does not the schematic show A9 connected to N3/10? I'm not a fan of MB schematics.

You could very well be right. In which case I stand corrected.

Trisha Edwards

Discussion Starter · #25 · 7 d ago

I appreciate all your responses and I will provide updates.
There is voltage coming from M18 only when the plug is removed. Once its plugged in, no voltage in the wires.

will update

Sneaky Pete

· #26 · 6 d ago

mchild said:
This is what I was thinking about. Compressor A9 and ECM N3/10. Does not the schematic show A9 connected to N3/10? I'm not a fan of MB schematics.

On 2nd thought,is it? OP has a 200,so engine is M271 meaning that in the diagram option U739 is applicable. A9 is connected to intermediate connector X26/25 and that is connected again to N10/1 front SAM connector 18 pin 1. At least that is how I read the wiring diagram. Don't think the ECM plays a part in it.
But agree, it is a bit nitpicking

, best thing OP can do is, as you said, have a good mechanic take a look at it.

Reactions: 1

mchild

· #27 · 6 d ago

Sneaky Pete said:
On 2nd thought,is it? OP has a 200,so engine is M271 meaning that in the diagram option U739 is applicable. A9 is connected to intermediate connector X26/25 and that is connected again to N10/1 front SAM connector 18 pin 1. At least that is how I read the wiring diagram. Don't think the ECM plays a part in it.
But agree, it is a bit nitpicking , best thing OP can do is, as you said, have a good mechanic take a look at it.

As I have mentioned I am not a fan of the MB schematics. Following your logic and looking back over the schematic I agree. I further confirmed that there is no A9 connection to N3/10 (ECM). Sorry I created a rabbit hole for us to spend time in.

Reactions: 1

mchild

· #28 · 6 d ago

Trisha Edwards said:
I appreciate all your responses and I will provide updates.
There is voltage coming from M18 only when the plug is removed. Once its plugged in, no voltage in the wires.

will update

That sounds like the wire may be shorted to ground. Quick test is to use a test light connected to battery positive and the other end of the test light connected to the disconnected wire. If that wire is shorted to ground then the test light would light.

Reactions: 1

efair

· #29 · 6 d ago (Edited)

My video of a scope trace - this was taken at the Front SAM connector.

mchild

· #30 · 6 d ago

efair said:
My video of a scope trace - this was taken at the Front SAM connector YouTube

Ed - link isn't working for me.

efair

· #31 · 6 d ago

mchild said:
Ed - link isn't working for me.

My bad - I updated the link in post # 29

mchild

· #32 · 6 d ago

efair said:
My video of a scope trace - this was taken at the Front SAM connector.

Nice demonstration.

Trisha Edwards

Discussion Starter · #33 · 5 d ago

Scanned
No codes
The technician even did an AC compressor initialization and everything seemed ok except no cold air.

photos.app.goo.gl

New video by Cam

photos.app.goo.gl

Trisha Edwards

Discussion Starter · #34 · 4 d ago

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mchild

· #35 · 3 d ago

@Trisha Edwards Have you made any progress on the issue you posted in #25? Connected a temporary wire between M18 and the compressor to see it the compresoor will then operate?

Trisha Edwards

Discussion Starter · #36 · 3 d ago (Edited)

mchild said:
@Trisha Edwards Have you made any progress on the issue you posted in #25? Connected a temporary wire between M18 and the compressor to see it the compresoor will then operate?

yes. When i do that the AC compressor comes in and freezes the pipes.

Sneaky Pete

· #37 · 3 d ago

Trisha Edwards said:
yes. When i do that the AC compressor comes in and freezes the pipes.

Maybe time to bring out an oscilloscope as in post #29 and see what signal comes from the SAM? It should be a PWM signal,so depending on whether how much the system would need to be cooling the interior it could range from nothing (no cooling) to 100 % (full cooling). And anything in between. If you put cooling on max it should show a straight voltage line. You demonstrated that the compressor works so your mechanic should take it from there? I mean,surely he could do more than just read error codes and then say that everything looks okay? The system is not cooling so everything is not okay. Can't say much more from this side of the internet,sorry.

mchild

· #38 · 3 d ago

Trisha Edwards said:
yes. When i do that the AC compressor comes in and freezes the pipes.

I reproduced your situation where you have voltage at pin 1 at connector M18 in the front SAM with the connector disconnected and no voltage with the wire connected.

My test: with system set to max cooling, the connector disconnected at M18 I had 10.5 volts at pin 1 at the SAM using a simple digit volt meter. I connected the conector to M18 then disconneted the power connector at the compressor and confirmed I had 10.5v at the connector side coming from the SAM. I then shorted that same power wire to ground. With that wire shorted I then read zero volts at the M18 connector pin 1 (back probed the connector).

That tells me you have a short between the M18 connector at the SAM and the compressor. To confirm if the short is in the wire or the compressor disconnect the power connector at the compressor and do a power voltage test at the connector side that is coming from the SAM. If you now have voltage at that connector then the short is not in the wire. So now it is likely in at the compressor. You can confirm this by doing a continuity test to ground at the compressor side connector power wire. If you have conntinuity to ground on the power wire at the compressor then either your compressor is shorted to gound internally or you have a short in the few inches of wire that goes from the connector to inside the compressor. Instpect that short secion of wire to see if there is anyway it is causing the short otherwise the short is internally.

I hope this helps.

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Trisha Edwards

Discussion Starter · #39 · 3 d ago

This helps so much! I’m going to try!
On a point of note, the first thing I changed when I initially had AC problems was the control valve on the compressor… I changed my genuine one for a non-genuine one off ebay thinking it was the culprit.

I will make the checks advised and update you.

Trisha Edwards

Discussion Starter · #40 · 3 d ago (Edited)

So far..

M18 disconnected; 12.4v at pin 1, 9.8v at pin 2

M18 connected; 12.4v at pin 1, -0.61v at pin 2

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