For anyone that doesn't know, PIV is peak inverse voltage. It's the maximum reverse voltage the diode will handle before starting to break down. 50piv is just right for this application.

Picoscope + Radio Shack + Russian ingenuity = Success ! Well done Ivan, great fix :-)

How many mechanics on Earth would have thought of this? You're one in a million Ivan.

I know for a fact straight from a Nissens specific training seminar that vintage of Mercedes AND Volvo with a solenoid valve compressor comes with and without a flyback diode. Their compressors come with a subharness included that contains a diode. I don't recall where you purchased the compressor or what brand, but that class immediately came to mind. This is why training is important, I only do BMW products however information like that will save your ass one day. Another fun fact, new Volvos with 1234yf have a capacitance sensor in their AC system due to the fact refrigerant flows through the battery packs on their electric cars. Absolutely never use PAG oil (because it is conductive), or you have just bought the shop a shiny new brick that it's start is disabled and needs it's entire AC system replaced.

I'm impressed that the controller survived all those negative spikes!

Always watch out for diodes built into relays as well. A relay swap can blow a PCM if no diode is on it.

My thought is that the original compressor has a diode built into it that shorted and then blew open. The replacement compressor doesn't have a diode in the harness, (probably a replacement full time non-PWM compressor). If you remember the original diagnostic code alluded to a shorted circuit. The diagnostic code wasn't 100% correct in this case but close to correct. If the schematics were more accurate, they might have shown either a diode coming into the compressor at the short pigtail or shown the compressor with a diode in it.

As soon as I saw the "Back EMF" spikes, I *knew* there was missing diode. As a general rule, ANYTIME a coil is driven with sold-state device, a snubber-diode to ground is standard electronic-design. Technically, the diode should be AS CLOSE AS POSSIBLE TO THE COIL so the backEMF has no chance to propagate down the wire and crosstalk into other circuits.

The manufacture used several suppliers for the AC compressor! This solenoid is only compatible with select Denso 6SEU16C, 7SEU16, 7SEU17C compressors equipped WITH A DIODE, held in with a snap ring and are visually identical.

Ivan, good catch and fix on the back EMF. However for a device that monitors current your test lights lights are problematic. When you first apply power to a incandescent lamp there is very little resistance and this causes excessive current to flow. As soon as the filament heats up the resistance increases and the current flow decreases. This is why you initially saw 4 amps on the scope and then it settled down to 1 amp. The controller likely saw this 4X normal current flow and shut down to protect the controller before the lamp reached it's steady state 1 amp draw.

Diodes are a very common fix in the electronic access control industry where virtually every door lock or hold back has some form of large coil that generates negative spikes when released

A better install of the diode would have been to solder pigtail wires to the diode. Cover diode and solder connections with heat shrink then connect pigtails to the wires. Also, a label attached indicating what the component does.

Great job Ivan! At 72, I'm no electrical genius anymore, but I can see common sense and understand things that are stupid, even bad designs! And yeah, ALL BRANDS have and continue to build bad designs after 100 years! Just amazes me!

Used in that configuration, it's called a "flyback" diode.... always good practice in circuits that drive an inductive load (even relays). There was probably one inside the controller that blew-out at some point in the car's history.

I have ordered my "Ai Ivan" model. Well done, I am at a loss at how this channel is not at 500K subs!

I'm glad my 1973 450SEL doesn't have all this electronic module nonsense that virtually all new cars have. Awesome diagnosis Ivan!

Good catch, Ivan! The internal diode could have been blown on a careless bench test with the wrong polarity, or just had a premature fail in the silicon. You definitely need an anti-parallel diode on inductive loads controlled by semiconductors - it was sheer luck that running without the diode didn't damage the output transistor in the controller. An oscilloscope (and the brain to interpret it) is fundamental to catch these faults. Great repair!

I have been watching for YEARS and I finally GUESSED ONE RIGHT! check the part 1 a diode was my guess.. YEA ME!!!!!

I once found an open diode in a Chrysler wiring harness. No AC clutch operation. It was well hidden in the harness. The dealer didn't have one and didn't even know it was there. I used a 1N4007 or something as a repair and taped it up again. I also seem to remember a diode installed on some Fords right at the clutch connector. Good job Ivan.

Kudos to the folks who were talking about diodes on part 1. I was way off with my front SAM commentary. Although someone did mention that there is a compressor control learn-in feature. I don't think you can "learn" back EMF but would have been interesting to dig into maybe.

I was thinking you were going to use an external solid state relay to fire that thing but the Flyback diode was cleaner. Well done sir.

Good job. Most auto technicians don't do diagnose like this; they simply remove and install modules because that's also what the manufacturers instructed them to do.

great fix.Flyback Diode or Freewheeling Diode is connected across the coil in reverse to shunt back emf to ground, possible bad diode in both compressors. a flyback diode is usually placed close to the coil. some compressors have a diode mounted in the clutch coil and some are external and some mounted in the connector. dorman sells pigtail for gm's with a diode across the terminals in the connector. Also diode cannot be tested in circuit easily with a DMM as it is in parallel with the winding. perhaps using a milliohm meter. Scope shows the failure

Ideally the diode should be as close to the source of the spike, ie. the compressor in this case. Excellent troubleshooting and repair as usual.

Most of the A/C compresors I have seen in vehicles, (even the old ones with manual controls, no electronics) have a built in spike supression diode installed in paralel in the connector wires that feed the clutch coil to prevent those back spikes of the magnetic field from the coil when is disengaged. Good work Ivan, as always.

The lesson was to use a scope especially on variable controls

Best place for the diode would be at the compressor. when the magnetic field in the coil that operates the valve collapses, that is whats causing the negative spike.

I am surprised there is no flyback diode displayed in the OEM diagram. This is the kind of issue that the dealer would fire the parts cannon at for 10-20 visits and then want to sell you a harness replacement.

A scope also comes in handy on systems with dc to dc converters or switched mode power supplies. Too much ac ripple can cause stuff to go absolutely crazy, and you may not see that on a voltmeter, even if it's set to read ac. Great job, as always, Ivan!

Great work Ivan, your gonna put the service writers with the tablets out of business 😂

I performed fixes like this (thinking outside the box) when working on alarm systems back in the day. This brings back fond memories.

Pretty much all MBZ of this Era use the driver front Sam module in the operation of ac. I have replaced many sams along with compressors. I am gonna look up that diagram and see

it's amazing how many educated people are in the comments who love to watch others who work in the same field dealing with electronics.

outstanding video series,, this is actually a hugely notorious and well-known issue,, for the clutchless compressor mainly found in luxury vehicles like this, there are two control valves available depending on application: one with an integrated diode and one without-- yellow dot on the plug indicates diode present. however, both genuine and aftermarket compressors alike are arriving DOA from the factory, fitted with valves either missing the diodes entirely OR, diodes of inferior quality and/or incorrect rating OR, diodes wired backwards or faulty solder joint or bent pins etc. you'll deliberately order a compressor with a valve with the yellow dot, or just a valve alone with a yellow dot, and it still may not work because quality control has slumped so hard in the last decade. scant few have ever even heard of a clutchless compressor-- horror stories abound of owners spending THOUSANDS on all new ac componentry to no avail, OVER A $.10 DIODE. the shops and diyers that are actually successfully servicing these things have taken to soldering a redundant diode into the control wiring exactly as youve done, most often right by the valve, or up top in the harness, grounding right to the valve cover. again, wonderful videos, really shows in depth whats actually going on electrically and electronically here!

I have notice that in some wiring diagrams they would not show the location of diodes. My 2004 CRV has some and I found them by mistake. Great solution, Radio Shack brings some old memories.

Epic diagnosis and repair! It’s really a crying shame that part stores don’t have brain cells for sale on the shelf. Awesome video!

When you showed the negative pikes, I realized a diode was necessary. This is not the first time I have seen diagrams omitting a diode here or there, especially if the load (compressor) is remote from the controllers. Very cool diagnosis, Ivan.

I've seen your creative fixes before, but this thinking and solution was exceptional. Wow, well done.

Great diagnosis and video Ivan. Also a great lesson to always check with a scope. I would have made the same mistake and probably spent more time and energy until my synapses kicked in. Now I’m chomping on my bit because you didn’t take the cover off of the controller and check for a freewheeling diode in the circuit that might have gone open to ground. Maybe a zener diode would be better suited over a standard diode. I know they used to place them in the coils of some of the compressor clutches. The older Mercedes models that just cycle the clutch have a speed sensor to monitor for belt slip to protect the belt drive. They cycle the clutch off and wait 3 minutes before attempting again.

All hail the mighty diode from radio shack - what a fix, Ivan. Well done

That Radio Shack package brought back memories of my first soldering iron and first second degree burn. Great job Ivan. Your electronics knowledge far exceeds mine. I cook, and I fix computers.

Ok, I watched it and wasn’t disappointed as you always nail down the issue! Glad this channel was recommended to me by you tube at least a year ago!

There sure are many very intelligent techs on Your channel and My hat is off to all of them when it comes to this more intricate electronic field, but it is amazing to Me how You tackle and sort this stuff out in short order. I know in My local neck of the woods, this owner would be up the creek without AC. Nice work. 👍👍🇺🇸

Yes a comment below said the diode should be as close to the inductive load as possible. I. E. between the compressor case and the control terminal. When the diode is remote, the inductive spike will travel through wires before being snubbed. I would also use a switching diode as they have fast response to catch the flyback pulse. Anytime the words pulse width power control signal is mentioned we should think transients can happen.

In this case, I have to agree that I can't think of another method to determine if this diode is present than using an oscilloscope. The fact that the diode should be somewhere in the circuit can be assumed, since it is PWM'ing an inductive load, but since the schematics don't show the diode, there is no way of knowing if the diode should be in the compressor or in the controller. If you would know that it should be in the compressor, you could measure it by applying a 500mA reverse current and see what the voltage is. The diode would clamp it to 0.6v (You could still do this with the old compressor) But if the diode should be in the controller there is no other method than using the oscilloscope to see if it working.

Mercedes uses press fit pins and super brittle lead free solder on connector pins. It's why most modules fail in MB cars. Re-working all the pin connections with regular solder on soldered and press fit pins fixes many failed Mercedes modules or prevents future failures.

A genius solution Ivan, well done. The original diode (build in) was shorted, the original short !!

"Hey hey hey" ! the sound of victory lol. Good ol RadioShack to the rescue. unfortunate they went out of business.

That was a nice little hack to get the results you needed Ivan. Well done. Your customer will be very happy that the A/C is working once again. But there has to be something else we are overlooking. Because not everyone will have easy access to electronic components like that.

That was an AWESOME lesson ! Super interesting problem / series of challenges you confronted and handled here ! It’s a great day when another one of your videos comes out !!

Nice I was just looking for something worthy to watch and you posted this, great timing Ivan!

Wow! A diode! I just serviced a fuel injection ATV for a worn out clutch and poor running motor. Replaced the clutch first then checked the motor. Would start and idle but no power and stall under load. Nothing seemed wrong, but a Google search said check the plug type "ATC" diodes in the fuse block. Found 2 of the 3 reversed. Switched back and engine ran like new!

Ivan, AWESOME WORK as always thanks for sharing.

Smart Diag and fix, as always! Thanks for sharing!

Scope for the win. I’m amazed at how many applications there are for it.

Slick fix there Ivan. My guess is there was / is a diode that is broken because you tried two compressors and two climate modules I believe. If a non functioning diode was present in either and replaced my guess is it would have worked. Bet they had a diode in the harness or some such thing. Regardless, excellent case study.

This is fairly common on the Mercedes AC control solenoid with the integrated diode but congrats on finding it independently.

Amazing diagnosis and I bet super satisfying when it worked amazing fix

Lesson of the day,we need to use scanners,understand the information,somewhere in that Mercedes there would have been a diode,probably on the original compressor or bcm,without the scanner ,you would not know about the return voltage,bloody good video,thanks for the video.

Ivan, you never cease to amaze me. Great solution call, my man.

Very clever; impressive diagnosis and repair.

Ivan, I have a box of those old Radio Shack parts. Keep watching your video's and I suspect I'll be using some of them. Nice fix. Thanks for Sharing!

Hi Ivan I use a lot of relays in my designs and they should always have a back emf diode when being fired from semiconductor drivers. Some relays have an internal resistor to reduce back EMF. Relays and solenoids are effectively the same thing. Coil of wire. The diode should be fitted as close to the inductor as possible. Helps reduce RFI. But as you say where is the diode in the replacement unit? On AC circuits I use an MOV.

Great video Ivan as always. Cool Diagnose and fix, can't beat the Scope hands down.

$.25 part fixes a thousand dollar issue. It's not about the parts but understanding the problem and getting to the root cause. Great job

Ivan, I used to work with DC power supplies that "made" a pulsating DC voltage up to ~100,000 Volts peak DC. We wired 80 diodes, (almost thumb size,) in series, on each of the 4 legs of a full wave bridge rectifier I don't remember the PIV for each Diode, in the string, but it was huge. Those components were submerged in "Oil" for cooling and insulation purposes. Loved working with them. I worked in QC and testing for a company that built precipitators: "NWL Transformer Company," Bordertown NJ." Nothing like listening to the "crackling" of an unloaded ceramic bushing, as you ran the unit to "30% over the AC power input, to the xfmr primary, for 1 minute." This was to see if it would make it or not. :-) Always had my hand over a BIG RED power off mushroom button. :-) Ivan, you demonstrated great brainwork to figure out how to eliminate those negative pulses. Had you not done this, that car would still be "down and out." Great job.

😂 Ivan you just resolved my charging problem in my jaguar.. never suspected this ground.. thanks

Normally the diode is connected across the inductor as close to the inductor as possible in a normally reverse biased condition, that shorts the spike that is generated, like when you see relays connected to a micro controller (freewheeling diode). the diode must have a very large piv compared to the working voltage because the negative spikes can be substantial. good call Ivan Rich

Ive been really looking forward to part 2 of the Mercedes GL air conditioning fault, Thanks Ivan.

Impressive. Thank you for another good one!

It takes real knowledge and experience to pull off such a cool repair

Maybe just for comparison you could just plug in the old compressor and hook up a scope to compare the signal with the new one. Possibly the old one had a mechanical problem, not an electrical one. I've scoped the pwm signal on a few compressors and have never seen a negative voltage spike. They must have a diode somewhere. Good find.

Great video, Thanks, Ivan.

The standard diode used in electronic applications like this on relays is a 1N4148, has thinner leads, easier to twist around that wire, but I understand, you use what you have available at that time. Good Job!

There has to be a diode in that plug on the compressor or in the wiring in the dash at another junction in the wiring where ever it is it's burnt out the poor old DIYer would never have found it & most won't spend the money on a scope. They (like me) would have to find someone like you to effect a repair out here in AUS the problem is finding a trusworthy tech .Thank you for this it has been an eye opener.Cheers mate be seeing you in the next video.

A "Cool" diagnosis, you say? Naturally, it was the air con system! Ayyyyyyyy, I'll show my self off the stage. But yeah, I never knew variable speed compressors were a thing for automotive applications.

Superb, Great work Bro😍👍🏻

Oh yeah, I forgot to mention that my hunch was right in my comment yesterday, that everyone, including me, thought that the problem was with a bad or missing ground, but I had a hunch that you were once again going to prove us wrong. Hahaha. Way to go, Ivan.

Ivan, This was by far the coolest video of the year 😎

Your hacks are great!

Nice find Ivan thanks 👌

Great find and nice fix! Almost NPR.

Very impressive. Another great video.

My thoughts are the wiring harness on the old compressor should be transferred to the new compressor. It might be a bad diode in that harness but it can be tested for that.

Great work and you are very honest !

That is a spike wupression diode that has been used since at oeast the early 70's (GM put it across the clutch connector - soldered in) and taped up on the end of the connector. Repaired many over the years. Diode test on the DMM, or old analog meter would show the fault.

Fixed Gopnik style! I love it!

Be very wary of using common types of "rectifier" diodes like 1N4000 / 1N5400 series etc. on PWM control systems - those and many similar ones have a slow response and can both allow brief spikes past before starting to conduct, and keep conducting for a short time after the polarity reverses again. That can cause strange problems, radio interference or even damage to other components in some circumstances. It's only a couple of microseconds, so won't usually show on a slow scope sweep. Fast recovery or especially Schottky diodes are fine at higher frequencies.

You live and learn. Good work.

Boom it's finally fixed you're genius Iven

nice! a freewheeling diode to clamp the back EMF from the compressor coil. radio shack used to make shrink tape to use when one forgets/ has no room to put shrink sleeve on.

Great work Ivan 👍

another banger of a diag and repair!! the Final Solution for the AC will make for carefree roadtrips in German MB.

A bad diode was the problem on my Ford pickup with the clutch not engaging. My co-worker and I beat our heads against a wall, trying to figure it out. In the short term, we had run a removable ground wire to the body that I would reconnect for the short drive home each day, for a week or so. I found the diode fix on a website forum and it fixed the issue. The funny part was that the factory OE diode was AC Delco, my local Ford dealer didn't carry it in stock, but the Chevy dealer had it.

Just a few brain cells, Ivan? Unfortunately, that is what many of us lack!🤣

Привет Иван! Интересно наблюдать твою логику мышления. Я в детстве, в советское время, ходил в радио кружок, и это хобби осталось.

Better schematics would have been good to show that diode in the control Circuit of the compressor but you figured it out anyway and the rebuilders should pay more attention to what they're doing and do it right but until things have more more quality controls behind their processes and the schematics of the corporations we will have to fight all this stupid stuff but you did a bang up job there awesome awesome awesome Ivan

I really appreciate this video . Its a great lesson..

One of your most impressive thinking outside of the box fixes so far - for me anyways. Skilllzzzzz

Well done, Ivan and his oscilloscope wins again!