Compressor speed sensor. - Page 2 - PeachParts Mercedes-Benz Forum

Diseasel300 · #16 08-03-2017, 07:45 PM

Pin 10 is a pulldown pin. The Klima outputs voltage on Pin 10 which is pulled down by the ACC to turn the compressor on. The Klima in '86+ vehicles using the 1 piece ACC use a darlington transistor setup to do the switching, so you'll never read a "hard" ground.

Mxfrank · #17 08-03-2017, 09:53 PM

That explanation sounds closer to what I'm actually seeing. But I have to add that all my measurements were made with the Klima removed from the circuit. The .75V was coming from the ACC unit. I suppose that could be explained as leakage. What this means that the Klima can never be circumvented with just relays. And it also suggests that I need to test the ACC unit, as the Klima could be fine.

Diseasel300 · #18 08-03-2017, 10:08 PM

Leave the wiring to the refrigerant switch open and install your Klima relay. Start the engine and measure each pin to ground. The one going to the ACC will measure some arbitrary value (fractions of a volt) and the one going to Pin 10 on the Klima relay should measure close to battery voltage.

*IF* you measure high voltage (battery voltage) on the pin going to pin 10 on the Klima relay, short it to ground and see if the compressor engages. If it does, you know the problem lies in the ACC unit. If you don't see battery voltage on Pin 10, the Klima is not allowing the compressor to engage for some reason.

I would suspect (but haven't tested on my own) that the Klima can be overridden entirely using a couple of ice-cube relays or even a custom PCB designed to fit in the same box. I'd be happy to draw up a schematic if anyone wants to guinea-pig. Based on the schematic, the only function you'd be giving up is the speed sensor on the compressor, which is what most people seem to want to skip anyway.

Diseasel300 · #19 08-03-2017, 11:30 PM

I've attached a PDF diagram of how to implement the Klima functions using 2 "Ice-Cube" relays. You want to use real "Ice-Cube" relays and NOT the Automotive style relays. The coil impedance is important, Automotive relays use a low resistance coil, ice cube relays use a high resistance coil. If you use a low-resistance coil, you will damage the ACC, it has a maximum burden of 1 amp!

There are 2 limitations here:

1: You will not have a "delayed" compressor engagement based on RPM when starting the car. This isn't a huge deal, the delay was removed from later Klima relays.

2: You will not have the belt-protection function from the speed sensor on the compressor.

Otherwise, all other functions of the Klima relay are left intact.

- The Kickdown is left energized and gets rid of that squirrely relay that was done away with in later models

- The "cutout" switch on the accelerator will cut out the compressor when closed

- The overheat switch in the head will cut out the compressor when it closes

- The ACC will cycle the compressor as it normally would

- The refrigerant switch is in the wire from #10 to the ACC, so the system is still protected from pressures too high or too low.

Mxfrank · #20 08-04-2017, 12:46 AM

Quote:

Originally Posted by Diseasel300

- The "cutout" switch on the accelerator will cut out the compressor when closed

- The overheat switch in the head will cut out the compressor when it closes

That's about the same as the diagram I posted. Thanks for the tip on limiting current.

If you don't use a diode to separate the cutout and the overheat terminals, then the fan will come on at high speed every time you step on the accelerator.

Diseasel300 · #21 08-04-2017, 10:01 AM

The Aux fan shouldn't come on at all. The power source for the fan contact in the temp switch is separate from that of the overtemp or cutout switch. The overheat switch is a 2-contact switch. One contact drops out the compressor, the other switches on the high-speed relay in the fusebox. They operate at the same time, but they control different circuits.

The overheat and cutout switch are both powered from the Klima, it doesn't matter which one of them close, they will both drop the compressor out. There is no common connection between the overheat A/C switch, the cutout switch, and the high-speed fan switch other than a common ground (chassis).

Mxfrank · #22 08-04-2017, 07:58 PM

You're right. The circuit is a bit confusing, but the overtemp cutout is parallel to the fan relay circuit.

Diseasel300 · #23 08-04-2017, 08:02 PM

That's all well and good, but they are not fed from a common source. Unless the fan switch is closed, the cutout switch cannot backfeed the coil for the aux fan. If the fan switch is closed, the fan is already on high anyway and the compressor already dropped out. The modes where the two can operate at the same time accomplish the same goal so the overlap is moot.

Unless the fan switch is already closed, there is no common circuit between the cutout and the fan relay.

howardjnl · #24 08-04-2017, 08:45 PM

Diseasel300 and Mxfrank: Guys, I've tried to capture what you guys have discussed, omitting Mxfrank's suggestion about a diode to isolate two leads fron one another. For now.
I've added diodes to suppress back EMP from the collapse of the coil magnetic fields when contacts are opened. This extends the life of relay contacts.
Diseasel300's comment about coil impedance is well taken.
Comments or suggestions about my attached diagram?
John

Diseasel300 · #25 08-04-2017, 10:58 PM

The freewheeling diode across the contacts will do nothing for you in this case since you are not connected across the coil. The point of the freewheeling diode is to dissipate the energy stored in the coil, not across the contact.

Adding diodes across the operating coils of the relays is nice in theory, but the impedance and EMF flash caused by them is so infinitesimally small that they don't serve much purpose.

To properly freewheel the clutch coil, you would want to move your diode between pin 7 and ground. Use a 6 amp diode.

Mxfrank · #26 08-05-2017, 04:49 AM

You're going to have a hard time finding automotive DIN relays with a very low current draw. If you do, they will probably be diode suppressed so the coil diodes would be unnecessary. Be aware that most of the "ice cube" relays used in Mercedes cars are resistor suppressed, which means they have relatively high current draw.

An opto-coupled SS relay or optoMOSFET would probably do the trick, and would need no external suppression devices. Also draws minimal signal current, so it would preserve your ACC head unit. Something like this:

http://www.mouser.com/ds/2/657/d_c60-1063099.pdf

Diseasel300 · #27 08-05-2017, 10:58 AM

Or - (just a suggestion) - KISS and use a simple PCB like Mercedes did with PCB style relays. Since they're designed for electronic switching, they have very high impedance coils. The Klima relay already uses them, I see no reason why they shouldn't work for bypassing it.

Automotive relays are designed to be switched with a set of contacts such as a switch, pressure switch, or temperature switch. That's not what we're doing here.

sixto · #28 08-05-2017, 11:40 AM

Might be overthinking it. I had a 30 amp Bosch relay in the SDL for years with no PBU problems.

Sixto
98 E320s sedan and wagon

Mxfrank · #29 08-05-2017, 05:37 PM

But there are differences even with "PCB" style relays. I happen to have three miniature relays that I cannibalized from an old OVP. They draw as much as any DIN relay. Sorather than guessing, it's better to hit the catalogs and pick one that has a good coil rating.

As for "Bosch" relays, there are a few things that complicate a selection. First off, Bosch hasn't made a relay since 2007, so anything you get is aftermarket or very used. There are good mfg's, like Tyco or Hella, but most of what you find is made by who-knows-who, who-knows-when and who-knows-where. Even when Bosch was making relays, they had 300 specs in their catalog. High impedence, low impedence, various current and voltage ratings, fused/unfused, and suppressed/nonsuppressed. Most of the ones used in Mercs were resistor suppressed, and draw more current. So which "Bosch" relay did you use, and where can I find it?

Given the trouble and expense of overhauling the ACC unit, I'm trying to be conservative with this mod.

howardjnl · #30 08-05-2017, 09:58 PM

Guys: The DS60 series certainly looks like it fills the bill with a 1500 ohms DC impedance as compared to the "ice cube" auto parts store relays at around 90 ohms. And the prices look good at a little less than $30 each.

BUT, the BWD version of an "ice cube" has a coil impedance of 85 ohms. At 13.8 VDC, it should draw 162 mA. Diseasel300 mentioned elsewhere on this site that the dash unit can handle 1,000 mA max. The "on" steady state for the dual relay configuration should only have a single relay active at one time. 162 mA draw on pin 10 from the climate control unit (CCU) doesn't seem excessive. Am I missing something?
Also, note Diseasel300 that I corrected my drawing mistake for the diode on pin 7 for the compressor clutch coil. Sorry about that. I've attached an updated drawing. Note also that my two 1987 cars do not use pin 12.
John