MyE28.com

What is the 1” by 1” brown square on the top of the AC Compressor? A wire coming from the coil attaches to it. I broke the wire at the base of this square piece and in attempting to fix it broke it. Help. I’m a big time novice so if you can help please dial it down to dummy level.

Greg
602.531.3891

I see this is your first post -- welcome! Please read and heed the Forum and Tech FAQ sections.

I moved your post because it's a tech issue. Next time, please include the year/model.

That's a capacitor to soak up the spikes in the electrical system when the clutch engages/disengages. Said spikes can damage the ECU. Will they? Maybe, maybe not.

Can it be replaced?

Greg

Thanks for saving me from embarrassment.

There's no embarrassment in asking questions about how to fix your car, that is what this forum and others like it are all about.
It's how you learn to work on cars. Good on you for dong your own work and for asking questions. Keep it up, and most everyone here is willing to help you along the way.

Nvrovrpar wrote: Jan 18, 2024 6:11 AM Can it be replaced?

Thanks for saving me from embarrassment.

I can't find a separate part number, so you might have to find one from a junkyard and test the capacitance, or see if the capacitance is rated on the part, or see if it has a manufacturer number on it.

Eh, shit happens. I've busted parts before.

Shawn D. wrote: Jan 19, 2024 6:45 AM

Nvrovrpar wrote: Jan 18, 2024 6:11 AM Can it be replaced?

Thanks for saving me from embarrassment.

I can't find a separate part number, so you might have to find one from a junkyard and test the capacitance, or see if the capacitance is rated on the part, or see if it has a manufacturer number on it.

Eh, shit happens. I've busted parts before.

What he said. Also, BMW does list a capacitor for the alternator:

https://www.ebay.com/itm/282828211411

Don't know if the specs are right, but it would look ok on the compressor

Federico wrote: Jan 19, 2024 7:32 AM What he said. Also, BMW does list a capacitor for the alternator:

https://www.ebay.com/itm/282828211411

Don't know if the specs are right, but it would look ok on the compressor

The specs are almost certainly not correct. The alternator capacitor has to deal with continual smaller ripples, whereas the A/C capacitor must deal with intermittent large spikes.

Shawn D. wrote: Jan 19, 2024 9:09 PM

Federico wrote: Jan 19, 2024 7:32 AM What he said. Also, BMW does list a capacitor for the alternator:

https://www.ebay.com/itm/282828211411

Don't know if the specs are right, but it would look ok on the compressor

The specs are almost certainly not correct. The alternator capacitor has to deal with continual smaller ripples, whereas the A/C capacitor must deal with intermittent large spikes.

Yeah, but. E28 alternators put out 65-90 amps peak, so even being 3 phase it could be a pretty big hit, but probably mostly to act as a radio filter. Frequency is different by a factor of thousands though.

While I'm sure they put it there for a reason, if it was my car I'd probably wing it and just run without it and see what happens, if anything. Might get a spike on the radio that comes as a Brrrt, but again, I'd wing it. Maybe not crank the stereo up for the first few trips with the A/C on though. YMMV.

As I said earlier, I think this is to protect the ECU. While it would diminish noises on the stereo, I don't think that's the primary purpose.

We need RonW to chime in on this!

Shawn D. wrote: Jan 20, 2024 4:57 AM
We need RonW to chime in on this!

Agreed.

I speculate the wires are a tee connection: one goes into the mystery block, connects to the cathode of an internal silicon diode (anode going to mounting screw ground), and then exits the other wire to supply +12V to the clutch coil, as shown on the E28 ETM schematics for 86 and onwards models (and IMHO, the earlier models needed this diode spike protection too but lacked it unless contained inside the AC controller).

Even if the wire is broken off flush or even slightly recessed, you should be able to use a multimeter applied to the mystery block to verify near zero ohms between the good wire that remains and the stub of the broken wire, probing inside with a needle if necessary.

If your measurement supports my conjecture, you can repair it just by splicing/shrink tubing to the good remaining wire, and a blob of epoxy to keep any moisture from entering the broken stub where it enters the mystery block. An external tee is just as good as an internal one as long as the diode and clutch both connect to the +12 activation command.

If you Internet search AC compressor clutch diode you will find much discussion across all car makes and models on this surge suppressing diode. When the clutch is activated it just draws up to 4.2 amps. But when power is removed the inductive kick nature of the clutch coil produces a negative voltage spike lasting until the clutch coil 4.2 amps of magnetic field energy is dissipated thru the diode if present. One pump manufacturer says up to -400V can briefly exist without the diode. Functioning diode limits the spike amplitude to less than -1V .

Inductive spikes are not good for any 12V connected electronics and the ECU especially in the fault case where the high pressure cutout switch opens (isolating any protection diodes inside the evaporator temperature regulator) and the Motronic ECU is looking directly at the clutch voltage as a sign the compressor is loading the engine. Capacitors can and are used to control voltage spikes but given the 3.6 to 4.2 amps and many turns of wire inside the clutch it would take a physically large capacitor to do the job of the diode.

If you like, you can temporarily isolate or unmount the block and use multimeter diode test mode to verify a good diode .7V or so in forwards direction, open in reverse. A 5A, 50V or more silicon diode would be a replacement.

wally wrote: Jan 20, 2024 4:51 PM I speculate the wires are a tee connection: one goes into the mystery block, connects to the cathode of an internal silicon diode...

Wow. Very good, my complements. Looked like a cap and I never thought to look at the ETM, which clearly shows a diode in there.