SQLGuy

The problem is that the squid is not part of the expansion valve; it's part of the evaporator.

dr bob

^^^ I'm aware, just lending a little WYAIT list item to your task.

If you have anything less tan total confidence in your fixability, pop the question about a used one from the group. I'll speculate that there are a few floating around, and you can always get a good used one from 928 International.

blown 87

It sounded to me like some one had used glue on the fittings, so just general tip.

any kind of sealer will clog up a recovery machine and can not be good for expansion valves.

sorry if I came across wrong.

SQLGuy

Hi Greg,

No, no glue on the fittings... I'm thinking 3M spray glue or similar to hold the carpet set in place.

SQLGuy

As for sealant, though, I may have spoken too soon. The rear expansion valve definitely has some sort of junk in it... some gummy deposits, probably a "system sealant."

I was trying to keep costs down by not replacing items I didn't have to, but I guess two new expansion valves will need to be added to the work order...

On the bright side, I pulled and flushed the condenser, which gave me the opportunity to remove several years worth of dead bugs and leaves from between there and the radiator. I also pulled the rear evaporator, and will be checking with local shops tomorrow to see if there are some pros in the area that could resolder the brass to aluminum connections.

dprantl

If I found gummy deposits in my expansion valve, I would not put the system back together without completely flushing out both evaporators and the condenser first. Then drain the compressor, add new expansion valves, new drier, put back the right amount of the right type of oil, then recharge. Oh, and during that process it would be pretty easy to replace every o-ring in the system. After that, your system should be good for 10 years or more.

Dan
'91 928GT S/C 475hp/460lb.ft

SQLGuy

Thanks.

Other than originally not planning to replace the expansion valves (especially since one is only two years old) that was my plan regardless of the deposits . The hoses, evaporators, and condensor all needed to be flushed, the compressor needed to be drained, and the drier needed to be changed anyway since I'm converting from R12 to R134a. New expansions valves are on order and I should have them tomorrow.

I also found a local welding shop that specializes in all the stuff that nobody else in town does (titanium, dissimilar metals, etc). They feel pretty confident they can resolder the evaporator lines. (fingers crossed). In my own experiments I used high pH flux and tried three solders (silver solder, Staybrite, and Alpha aluminum solder) with some scrap pieces of copper and aluminum. Though it's rated to solder Al to Cu, I could not get a joint between the two with the Alpha; it flowed well between the two pieces, but didn't adhere to the copper. I also didn't like its very high melting point (1070F). With the Staybrite, I was able to get a decent joint between the two, but still not as strong as I would have liked. The silver flowed OK onto the aluminum, but I couldn't get it to flow between the two pieces. I feel better leaving this to a pro.

For Doc, here's a shot of the mostly finished new motorcycle fuel rail:

SQLGuy

Here's the current status:

The local specialty welding shop did not feel that they could do anything with Cu/Al bonding. I then checked with my local welding supply shop. While they carried a number of silver and soft solders, and some "dissimilar metal" welding rods, they carried nothing for soldering Cu/Al.

Next step was to order something made for this: MuggyWeld Super Alloy 1. This stuff was advertised as being able to solder aluminum, copper, pot metal, and pretty much anything else, in any combination, with good strength and at only 350 degrees F. I called them and they confirmed it would be a good choice for repairing copper to aluminum joints in an A/C evaporator. Pretty pricey stuff for solder ($50 with shipping for five rods), but it sounded like the right stuff.

Got it in yesterday but didn't have time to try it right away. This evening, I first went back to my test pieces of scrap copper and scrap aluminum. It's a bit tricky to work with... doesn't exactly flow; instead you need to get the heat just right and rub/drag the rod tip over the surfaces to get it to adhere and spread. However, it pretty easily, and strongly, bonded the two scrap pieces - where, with the Alpha Al/Cu solder/braze, I couldn't get any bond at all even at a MUCH higher temperature (1100F +).

On to the evaporator: set up my new HF blasting cabinet and soda blasted the areas where I'd be working, then blew off the dust and cleaned with brake cleaner. Soldering the evaporator joints was trickier than the scrap pieces... either the alloy wasn't as solderable or the surface texture wasn't as conducive to soldering, but, after a bit of work, I ended up with joints that are certainly not pretty but that I think are sealed. I didn't have a good setup to pressurize and immerse the evaporator, but I put soap over all the joints and pressurized the unit several times to see if any bubbles formed or blew away - nothing.

Tomorrow, I'll install it and see if the system draws a good vacuum... then see from there.

Cheers,
Paul

dr bob

Hi Paul--

Looks interesting. I'm always hesitatnt on repairs like this, mostly because I don't like to depend on soldered connections for mechanical strength. Was it apparent what the Behr guys used to hold it all together originally?

If you have a chance to pre-charge the system with dry air or N2 for testing, that would be a good thing.

SQLGuy

Hi Doc,

Well, the MuggyWeld is rated 20000psi, but there shouldn't be any reliance on the solder for strength here anyway... the connections were not loose or unsupported without the solder; they were just leaking from between the metals. It looked like the original connections were pressed in. There may have been some shellac or something involved, but I'm not sure. The Al/Al connections at the ends of the coils appear to be factory soldered.

I don't have nitrogen at home, and don't really have any other uses for it that would justify going out and buying a bottle. I'll have to rely on the vacuum to see how well the system's sealed prior to charging it.

Cheers,
Paul

WallyP

Many years ago, Government Motors - sorry, GM - started using aluminum/copper A/C evaporators on their cars. They failed. The aftermarket promptly came up with a fix for all of the leaking units that had been replaced with new ones. Basically, you clean the area that you pictured, then pot the entire area in epoxy. You end up with the evaporator having each end completely sealed into a solid lump of structural-grade gray epoxy. Of course, you need to make sure that the epoxy doesn't interfere with mounting the evaporator, attaching the lines, etc.


Seemed to work OK.

SQLGuy

Well, the system pulled and held a good vacuum, so I went ahead and charged it. Still some other problem, though: if I jumper 12V to the compressor clutch the system runs and cools fine; but the clutch does not engage otherwise.

I have verified that the low pressure switch is closing. The AC switch on the console, FWIW, lights up when engaged. I also tried switching the system to defrost (since my previous 928 initially had a non-connected AC switch but would still run the compressor when on defrost). I also verified that the 7.5A AC fuse is good and that the clutch wire is connected.

My wife's out with the car now, so I can't do any troubleshooting at the moment, but I guess tomorrow I'll go back and verify that I don't (as I expect not to) have 12V at the low pressure switch.

Any suggestions on things that typically go wrong like this?

By the way, how does the system combine the signals from the front and rear evaporator ice sensors?

Thanks,
Paul

dr bob

Paul--

There's a history of problems with the compressor relay in the head unit. It's undersized for the duty and will fail eventually. Diagnostics are on the normal tips pages. I did a repair with an external relay, and there are others with different suggestions/options.

The freeze switch disables the compressor clutch when the main evaporator freezes. For the rear, the thermosttic control disables the rear system by closing the solenoid valve under the passenger seat. That interrupts the flow of refrigerant to the expansion valve, effectively stopping the cooling in the rear unit. I'm not sure there is actually a freeze protection function there; the system as I know it is just on/off flow control on demand. Hotter gets flow, cooler shuts it off. Maybe the lowest temp setpoint is high enough that freezing isn't an issue. Remember that inside air in the car, already dehumidified, is what gets re-cooled in that rear unit. Moisture load is much lower I'll guess. I'll also speculate that there isn't enough heat transfer capability to freeze either evaporator when the rear unit is in service. Suction pressure is too high.

SQLGuy

Hi Doc,

Thanks. The relay in the head unit was indeed the issue.

Also thanks to John Pirtle's nice writeup on disassembly of the center console (http://members.rennlist.com/pirtle/svc_cons.html).

I was looking for some internal pictures of the head unit, but didn't find any, so I snapped a couple while I was in there. The octal layout of the stock relay doesn't look to be too standard (most relays I've seen have an assymmetrical pin layout), so I wired in a retrofit with a 5A DPDT I had lying around.

In one of the pictures, BTW, you can see where one of the contact sets in the original relay has been discolored from overheating.

Now I just need to finish putting all the trim back, and wait for a decently hot day to see how well things are working.

Cheers,
Paul