For our two 2000 Audi A6 project cars, the model with the 4.2 V8, I’m fantasizing about replacing the solenoids for the ZF 5 HP-24A. Each car is misbehaving and I haven’t figured out why.

As parts go, the eight solenoids seem to be relatively easy to replace.  Instead, were we to get into replacing clutches etc. then I’m way out of my depth.

New solenoids as a set are … okay, wow, about $500. Not cheap. When I discover something like this, I start to wonder how I can test a used solenoid to see if it’s good beyond the obvious.  I could get an entire used transmission, albeit in unknown condition, for less than the price of a new solenoid kit.

Ditto as to the other internal components. They are pricey.

I also recall reading “DO NOT PUT ANY USED PARTS INTO YOUR ZF TRANSMISSION” so … there’s that. Whoever wrote that probably had some good reasons, maybe for the same reason one shouldn’t use previously used … what’s an example … geez … toilet paper. Or condoms.

Still, given the high price of these new ZF internal parts, I’m tempted.

It’d help a lot to know what I’m doing, which right now isn’t the case.

For example, until recently I worked on the principle of “don’t ever re-use manual transmission clutch components; use new stuff only.”

So one fine day this summer, my friend (who is a professional mechanic) and I had just removed a BMW manual transmission that I was about to buy when he inspected the clutch. The more he looked, the more he liked. The conversation went somewhat like this:

• Him: Wow, look as this clutch. Buy this, too. Oooh, and the flywheel.
• Me: Nah. I don’t ever re-use used clutch stuff, on principle.
• Him: Wait, this IS just about brand new. The owner of this car must have just had this installed when something else broke on his car. I mean, wow, look at it.
• Me: I can’t really tell what makes a clutch & flywheel new vs. not.

And so, patiently, he explained it all to me, pointing out various technical details. He convinced me, so now I own an almost-new pressure plate, clutch disc and flywheel for a BMW M20 engine. New, this stuff would have cost me several hundred dollars. Instead, I got the parts, gently used, almost new, for next-to-nothing.

You can probably see where I’m going with this. I’m really enthused to find a way to strip down a ZF transmission, scrutinize (or better yet: test) each part, and then keep the good ones and toss the bad or well-worn ones.

I’m aware that there can be subtleties. For example, often, parts wear as a set, and so if one installs a used camshaft along with used rocker arms, and they didn’t wear into that not-new shape together, then their surfaces won’t touch as a broad plane but more at an angle, and then both parts will wear out way faster. That’s why, when disassembling a cylinder head, it’s crucial to label what went where. And probably a cylinder head is primitive compared to the high precision of ZF transmission internals.

Still … it sure is tempting. I mean, at the very least, the transmission pan and its hardware should be re-usable if it’s a good, used part, without antagonizing the ZF gods. And maybe some speed sensors and cables too. And so on. Somewhere there’s a tipping point, but my guess is that it depends on the relative merits of each part, as opposed to using a broad principle that is unnecessarily dismissive.

Applying false absolutes and buying everything brand new would explain why a transmission rebuild can cost thousands of dollars, which hardly anyone wants to pay since one can buy good A6 cars for less than $3,000 — for the entire car, which explains why I see SO many dead A6 cars in junkyards: once the transmission misbehaves, people give up on the car.

Resurrecting dead ZF transmissions affordably, using good, tested, gently used parts  … that is becoming very tempting to me. I’d be the first beneficiary of his policy. I’d have two healthy Audi A6 project cars.