Midnight Freeway E30 Rescue Puzzle

Puzzle scenario: You lend a close friend your BMW E30 automatic to make a much-needed long-distance trip. The friend is on his way home and calls late at night. He says he’s still an hour away from home.The lights on the E30 got dimmer and dimmer and then the check engine light came on a few times and then the engine cut out. Now the car is dead by the side of the freeway somewhere in the boonies. You call around, find a nearby 24-hour towing service and arrange to have the car towed there and your friend taken somewhere safe where he’s welcome to wait but not spend the night. No problem, says the towing guy, and there’s even an auto repair shop right across the street. Now what?

Hints: Screwing this up will …
1. Be very easy because if you let events unfold in their typical way, you’re pretty much doomed.
2. Be done by others, most likely rather then you personally doing the damage
3. Destroy the transmission on your E30, or the E28 you are about to drive to go get your friend, or both.

For extra credit, to make this harder, let’s also say:
A. You’re a charming, pretty girl in whose presence guys such as tow truck drivers and auto mechanics act all macho, wise and helpful
B. This is happening in the American South
C. You’re so smart that there’s s a good chance you’re likely to be the most intelligent person in the ZIP code, especially that ZIP code

The combined effect of A, B and C is that you’re accustomed to pointing out well-founded concerns to guys, and being ignored except for literally hearing the response “don’t you worry your pretty little head about that.”

And no, the pretty girl isn’t me. It’s someone I care about very much.

Wait, what? What does any of this have to do with the automatic transmission on each car?

Well, here’s a one-paragraph explanation. These ZF 4HP-22 units are wonderful automatic transmissions. They were used in almost all BMW models from the mid-1980s to early 1990s, including the E30 325i in our story, and the E28 528e rescue vehicle. Properly serviced or sometimes even when not, they might be 30 years old yet still going strong. The one in my 1989 E30 has probably gone a quarter of a million miles and it’s still behaving perfectly — and I drive my E30 hard. Typically the only way you can kill them is by revving the engine in Park, Reverse or Neutral. It doesn’t take much. Think of the revving that happens during a smog check or right before a juvenile stop light drag race. Yep, that’s enough. In less than a minute, sayonara transmission. Some models have had that vulnerability fixed, but it’s very hard to figure if your car is one of these. Now, let’s think this through some more. What do people traditionally do as part of jump-starting a car, or starting it after it’s been dead? They rev the engine. Not that they need to … the alternator charges well enough at idle speed. But stupid is as stupid does.

So your mission is to somehow get the guys to not destroy the transmission in either car. And you already known you can’t tell the guys not to. They’ll just blow you off and do it anyway. You have to shape the events to prevent the damage.

Slightly edited, here are the instructions I sent to the pretty girl on her phone that night. Did they help? That’s another story. Whether they did or didn’t, I hope they help YOU in a similar predicament. That’s why I’m posting this.

* * *

The highest risk here is that someone will rev the E30 car or E28 car in park or neutral and destroy the transmission in less than a minute. Left to their own initiative, guys will rev a car in park or neutral as part of jump starting, or seeing if the charged battery works, etc. Don’t trust them. Say no and make it impossible — otherwise with the best of intentions they will totally screw it all up.

Preventative measures:1. Ask the towing guy to disconnect the battery from the car and then take the battery home with you. Trickle-charge it overnight at the slowest rate possible so as not to damage the battery. If you don’t have charger then go buy one.

2. Take the keys to the E30 car with you tonight.
3. Make it emphatic that the guy shouldn’t put in another battery or do anything with the E30 car. You’ll be back.

Then, tomorrow when you go there, bring along that battery and wrenches in sizes 10,11,12 and 13, plus a vice-grip, screwdriver, jumper cables, multimeter, gloves and safety glasses.

Install and connect the battery into the E30 car while the key is off and the hazard lights and headlights are all off. See if there’s a spark when attaching the terminal. If yes, there’s a short in the wiring so then it’s time to make sure everything is off. Start removing fuses and relays one by one until you’ve resolved the cause.

Measure the voltage over the battery terminals. Then start the car again. Do not rev it. Measure the voltage again. If it’s no higher then the alternator isn’t charging the battery. Could be dirty or loose terminals, cables, wiring … a loose or broken belt, a loose clamp … not necessarily the alternator.

Let the car sit and idle for 15 minutes to see if the battery can power the car for 15 minutes without being charged.

Then turn the the E30 car off. Connect the E28 car and the E30 car battery terminals (pos to pos, neg to neg) while the the E30 car key is totally off and the E28 car is idling. That will charge the E30 car battery at the right speed. Don’t rev the E28 car and don’t let anyone else do so otherwise good-bye transmission.

Then disconnect the cables, connect the battery terminals on the E30car. Start it and do not rev it. Drive it away. Every fifteen minutes or whatever, pull off the freeway and repeat the car-to-car charging process until you get both cars safely home.

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