Could a mechanic have removed the oil from my differential if it's a "closed system" without a drain plug?

Dear Car Talk

Dear Car Talk | Oct 01, 1998

Dear Tom and Ray:

I have an Olds Bravada with 85,000 miles on it. Last month, I had the car
serviced (oil, lube, brake work) and set out on a trip. After 500 miles, the
front end differential went out. I had it towed to a GM dealership. Two weeks
and $3,100 later, I got the Bravada back. They said there was no sign of a
broken gasket or a leak, but the old differential had no oil in it, and that's
why it had burned up. They suggested that perhaps the other dealership had
drained the differential oil and forgotten to refill it.
I asked my local dealership if they might have done such a thing. They said that
the differential on this car is a closed system and doesn't even have a drain
for the oil. They said they've never touched it. What do you think happened? --

RAY: Hard to know, Steve. To the best of my knowledge, there is no drain plug on
this differential. But that doesn't mean the oil can't be removed.

TOM: Right. There's a way to ADD oil, and it's possible to suck the old oil out
in order to change it. And your local dealership may have removed the old
differential oil with a suction pump and forgotten to refill it.

RAY: Why would they be changing the differential oil in the first place? Hmmm.
To make money by performing additional work on your car?

TOM: But in order for this approach to work, they'd have to do, what? Bill you!
In which case you'll have them dead to rights, because your repair order will
say "change differential oil."

RAY: So you need to check their receipt from before your trip.

TOM: Bear in mind it's also entirely possible that the oil just leaked out very
slowly, and the other dealership didn't see any evidence of a leak. In that
case, the timing was just coincident.

RAY: And unless it says "change differential oil," "service diff.," or something
similar on that repair bill, you'll never be able to prove otherwise. Good luck,

* * *

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Ray's latest pamphlet, "How to Buy a Great Used Car." Send $3 and a stamped (55
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