I DO have classics; two '72 Challengers in the restoration shop, two more at home, and two '80 Volares. Plus my uncommonly reliable '88 Grand Caravan daily driver up here in the heart of road salt country. I know what you're saying about the inappropriate overuse of unnecessary technology. I'm on my way out of town shortly to help a very knowledgeable friend figure out a simple brake light problem on an over-technologized computer-controlled vehicle.
What I'm referring to about returning electrical parts is when that new sending unit didn't solve the problem. They don't want people buying parts to try, then returning them if it didn't solve it. Nobody else wants to buy a part that someone else had first. At that point it isn't "new" anymore. The parts stores aren't in the "here, take it and try it, then bring it back" business.
The proper thing for the shop to do is continue with the diagnosis, fix the problem, THEN remove any parts that weren't needed, however, some parts are hard or not practical to remove, or are so low-cost that the labor time to remove them would cost more than the cost of those parts. In the case of the sending unit, the shop would typically put it back in their inventory and hope to sell it to someone else in the future. Problem is they can't do that real often or all their profits would be tied up in unsold parts. Oil pressure sending units do have a well-known failure rate on all vehicles so the argument can be made that it's better in the long run to just leave the new one in.
My cousin learned early on at his tv repair shop that you can't just buy lots of parts to "try" on each tv. We had to become very good at diagnosing exactly which five and ten-dollar parts were needed rather than just buying a dozen suspect parts and only using two or three of them. The boss was so pathetically honest that he would never leave in a part that wasn't needed, but that meant for every tv or vcr repaired at a 20 dollar profit, there would have been thirty dollars worth of leftover parts on the shelf representing breakfast not on his table the next day. Most of those parts were specific to one brand and model that the chances of them ever being needed again was remote. No one buys three house insurance policies in case you need one. No one buys three different computers, all involved with turning on the head lights, in hopes one will solve the problem. Few people, (other than me) buy more tomatoes than you can eat before they spoil. The point is a business can't afford to stock a lot of unneeded parts. Their parts room would start to look like a Walmart store with no customers.
Of course it's a different story when a new part is defective, but the clue here is the symptoms are exactly the same with either the old or new sending unit. Even a defective new one would act a little differently in some respect. Since the symptoms are the same either way, we can deduce the real cause of the problem is something that wasn't changed which is everything in the circuit other than the sending unit. That leaves the actual oil pressure when the engine is warm, the wiring, or as you reminded me, a computer. Transistor driver circuits are notoriously unreliable and one is used in computerized cars to operate the warning light, unlike in the days of our old reliable cars where the sending unit ran the light directly. A little corrosion between two terminals in a connector can trick the computer into thinking it should be turning on the warning light. The sending unit was still the logical first suspect, but there's a whole bunch of other variables and complexity to consider on newer cars. The first shop should be given the chance to continue on with the diagnosis rather than having new shops start over at "step one". Running to new shops is like a record, (remember those?) With a skip backward on the first song so it never gets to the final song. The mechanic needs a chance to get to the final song, the solution.
Sunday, September 16th, 2012 AT 8:15 PM