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Hi Will,
Your story about towing through the mountains intrigues me. I have not tried that yet with a Volvo. But I thoroughly enjoy driving through the mountains when the weather is nice; we make the effort to choose a different path to Ohio on every trip, looking for the most scenic West Virginia, Maryland and Pennsylvania mountain roads.
On each of those trips we try to use a different Volvo; though now we will need to go into re-runs. I think our favorite in the mountains is the 242ti/M46- at least it is my favorite; my wife thinks I take the curves too fast.
But on all of those mountain pulls, up 8 to 10% interstate highway grades (you Westerners don't laugh now!) when using the N/A cars with AW-70s, the kickdown would jolt us into being aware we were falling behind the bimmers while passing the semis, whether on cruise or not. Then, after a few episodes with the momentary racing brought on by the kickdown, the OD off button would take us right to the top without the jerking.
Not towing anything, I probably didn't need it coming down the hill, but somehow I felt more comfortable heating up the transmission fluid in third, than warming the brake rotors.
Now, an admitted nerd, born knowing which end of a soldering iron to grab, I did not find the OD control circuit to be ill conceived. But there are two, and only two that I know of, bugaboos with the implementation of it.
The first, I believe, is the white wire under the car. Thankfully, it was well supported from the solenoid itself to the connector, otherwise we would be changing a lot of solenoids just because the wire broke or dissolved off of the solenoid case. But the rest of the wire, to and through the shifter box tends to look like some of those poorly formulated 80's harnesses after 20 years under the car. It usually shorts to the tranny case, but as Don's reply indicates, can fall apart altogether.
The second bugaboo is one not confined to just the OD. It affects all of these small electronic control circuits we refer to as "relays". It is a manufacturing defect that occurs when the circuit board assemblies are wave soldered in mass production. The problem has been known since the very first production soldering wave machines of the sixties: a large component with a large thermal mass, such as an open frame relay, requires more time in the wave than its neighboring resistors, capacitors, diodes and transistors. But the assembly only makes one pass, so the time (heat tranferred) must be a compromise with the maximum allowed to the sensitive small components with little comparative thermal mass. The result is cold solder joints that later fracture, as they are subjected to mechanical and thermal stress.
Best p-ography I have of a candidate for microsurgery.
The fact these high-mass components were not reflowed at the point of manufacture surprised me, because I remembered this practice and its "extra" labor being standard where I worked in 1969. I wondered what exempted these 80's devices from the wave compromise problem. Then I found this forum where the bugaboo was well known, discussed and the repair's most vocal proponent, and pioneer in teaching us how to deal with it, even had his name turned into a verb, describing the "$0, clean and easy work".
So the stock OD relay, truly a circuit to provide power-up state selection and then latching, plus a relay, suffers from bugaboo two virtually from the day it was made.
But a third condition that frustrates many of us results when we attempt to cure #2 without curing #1: The wire under the car loses its insulation. It rubs once on the transmission housing and kicks the fuse in the panel. No OD now. Three fuse replacements later, I put in a bigger fuse. Aha! Now when the wire rubs, the current is too much for a tiny trace on the OD relay circuit board, (yes there are photos of that too in the FAQ) so for most of us, a new relay gets put in.
I can well imagine a terrible impression of the initial conceptions involved in that OD's design, after a few rounds like this. But the fact is, getting that white wire fixed under the car is not clean and easy work. And it shouldn't have fallen apart in 20 years.
An industrial quality magnetic latching relay might be well applied as a replacement for the circuit board relay assembly. In my own mind, I wonder how that would be extrapolated to all the other relays; fuel pump, directional flasher, wiper delay, bulb out sensor... practically all have the same cure.
Skip this if you don't care for digressions, but you bring up latching relays. There's one piece of old-time technology that seems to be very reliable in the harsh environment near the 240's battery and hood seam: the headlight hi-lo relay. Here's a latching relay with non-volatile state memory that owes its time-tested design to Walter Strowger's 1888 patent on a telephone switch. (Look that one up for a cute story about the mother of invention.) I was just remarking to a friend how the simple pawl-and-electromagnet mechanics needed hardly any improvement 100 years hence. And, it is so reliable, the one you take from the boneyard will work just fine, unless the battery acid got to the aluminum cover. Sorta like saying "we don't need no stinkin' transistors".
I was looking at IDEC's data sheet for the RS2L series but I could not find a supplier price for a 12V version. I am always interested, but for the BB community - so post the alternative. Also, I have internal schematics of a couple OD relays if anyone is interested in those.
--
Art Benstein near Baltimore
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