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Just finished fixing the heated seat wiring in our 242t. What a joke. They wired the driver's seat only right to the back glass defroster switch.
In determining which fuse to use, keep in mind that the number of items powered is not important, it's the amount of amperage drawn by each item. If you have seven things plugged into a fuse and none of them draws more than 1 amp, an 8 amp fuse is fine. Fuses are usually rated based on inrush current. Many things will draw more current when first turned on (though in our case, not much more).
If you look at enough fuse box covers, you'll see that various years change based on what equipment was offered. In some cases fuse 8 has nothing on it except a bunch of little lights like the engine bay lamp and courtesy lamps. Some years show heated seats on 10, some don't.
Here's the correct process: First, perhaps with a battery, power up your seats, both of them, and put a meter on the power lead. Figure out how much amperage is being drawn. You must make sure the thermal switch is closed in each seat so current is being drawn. Check the board, I can't recall what the temp is at which they close.
Next, decide which ignition hot fuse you want to use. Your heated seats need to be on an ignition hot fuse. Hot only with key on. Turn on all the items that run from that fuse and put a meter on it. If there is insufficient margin left in the value of the fuse to accommodate the additional load you found from both seats, then you can't use that fuse.
Yes, there may indeed be fuses in your box that are not being used. Look for day running lamps.
One should not arbitrarily upgrade a fuse on a circuit to provide for your seats. If you run a larger fuse on a circuit designed for a smaller one, you remove the protection for items already on that circuit. Remember, the fuse is there to protect the wiring, not the device. If the dome lamp is on an 8 amp fuse and you upgrade that to 16 amp for your seats, you loose protection for the low draw item. Imagine what will happen if your dome lamp shorts. You may fry the wire to the lamp before the fuse pops.
If your fuse box shows a different fuse for each of the seats, that's for two reasons: First, in the early cars, both seats were not necessarily heated with the heated seat accessory. I've seen many GL's and turbos, pre-86 cars, that had only one heated seat (like our turbo coupe). There is also the issue of load, again. There may not be a combination of items that allowed Volvo to put both seats on a single circuit without overloading that fuse, thus two circuits. If you are running wires anyway, I might suggest you go by what's in your fuse box. That is, if it shows one circuit for each seat, go for it that way and run a separate wire to each of the two seat switches. Only a few feet of wire, and you'll have the advantage that your wiring and fuse box cover will concur.
DS
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