This might be the same (or a similar) problem to the one posted by 94GmDeville on 7/12. It's a 96 Seville STS. It blows the rly ign 1 fuse in the trunk after 5-10 minutes of driving in the 65-70 MPH range. Driving locally, or continuously at 50 MPH or below never blows the fuse. Everything is fine after replacing the fuse. There are no new codes on the DIC, nor when the shop connects their computer to the diagnostic port.

I know my ignorance is going to shine like a freshly waxed kitchen floor, but what's on that fuse? That's where I would start a process of elimination. For example- if you use the cruise control at 65, but never at 50, and the CC is on that fuse, well, there's something to look at- the drive unit for the CC may be binding up, causing a slow blow.

Thanks for the reply, but I've been down that road. I've looked at each thing connected, and can see no reason why any of them would cause a short or extended overload under the conditions. The fuse feeds the CC switch in the stalk (open during fuse blowing; the CC was off), the instrument cluster (replaced) the fuel level sender (fuel gauge behaves normally), a brake switch (open during fuse blowing), and two electronic modules, the BCM (aka PZM) and the SDM. The two modules pass self test. There are no codes and everything seems to work properly after fuse replacement.

I have mostly (but not completely) discounted a sustained overload, since it takes about 12A to blow the fuse in five minutes or so. That's 144 watts going somewhere for at least five minutes. I think that would cause permanent damage in any of the things connected to the fuse. (The fuse blowing has happened five times.)

If you can shoot holes in any of this, please do. We're at the grasping at straws stage.

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Believe me, I understand the frustration. OK- you get no codes on the cluster, and the shop's Tech2 or whatever-they're-using finds nothing. I'm guessing that's because when the fuse blows, the various modules, or at least that section, is thinking that the ignition has shut down- nothing unusual there. My next step would be look for a chafed wire, or, considering the fine wire gauges involved, a possible nicked wire, reducing the amount of area for power transfer- basically trying to push the same amount of power through a smaller wire, which will cause a blow on a small fuse, or an overheat and melt on a larger one. I would pay special attention around the PZM- the various computers in a car today suck ungodly amounts of power. IIRC from my days of wiring Devilles as limos and hearses, even though most of this stuff is on a plastic board in the trunk, the rather sloppily made harnesses pass through an awful lot of sheet metal. Try a simple diagnostic before you tear your car apart- disconnect everything connected to that fuse, pull the fuse out and, starting at the 'cold' side of the fuse, check for a short to ground. This isn't guaranteed to fix your problem, but it will give you a direction in which to head.

We had already tried disconnecting everything, but since the problem only occurs at cruising speed, everything looks fine with the vehicle standing. If whatever it is would short, and stay that way, it'd be easy...but Murphy is alive and well. It doesn't seem to be an intermittently bad connection or insulation problem, since I can drive the vehicle over washboard roads, potholes, and RR tracks, and nothing happens. It happens only as I am cruising in a straight line on a smooth Interstate. I suspect it has something to do with heating somewhere, but so far, no idea where. All but the fuel level sender is inside the vehicle, and all of that inside (except the PZM) is in the air conditioned space.

I'm considering a couple of things. One is to kluge in an ammeter and an external fuse into the circuit, to see if it is a steady overload, as opposed to a short. The second is to disconnect everything I can (one thing at a time), and then try it. The vehicle won't start with the PZM disconnected since it's included in the theft deterrent system. I don't know if it'll start or not with the SDM disconnected, but I won't drive at 70 with the airbags disconnected anyway. The rest can be disconnected, at least for a while. I'll let you know if I find anything.

Thanks for your thoughts and input. It's easy to get stuck in a train of thought, and someone else can often blast you out of it.