My 1997 Sierra C1500 has a strange problem. The tach and the speedo go wacky, the idiots lights flash, the transmission downshifts out of OD, and the engine stutters and feels as if it is going to completely stop. It's like the PCM goes through a reboot and then runs normal - for awhile. It only seems to happen while towing a trailer on long trips. Sometimes, it happens early in the travel day before everything gets hot. Sometimes, I've had days without a problem. But, usually this problem occurs 2 or 3 times during a 8 hours drive. Scanning shows no trouble codes. I've replaced the ignition switch (which seem to help somewhat) and the VSS. I also lose my cruise control. It stays out until shut down the engine and then start it back up again. I'm wondering if the trans is slipping and causing a PCM reboot. Any help will be greatly appreciated. Thanks.

 

It almost sounds like a loose ground. It's good you already replaced the ignition switch, cause that was my absolute first thought while I was reading your post lol

Check that your battery cables are tight and clean - that little spacer on the positive battery cable eventually crushes down and you can get a poor connection over time.

 

It does sound like a loose ground except it only happens when I'm towing my trailer (before I replaced the ign sw, it happened occasionally without towing). I've checked all the grounds I can find. I shimmed the battery connections awhile back so they're tight. I've never heard of this, but is it possible for a battery to short intermittently? Maybe I've got a lot of debris in the battery that shifts when the trailer weight lowers the rear. I wish I had a scanner I could leave plugged in during a trip to record the event - mine is software (laptop) based.

 

Are you using a hardwired connection to the trailer connector, or are you using a plugin style connector? I ask because I'm wondering if the trailer connector puts too much strain on the fuses under the hood. My truck experienced something similar but my stop lights went out completely. Turned out that my connector corroded and shorted between positive and negative.

Is it possible to eliminate the connector for a time, in order to do some testing? Like remove the connector for a time and test the truck out for a week or so to see if the problem persists.

 

NullHead, thanks for the input. I only make a towing trip twice a year - in between trips the truck runs perfect. The trailer uses the large 7 flat pin connector to power the brakes so it might be dangerous to run disconnected. There's no corrosion and the trailer lights are all LED so the load is light. It also has a remote security alarm which I thought might be causing a problem so I've been running with that system disconnected but the problem remains. I think it only occurs when the trailer is heavy that's why I keep looking at the trans. I just don't know how much data from the trans is sent to the PCM. I could do an 8 hour test run with an empty trailer to see what happens. That would be quite expensive at 14 mpg - unless it hiccups early on. This is a tough one.