I think I spelled phenomenon right...

Anyway, for a while my '88 351w has been running a little hot according to the gauge. Which is weird because the water pump only has about 6k on it and the radiator has 3k tops. So I kept an eye on it and made sure the coolant was full. Then one day I noticed that the gauge went down a bit when I turned the blower fan off. Then it went down more when I turned the lights off. Apparently, any electrical load will cause the temp gauge to rise. So I don't think it's actually overheating since it drops right back to normal range when I turn everything off.
Yesterday the oil pressure gauge did the same thing, but didn't seem to do it today.

The only explanation I can think of is that the gauge(s) are somehow trying to ground through another circuit?
Anyone have any ideas? I'm at a loss as to what to check first, except maybe the engine to chassis ground.

Thanks!

 

How are your ground cables? I had a Rodeo do this to me once too, the ground cable broke to the point of hanging by a thread, and the temp gauge pegged out of no where, the fuel gauge pegged on way or the other as well, all the others died, but the air was still cold so i knew it wasn't really hot or else my ECU would have shut it all down. When I looked, i had found that the ground to the body was hanging by a thread, so of course the starter worked fine, anything else that used the body ground wigged out. So check your ground cable first, if its even the slightest bit in question, take care of it, grounds are our friends, there's no such thing as too much ground, if that's not it, check a schematic and follow the grounds from there as well.

 

Ok, I will check that tomorrow. Maybe even add another ground to be sure that's not the problem.
I'll let you know what happens.

 

Well, I checked the ground on the passenger side that goes from the firewall to the plenum and that seemed good, so then I checked the one on the other side and it was only connected to the firewall and the other end was dangling! So I took a wire brush to it to get the loose rust off and then attached it with one of the plenum bolts. I think that fixed the whole problem! I just drove it the 3.5 miles to town and back, but the gauge never even got to midrange. Before it would go up almost 3/4 in 3 miles. And turning my headlights on doesn't change it anymore.

 

Good, glad to hear that all ended well.

 

Matt,

There should also be a ground from the cab to the frame gusset.
Beneath the drivers feet.

 

Wow, you still have a floor under your feet! I had to use a 1/8" piece of diamond steel since the floor was all rotted out when I got the truck. So I'm pretty sure that ground has been gone for a while! It seems to be working ok how it is, but if I have more issues with it I will add one.

BTW, my name's Glen. My truck is Mater.

P.S. Yay 25th post! Adding my signature now...

 

Welcome to the forum Glen!

I've replaced the floor....

Salty New England winters can be hard vehicles.
Check your rear spring hangers.
They like to fill up with slush and rust right through.

 

Yup, already replaced them as well as one radiator core mount. The other should be done soon. Had to replace one of the front spring shackles as well. It was broken right in half and the truck sat crooked when looking from the front.
Gotta love the salt on the roads!