95 f150 300 engine temp gauge problem
Thread Starter
|
Junior User
Joined: Jan 2008
Posts: 61
Likes: 0
95 f150 300 engine temp gauge problem
So I bought this f150 just a while ago and it's always been showing pretty high temp. Gauge would start off in the middle when car was cold and move to near red when warmed up. I figured maybe i just need a thermostat, so I replaced it (it was bad, btw) while also doing a flush of the system. This got a lot of seriously dirty stuff out of the system. Well since then it gets to red even faster and goes pretty much all the way. It still starts in the middle when cold.
I thought maybe the temp sending unit is bad in the back of the block, but it measures resistance within limits. I then measured resistance on the wire that's on the unit to the ground and measured only 20ohms. There appears to be a short on the wire somewhere. I tracked it down to be past the firewall somewhere inside the cabin. It gets much harder there to try to find out where the short is. Should I be looking at a short on the wire or is a short directly in the gauge cluster common? If it's in the gauge cluster, can it be repaired or do i need a new one? Thanks!
I thought maybe the temp sending unit is bad in the back of the block, but it measures resistance within limits. I then measured resistance on the wire that's on the unit to the ground and measured only 20ohms. There appears to be a short on the wire somewhere. I tracked it down to be past the firewall somewhere inside the cabin. It gets much harder there to try to find out where the short is. Should I be looking at a short on the wire or is a short directly in the gauge cluster common? If it's in the gauge cluster, can it be repaired or do i need a new one? Thanks!
Thread Starter
|
Junior User
Joined: Jan 2008
Posts: 61
Likes: 0
So I went ahead and pulled the gauge cluster out and started measuring resistances on the needles. Each needle has 3 terminals...GND, IGN and SIG. Ground to ignition resistance on both coolant and oil gauges is ~58ohms. Ground to signal is ~31ohms (pretty much the thing I measured on the temp sending unit side) and ignition to signal is ~60ohm on coolant and ~48ohms on oil gauge.
I never paid much attention to the oil gauge behavior, but it does hover around the middle most of the time...not sure if that one is bad, too.
Last I measured the resistances on the fuel gauge and they're very different: Ground to ignition is 290ohms, ground to signal is 200ohms and ignition to signal is 100ohms.
These values make much more sense since the temperature sending unit spec is 74ohms (cold) to 9.7ohms (hot). When you have a 200ohm parallel line to ground it's not as bad as a 31ohm parallel line to ground. Then again, maybe it's all calibrated for it.
Does anyone know what the correct resistances for this are supposed to be?
I never paid much attention to the oil gauge behavior, but it does hover around the middle most of the time...not sure if that one is bad, too.
Last I measured the resistances on the fuel gauge and they're very different: Ground to ignition is 290ohms, ground to signal is 200ohms and ignition to signal is 100ohms.
These values make much more sense since the temperature sending unit spec is 74ohms (cold) to 9.7ohms (hot). When you have a 200ohm parallel line to ground it's not as bad as a 31ohm parallel line to ground. Then again, maybe it's all calibrated for it.
Does anyone know what the correct resistances for this are supposed to be?
