Temp gauge over clocked, past pegged, not overheating 1995 Ranger 2.3L

[Jimbo45]

I have a 95 ranger 2.3L with 90k miles, that I use to commute to work. I noticed one day on my way home, the temp gauge was pegged off the chart, way past H, at about the 11 o'clock position. I panicked for a second, but knew the engine wasn't really that hot, since I had only driven about a mile, from a dead cold start. I stopped anyway, to ensure the engine was cold....it was. Ironically, I noticed the gauge issue after jump starting a VERY DEAD vehicle that sat for a year, at work. Not sure if that caused the gauge issue though.

So, I first mistakenly replaced the temp sender (for the ECM) with no change. Still over pegged gauge at cold, that maybe moves a hair farther when up to operating temp. I then replaced the temp sender in the block, thinking that would fix it. Nope. Same over pegged reading at cold with new sender (maybe even farther up). So, then I unplug the one wire from the sender, and the gauge needle drops to the bottom, to full cold.

It seems like a short in the sending wire, but if that was the case, why would the gauge drop to the bottom when I unplug the wire from the sender? Nearest I can figure, one of 3 things is happening:

1. I replaced the bad sender with another bad sender ( unlikely)
2. The gauge is bad
3. A resistor between the sender and gauge has gone bad, throwing off the calibration of the needle

Is there a resistor associated with the temp gauge I can test? Do gauges just go bad?

 

[RonD]

The dashboard temp gauge is pretty easy to test, and I think you have done this.

With key on engine off, disconnected sender wire, gauge should drop to COLD or go to HOT, either is fine.
Now connect this wire to a ground, it should do the opposite of above, if it was COLD when disconnected then it should show HOT when grounded or visa versa.

If it doesn't do this or gauge needle doesn't go all the way up and down then gauge or wire to gauge is at fault, temp gauge failing is not common, but can happen, yes i do think there is a resistor on the backside of the gauge.

If gauge does respond as it should then sender is at fault, yes even if sender is new.
Each model of truck can use a different sender, "one size fits all" doesn't apply, some use high OHMs when cold others use low OHMs, so if you got a new sender it could be the opposite of what your gauge needs.

To test sender you need an OHM meter, read OHMs when engine is cold and then when engine is warm, or use you old sender and a pan of boiling water to test it.
Low OHMs is the same as grounded wire, 0 OHMs is ground, high OHMs is the same as disconnected wire, infinite OHMs is disconnected.

One thing to know about 1 wire senders, never use sealant on the lower threads, a 1 wire sender is using the engine as its ground(second wire for compete circuit), so it must have a good solid ground to work properly, you can use sealant just leave some bare threads at the bottom for the ground.
Also check engine to firewall ground strap, if that is loose it can cause temp gauge issues.

 

[Jimbo45]

For those interested, I got it fixed today.

After taking out the instrument cluster, and checking all the resistors on the back of the panel and gauge, I determined it was fine. I then checked the sender wire, at the plug into the cluster....only 62 ohms. Should be like 300-400 when cold. So, I got another new sender today and testing it, it showed about 400 or so ohms, in my hand.

I put in the second new sender, and it works fine. Who'd a thought I would get a bad sender, that would throw the needle way high, to almost the exact same spot as the old, bad one that I replaced????? Jeesh. At least its fixed now.