so i recently swapped out my main harness in the truck out because mine shorted and had a small electrical fire. ive got mostly everything in and back to gether but now im noticing my temp guage pegs maxed out almost as soon as i crank the truck.

relevant info:
harness is out of a manual 1984 F150 L6. mine is an automatic 1984 F150 L6
the temp guage was working fine before the short
ive tried both the cluster from the donor truck and my original cluster
unplugging the connector from the sensor has no effect and still pegs out
ive checked all my grounds (i think)

im kinda running out of ideas and need someone elses opinion on it

 

It's either the red/white or white/red wire, I forget which... one is for the oil pressure, the other for the water temp. Locate these 2 connection points.

Long story short, these wires connect to ground via the sending units. Turn the ignition ON, then briefly connect each of those wires - one at a time - to a good ground; the respective dash gauge should peg out to the right (1987+ will max out to the left).

@Franklin2

 

red w/ white stripe. i was looking at those yesterday

 

Here's a wiring diagram for 1985, which should be similar:

Fuel Tank Selector & Gauges (Gas) - Gary's Garagemahal (the Bullnose bible)








As previously mentioned, if you ground the red/white wire (#39), the gauge should drive full scale high. If the gauge had been operating properly with the old harness, this would suggest something is funky with the new one. A break in the circuit, such as that wire not being connected, would cause the gauge to read full scale low.

I'd suggest disconnecting both ends of the wire run, at the sender and back of the instrument cluster. With neither end connected, check the wire run for continuity end to end. You should have no more than a fraction of an ohm, basically "perfect" continuity from the instrument cluster to the sender. Next, check this wire run for continuity to ground. There be none, zilch, nada. If you do find a problem with either test, the fun part will be determining exactly where. The diagram shows connector C325 in the middle, but doesn't show details of exactly where it's located other than on the LH inner fender near the ignition module. The EVTM does say it is black with 4 wires, so you might be able to find it that way. Disconnect this connector and see which side of the wire run is the culprit.

Is the fuel gauge operating properly? If the tank(s) is full, that gauge could also be pegging high and you may not realize it. Do you have an oil pressure gauge or idiot light? If any of the other gauges are also pegging high, that could be a problem with the IVR.

In theory, if you find a fault in the harness, you should repair it. In practice, though, if everything else is good and this wiring fault is limited to the circuit feeding the temperature gauge, you could just run a new wire.

One last thought: Are you positive you have the new harness connected to the correct sender on the engine? You mentioned disconnecting the wire from the sender, but are you positive you pulled the correct wire? I was thinking you may have some other senders or switches, such as for controlling ignition advance.

 

If you had the sender unplugged, it is to the rear of the block under the manifold and hard to get to, and it still pegged then it is either the sender wire to ground or a bad gauge.

Karl brought up a few good things about the other gauges doing crazy things and unplugging the temp gauge but I dont see an easy way to do this as the big plug on the cluster feeds everything.
Same with running a new wire for the sender, how do you get it to the back of the cluster?

BTW what caused the cook out?
Dave ----

 

My thoughts were to unplug that big connector to check the wiring from that connector, through the firewall, and out to the sender. Check end to end for good continuity, and that it isn't shorting to ground. Of course you can't operate the gauges with that plug removed. I suppose you could remove that one wire from the connector (usually releases with a small screwdriver or pick) and then reinstall the connector. If the gauge now stays low, the fault was in that wire.


If you determine the wire run (between the instrument cluster and sender) is at fault, you may not be able to find the exact location of the fault. In that case, cut the wire several inches from the cluster and splice a new length of wire to reach the sender. Cap the wire in the harness that you just cut.

 

ok to answer a few questions.
1)what caused the cook out: the fan switch shorted out
2)do any of the other gauges work: yes all of them work as normal. this issue is exclusive to the temp gauge

i would think the fault might be in the cluster but ive tried multiple clusters and they all show the same, everything works fine except the temp guage

 

Then I would think the temp sender wire is grounded some where if you unplugged it from the sender.
Do you know if this happened when the harness was in the other truck?
Dave ----

 

honestly i have no idea what happened to the donor truck as it was in a junkyard. the people who owned the yard just knows it was sat there as a runner when it came in, but they couldnt remember anything else

 

ok so im an idiot. i thought the sender unit was on the water pump its self. apparently its in a much darker place to tread. will test tomorrow if the weather holds.

 

LOL that is why I pointed out where it is on the 300 six as you cant see it on the rear of the block below the manifolds.
The sender on the Tstat housing is for the feed back computer (83-86) or the EFI computer on the later year trucks.
I did not think this sender had the right type plug?

If the wire was unplugged and the key turned on the gauge should not of pegged.
This way you know from the sender plug wire to the gauge is ok.
THis is also how you check the temp / oil / fuel gauges if not working, ground the sender wires and turn the key on.
If the gauge pegs it wire to gauge is good and the sender could be (most likely) bad.

Good luck
Dave ----

 

so i did the simple unplug the sender test. temp guage didnt move from cold. i think that with the fact the nipple on the sender looks bent over (dont know how long its been like that) i think its safe to assume my sender went out. i would think if it was a wiring problem it would still peg out even if it was unplugged

 

There are differences between senders vs. sending units; one has a single wire, the others (connected to computers) have multiple.

Not until 1987+ would it behave like that; ground that wire with the ignition ON, it should peg the gauge.

 

ill do that tomorrow when i get home from work. but im fairly certain it was the sender since the peg was snapped. but ill double check tomorrow

 

If you unplugged it and the gsuge did not move that's a good thing.
Ground the wire, key on it should peg. Dont do this to long as it can hurt the gauge. That's good too.
Yep sounds like the sender is bad. It's a little hard to get to under the manifolds but it can be done.
Drain the coolant as it will come out when removed and be careful not to cross thread it going back in.
Dave ----