Temp / Fuel Gauge Short?

xumFJ40 · 03-07-07, 12:49 AM

This is a little long...so sorry.

Background:

I have a '75 FJ40, when I bought it, to the best of my knowledge my Temp Gauge and Fuel Gauge were not working.

To start the process of getting my truck running, I re-wired the entire loom.

I was not overly worried about the Fuel Gauge [just kept a 5gal jerry can handy :-) ], and ran an aftermarket Temp Gauge and sending unit for a while to make sure that I was not going to overheat.

Anyway, I have gotten to the point where I wanted to get the factory Temp Gauge working, so I bought a new factory gauge and tested the sending units to make sure that they were putting out the correct ohms...however, the Temp Gauge still did not seem to be accurate.

While installing the new Temp Gauge I noticed that the problem with the Fuel Gauge was that the wire running across the top of the gauge was broken, or burned in two.

Doing some research as to why my Temp Gauge was not working I read on Mud that the two gauges have a common lead and that if your Fuel Gauge is broken, your Temp Gauge will not work correctly. So I bought and installed a new factory Fuel Gauge.

Still no joy!

Problem:

With everything hooked up when I turn the key (engine not running) my Fuel Gauge remains on Empty and my Temp Gauge slowly drifts towards Hot settling at about 2/3rds of the way.

If I unhook the lead to the fuel sending unit the Temp Gauge will settle back to Cold as it should be...but the Fuel Gauge never moves.

If I ground the lead...the Temp Gauge behaves the same as if it is hooked up.

I have a full tank of gas and the ohms on the sending unit are reading 16, but just to be sure I removed the sending unit from the tank and the Fuel Sending Unit tests true through the range of ohms.

Question:

So...where do I look for this short?

I re-wired the entire loom, and tested before re-installing so I am pretty confident that the short would not be in the wiring itself.

Could the short be in the Gauge Cluster itself?

How do I identify a short in the Gauge Cluster?

Any suggestions?

Thanks!

Trollhole · 03-07-07, 07:26 AM

Welcome to guage hell. Hope this helps. Mine had the wire burned also. I soldered a new wire back.
Problem I found is that if you ground any of the guage sensor leads you will screw up the guages.
I just bought a new guage panel and was done with it.

76 Fj40 Guage problems

xumFJ40 · 03-15-07, 06:24 PM

Ok...just a sanity check here.

From looking at Tolls' diagram of the gauge cluster and the wiring diagrams available in Tech I am working on this assumption.

http://www.yankeetoys.org/documents/...am_78_fj40.pdf

The Yellow / Red wire that attaches to the Fuel Sending unit should correspond with pinout # 5 on the back of the gauge cluster.

Can another set of eyes confirm this before I yank out the entire wiring harness to locate the disconnect?

Thanks,

-R

xumFJ40 · 03-15-07, 09:26 PM

Ok gang,

For the record...

I decided that I would test the theory before removing the wiring harness by bypassing the fuel sending unit lead in the harness.

When I looked at the back of my gauge cluster the wire going into #5 was Yellow/Green and the wire going into #6 was Yellow/Red!

So...after switching the two wires, everything is working correctly.

outdoork9lady · 04-09-07, 02:12 PM

I have searched for JUST the wires from the sending units to the gauges. That is what is missing for my 78 oil gauge and temp. I can find the sender units and the gauges, but I need the actual connections from one to the other. Can anyone help me with where to find them? Maybe my "thinga ma jig" description is the problem LOL.

papagyo · 09-25-07, 09:31 AM

Hi All,

I am now working on getting my factory gauge to work also, the problem I have is the PO has cut and removed the circular plug that plugs into the back of the cluster. In the above photo is #6 the wire from the sender? Also it says that 'A" and "B" are connected, (right now nothing is connected to "B") which wire in the round connector corresponds to "B"? is it power or ground or something else? Anyhelp would be most appreciated. I do have a circuit diagram but due the the PO getting happy with the cutters not much matches up.

Also I am looking over a 72/73 wiring diagram and it clearly shows only one connection (unless I am missing something) to the temp gauge..is this correct? Were the backs of the clusters different on a 72 compared to a 78 as the round black connector is not shown on the wiring diagram for a 72/73.

PS. All other gauges do work correctly.

Brian

papagyo · 09-25-07, 07:16 PM

TTT

Brian

jus cruzin · 09-25-07, 10:17 PM

pretty sure your right with #6 being the temp sender it should be a Yellow wire with a green stripe or a plain yellow wire,

with A + B being connected is due to the temp and fuel gague haveing a common earth, so check the earth wire at the fuel tank sender + clean or repair as req, and check the continuity between the A and B posts on the cluster

papagyo · 09-25-07, 11:36 PM

jus cruzin,

Thanks for the response, I will tell you what I currently have: I have a YR and WB coming from fuel sender, the WB is grounded to the frame under the truck (appears to be factory done) and only the YR(#5) goes to the fuel gauge. Wire #4 is power and is connected correctly at the fuel gauge and the oil gauge. I have the YG(#6) coming from the temp sender. So far all the gauges work except the temp gauge with these connections. Now since both A and B share a trace on the board should there be a wire connected to either A or B to make the temp gauge work. Does the temp gauge need power to work like the others? Thanks again.

Brian

jus cruzin · 09-26-07, 12:18 AM

the power to the gague cluster (temp, fuel and oil) comes from the light blue wire with red stripe (comes from the heater fuse 15A) , no need for seperate power to either post (as the temp works off a ground resistance at the sender).
I'd remove the temp sender wire off to sender itself, ground to an earth and check the temp gague for movement (be carefull not to ground for too long as dash cluster is very tempremental, if you want to be carefull about this process throw a 5W festoon globe between the earth and the temp wire (have been told this works)),
if you dont get movement remove the cluster and the temp gague check wires etc on the gague there's one that blows out if there has been a short in the cluster,
if all is good you could pretty much gurantee that it is the temp sender, replace as req.............
been going through the same process myself recently

good luck and keep us posted

papagyo · 09-26-07, 12:40 PM

jus cruzin,

Did what you suggested and disconnected the temp sender wire and grounded it to the chassis and the temp gauge started to move up. So I am assuming as you said that the gauge does work and the problem is sender which I will replace and see if that corrects it. Thanks.

Brian

jus cruzin · 09-26-07, 04:11 PM

Quote:

Originally Posted by papagyo

jus cruzin,

Did what you suggested and disconnected the temp sender wire and grounded it to the chassis and the temp gauge started to move up. So I am assuming as you said that the gauge does work and the problem is sender which I will replace and see if that corrects it. Thanks.

Brian

spot on Brian, give a new temp sender a go, should fix your porblem, remember not to put any PTFE tape (teflon tape) on the sender as it requires a good earth to complet the circuit