AAAAAHHHH!!! Wiring

[qmcdugan]

It's been five years Labor of Love on this 70 F-100,. But I think we got'er boyz! "IT'S RUNNIN"!
Cause:
Missing Resistor wire. HUNDREDS of dollars $$$$in trial and error... then pitched it to the Pros.

Ignorant POs and MORE stupid current owners.
An aftermarket set of gauges were install, but only the oil gauge works.

Here's what I need:
I'm hoping that the gauge wires are still there and won't requiring replacement.
I'd like to find the wire(s) left / right side, that will make the factory gauges work.

Fuel gauge does work

This truck has a transplant. A 351M ( 400ci ) from a 70 Mustang. But the wiring was never hooked to the engine by the PO. With the Instrument Cluster removed after disconnecting the Speedo cable I've looked but can not find the following wires for the :
WATER TEMP GAUGE 39 = RED - WHITE STRIPE
OIL PRESSURE 31 = WHITE - RED STRIPE

Located the following, where it joins the VOLTAGE REG
AMMETER 904 = GREEN - RED STRIPE

[6Delta]

I can't help you with the wires, I haven't pulled the cluster yet. Are you talking about the wires coming out of the plug in the cluster going to the fuse box, or the wires under the hood? Also, if you have a 351 M code from 1970 it is not a 400. While the heads are basically the same other parts are not. The 351 M code is a Cleveland. The 400 is almost a Cleveland. Some parts interchange, some do not. They both can really run, however if you "hot rod" the Cleveland there are a few items you may need to check.

[slitherfast]

First you will need a meter. Set it for 20vdc or less. Your trucks runs on a 12 volt DC system.

Second Wire color really does not mean a thing. The wires may have been replaced at some point with a different color wire. When Ford had the wire harness made they did not always use the same color wire every single time. This is especially true when going from one year to the next.

Third I have some quick questions.

When under the hood is the temperature sending wire connected to the sending unit. Is the the oil sending wire connected to the unit.

We will start with those two since they are easy to reach and trace down.

If both wires are there turn the ignition to run (don't start the truck). Pull the temperature wire from the sending unit. Put the red lead into the opening for the sending unit. Put you black lead to a good ground source. The core support, frame or a stainless bolt will do great. You should get between 10vdc to 12vdc. Anything less than 1vdc means you aren't getting power.

The sending unit for both oil and temperature is the ground for the wire. If these are painted it can cause a loss of ground. You could even use the unit as a ground source for testing. If painted use some sand paper and try and clean it up. The temperature sending unit is two pieces. The unit screws into nut that screws into the intake. If any kind of thread sealer or tap was used to put the sending unit into the nut it will not get a good ground and therefore not work.

If we are getting power at the sending unit that means power from the fuse box is making it's way to the unit. So the problem must be between the fuse box and the gauge. If your gauge cluster lights up when the lights are turned on that means power is getting the cluster. Also since the fuel gauge works some power is getting to the cluster.

The next is to set you meter to continuity test. The ignition can be off for this test. Now follow you're working temperature wire from the sending unit to the spot in the firewall. Look for any signs that some one has tapped or gotten int the pack of three wires. The three wires should be your coil from ignition oil sending and temperature sending unit. It should go into the fire wall near the center of the firewall just right of where it dips in for engine clearance but before the reach the master cylinder. Assuming the wiring still runs like the factory did it.

By now you should have noted the color of your temperature wire. Find it inside the cab. The coil wire should go to your ignition switch. Your oil and temperature wire should join into another group of wires the connect to the gauge cluster. At this plug/junction you can test for continuity by putting the red lead into the plug then on the other side use your black lead and touch the wire. If you don't get a reading you don't have continuity. This would also mean power does not go through. If you don't have continuity test you can leave the truck ignition to one and do the same steps as before when under the hood. As before anything less than 1vdc is no power. If all checks out there continue to follow the wire until the problem is found.

If all that checks out up to the plug for the cluster then I would do a simple test. You will need a second person for this test. One person will need to watch the gauge. Take the wire from the sending unit and tap the inside of the connector to the unit or a good know ground source. If the needle on the gauge moves this means the gauge is good and the sending unit is bad or not making a good ground. You will need the ignition set to run for this test.

Wiring on these old trucks really is not very hard. They are very simple once you understand the basics. One thing that helped me the most is when a friend of mine (he is like Stephen Hawking with wiring) explained to me that electricity works like circle. One half of the circle is your positive. The other half is your ground. You can't have power with out both sides of the circle. On these old trucks if you don't have power it typically means you are missing a ground.

Don't forget the gauge cluster must have a good ground. If not it will not work correctly.