Hey, I've got a 1966 F600 dump truck I'm using on our farm and would like to get the gauges working. None work, and don't peg out when grounded. I'm getting 12 volts to the new constant voltage unit which stays on no matter what position the key is in. Is this normal? Strangely, the fuse tha's labeled instrument cluster doesn't have power on either end. Any guidance?

 

I'm assuming that "don't peg out" means they are on the low end of the gauge, e.g. 'E' on fuel? If grounding the wire to the sender makes no difference, you need to check the circuit up to that point. You said that you have 12v to the CVR, but is it working? New doesn't mean good. Do you have voltage to the sender? An easy test would be to disconnect the wire from the sender and see if a test light will flash. That's not a 100% test, but it's a starting place. Which gauges do you have, vs. warning lights?
I don't have a schematic with me. May have one at home but you should have this figured out before I get back next week.

 

The standard round speedometer metal dash bezel is the ground plane for the ICVR (Instrument Cluster Voltage Regulator) or CVR (Constant Voltage Regulator) or CVU (Constant Voltage Unit) same no matter what they call it. The whole bezel either needs to be screwed into place in the dash - grounding it to the dash panel - or if you pull it out far enough to work on things then you need to ground it well to the metal dash panel or other grounded surface while you work on things. A small jumper with two clamps is all it needs. Start by tracing the wire that brings 12V to the ICVR and see where it comes from. It should come from the ignition switch like shown in this partial wire diagram below as wire number 30. An ohm meter or continuity check should be able to isolate that wire from end to end. I'd recommend you sort that first before going further into the gauges as that's critical stuff to a good working gauge system. The output of the ICVR is either a constant 5V if using one of the newer digital types or a fluctuating voltage that's supposed to average out to about 5 volts if using one of the old school analog types. The digital types you can measure with a digital voltmeter, but the analog types will cause many digital meters to constantly fluctuate and not show what's going on. In that case I recommend connecting up the oscilloscope and...wait do you have an oscilloscope? They are handy to see the maximum voltage output as well as the wave form.
If not, then the test light from ground to the output of the ICVR should show a strobe effect and an analog voltmeter should show a needle that bounces between 0 and 12 never quite reaching either one.

 

Thanks for the help. My CVR is showing 12v on the ignition side and 5 on the gauge side. However, it’s hot not matter if key is switched on or not. Is this normal? The ignition switch is new and I’ve traced the wire to the ignition.

 

The center post also called the A post should only be hot with the key in and turned to either the Run or Accessory positions. Might be good to pull the switch and test it independent of any voltage or wiring. If it tests OK, then there may be a short between the wire going to the ICVR and some permanently hot wire.

 

Pulled the switch and the center post is hot no matter the position of the key

 

That center post is where everybody connects extra electrical stuff to. It is possible someone has connected a hot wire to that post. I suggest removing everything connected to that post except the heavy black/ green stripe wire. That wire supplies power to the ICVR among other things. That wire may also have a small black wire paired with it at the connector. If you have that small wire also it is a 15-ohm resistor wire that goes to the voltage regulator, it isn't on any wiring diagrams, it is only depicted in the shop manual. It is there to supply exciter voltage to the ALT. if the ALT. light burns out in the dash. As far as the gauges they are easy to sort out, but you need to fix the center post being hot all the time first. And please no matter what you read do not ground out the gauge to test it. Those gauges were designed to operate on a nominal 5 volts if you put 12 volts on them for very long you run the risk of damaging the gauge.

 

What I suggested yesterday was to pull the switch and test it independent of any voltage or wiring. You mentioned the center post is hot no matter the position of the key? Sounds like you still have wiring and voltage going to it in that case. What I was recommending is to pull that switch completely and test it like any other switch. Look for continuity between contacts points in different key positions. In the OFF position, which is the first click from far left (far left being the ACC position), you should have no continuity between the B - Battery post and the A - Accessories post. If you do, then you have a bad switch it would seem. If not, then the problem must lie in the wiring.

 

Found the problem with it being always hot. Someone had rigged it that way. Ordered a new pigtail for the ignition switch because the yellow hot wire has been pulled out the old one. I don’t think that will fix the gauges not working but it’s a start.

 

You should be able to test each gauge directly by applying 5V DC across the terminals with the correct polarity. If they don’t respond then the gauges may be bad. You can use the 5V output from the CVR and a good ground to test them in place. I’d suggest temporarily pulling any existing wiring from each gauge for the test so something else doesn’t impact the test results.