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CVR or ???

LNimoy

Member
I spent the afternoon putting everything back into my dash after the harness install.
When I installed the instrument cluster, I started the car to check the gauges.
The Oil and Water gauges are not working. They are not pegged in either direction. Needle on both sit at the first line of each gauge. I know they get power from the CVR.
The Fuel gauge shows 1/8 tank. it should be a bit higher with 4 gallons in the tank, but it always read a bit low.
I have an analog multimeter and I checked the CVR and gauges.
I have 12 volts on the BATT side of the CVR and 5.5 volts on the outgoing side.
It has a good ground, shared with the lug with black wire on the back of the cluster.
With all the wires to the gauges plugged in, and the car running, I tested each of the gauges.
Multimeter set to 10V, I probed each post of each gauge.
I am getting 5 volts on each post of each gauge. I believe that this is correct, no?

I am going to check to see if my white/ red and red/white wire for the Oil and Temp gauges are swapped.
I did have to add pigtails under the hood to fix both sending unit wires, they were cracked and frayed near the plug for the sending units.
If I remove the wires from the sending units and ground them and they peg high, my sending units are bad and the wiring from them to the gauges is good, correct?

My initial problem was a severely shorted out under dash harness, Lights were going on by themselves, the car tried starting, without the key in the ignition.
Could that have blown both sending unit, could it have burnt both gauges? Can I test the gauges to determine if they are good?

Thanks,
John
 

Midlife

Well-Known Member
Staff member
Moderator
5V on each post is correct.
It is pretty hard to swap white/red and red/white wires, as on a 66, each wire breaks out in different bundles.
Correct, if you ground the sending unit line and the gauge pegs high, the sending unit is bad.
It's also not a good idea to use teflon tape at the sending units, as the threads act as the ground path.
 

Mach1 Driver

Well-Known Member
I am getting 5 volts on each post of each gauge. I believe that this is correct, no? YES

If I remove the wires from the sending units and ground them and they peg high, my sending units are bad and the wiring from them to the gauges is good, correct? Yes

Could that have blown both sending unit, could it have burnt both gauges? Can I test the gauges to determine if they are good?- you already tested them and they were OK- see the first line, however you can check them with an ohmmeter- disconnect one side of the meter and you should get 16-18 ohms through the meter for a car built prior to 1969.


Thanks,
John
This is a schematic of the gauge circuit for a car built prior to 1969:
1659950314832.png
It sounds like a sender problem (or the wires), but I would also check the engine ground wire from the back of the passenger side head to the firewall. Without a return path the circuit won't function.
 
Last edited:

LNimoy

Member
5V on each post is correct.
It is pretty hard to swap white/red and red/white wires, as on a 66, each wire breaks out in different bundles.
Correct, if you ground the sending unit line and the gauge pegs high, the sending unit is bad.
It's also not a good idea to use teflon tape at the sending units, as the threads act as the ground path.
Thanks for the confirmation and the tip on the teflon tape. I am going to get to it tomorrow and report back.
 

LNimoy

Member
This is a schematic of the gauge circuit for a car built prior to 1969:
View attachment 34639
It sounds like a sender problem (or the wires), but I would also check the engine ground wire from the back of the passenger side head to the firewall. Without a return path the circuit won't function.
Thanks for the reply. I'll check the wires and the resistance tomorrow and report back.
The schematic makes it easier to understand.
 

Mach1 Driver

Well-Known Member
The schematic makes it easier to understand.
Yes, that's the whole point of a schematic. Wire diagrams are fine for wiring the car, but not necessarily for understanding the circuit. I laughed when one guy claimed that wire diagrams were like cave drawings when compared to schematics. Ford didn't release any schematics, just wire diagrams, so I had to reverse engineer it back into the original document that it all started from- a schematic. You can find a full car schematic for a 66 or a 69 here:
 

LNimoy

Member
Picked up both sending units today. Had to settle for CarQuest units as I's like to finish this up while I can.
Brought my multimeter with me and tested the units in the parking lot, both tested good.
That will now be the first test I use if I ever have any more issues with the gauges not working.
Installed the water temp unit, without Teflon tape and it seems to be working fine.
Ran the car for 4-5 minutes and the gauge went up to the about 1/4 of the way on the gauge.
Went to install the oil pressure unit and wound up having to change the oil first. Was going to do it anyway because the car has been sitting.
Power steering hoses and the oil filter didn't allow any room to get a wrench on it.
Replaced it, the oil filter and filled up the oil.
The gauge slowly climbed for about 10 seconds and stopped at the small line at the end of the scan range.
It did not peg. When I tested the old unit to ground, it pegged in about 3-4 seconds.
With all the reading I've been doing, I will be checking the oil pressure with a real gauge.
I'm going to assume that both units are good but may not be 100% to original Ford specs.
Thank you Mach 1 Driver for all the schematics. I am going to print them out and put them in a binder for future reference.
Thank you Midlife for the harness and all the info you have supplied.
Thank everyone for the education I've gotten over the past couple weeks.

John
 
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