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Gauges Inop. on '69 Mach

D

drummingrocks

Member
I've been working on a friend's '69 Mach 1 lately. The car came from the factory with the tachometer, so it has gagues instead of the usual idiot lights. Soon after buying the car, my friend mentioned that although the tach worked, the fuel gauge never read above 1/4 tank and that the temperature gauge never moved off the "cold" mark. Hoping for a simple fix, I manually grounded the fuel sending unit wire, and the gas gauge needle moved through its full range of motion. Secondly, I performed the same test with the temperature sending unit wire; the temperature gague needle also went through its full range of motion.

Attempting to troubleshoot the problem further, I:
1. Replaced the temperature sending unit twice--neither unit caused the gague needle to move.
2. Pulled the dash and instruments and checked the printed circuit--someone had run a hot wire directly from the constant voltage regulator to one of the studs on the back of the fuel gague.
3. Replaced the printed circuit with a new reproduction for a car equipped with a tach and gagues.
4. Replaced the constant voltage regulator.

At this point, the owner also requested that I install a new fuel tank. The tank came with a new sending unit, and before installing it, I hooked up the sending unit wire and moved the float on the new unit through its full range of motion. The fuel gauge worked perfectly, and was reading the fuel level correctly, even up to a completely full reading. Thinking that the problem was solved, I reassembled the dash, only to find that I'm still fighting the same problem. The tach works, but the temperature gauge stays on "cold," and the fuel gauge is stuck on 1/4 tank.

I can't understand how manually grounding each gague will cause the needles to move, but none of the gauges will read correctly when hooked up to their respective sending units. Any other ideas, or things that I might've overlooked?
 
Only two things come to mind without thinking too much about it.

The float in the tank is hanging up or has a hole in it and it sinks.

You have a sender for a dummy light on the temp gauge.

Did you ground the fuel sender?
 
I looked over the float pretty closely before I installed it. I could've missed a tiny hole, but I still think the fuel gauge problem is related to whatever's causing the other problems with the dash instruments.

I made sure to get a sending unit for a gauge, not an idiot light.

I did ground the fuel sender when I first started troubleshooting the car. When grounding the fuel sender wire, the fuel gague needle went all the way to full.
 
Remove the cluster and leave the wiring connector plugged in. Check the voltage wire coming in with a voltmeter and let us know what you get.

Also make sure your cluster ground is good. IIRC the metal gauge housing is bolted to the body of the car below the windshield. This connection provides the ground for the gauges, so make sure the body has bare steel. Also make sure the ground screw/bolt is installed on the printed circuit that grounds the printed circuit to the center metal gauge housing. Looking at the back of the cluster it should be in the upper right corner.
 
According to the diagram I got from Midlife's site, the constant voltage regulator wires are red-yellow and violet. With the instruments and tach plugged in and the key on, the red-yellow wire has 10.5 volts, and the violet wire has .04 volts.

I tested the ground wire, and it tested fine. The ground screw/bolt is also installed on the circuit board.
 
It's likely an electrical problem, but are you sure the engine is coming up to a normal operating temperature.... ~180-200.... ?

I thought my temp gauge was inop for a while because the gauge barely, if ever, moved. As it turns out I had installed one of those "failsafe" T-stats that locks in the open position if the engine ever get's too hot/T-stat fails. I messed around with the gauge for what seemed like an eternity before putting a thermometer into the radiator fill point.... getting a reading of 100 degrees and realizing that the coolant was flowing through the system constantly. Once I replaced the T-stat the engine came up to temperature and the gauge worked as advertised.

Since you have 2 gauges with roughly the same problem, it's doubtful that this is the problem, but it's worth isolating just to be sure.
 
Thanks Dave, but we've verified that the engine is reaching operating temperature.

Just a couple things to add to the mix after troubleshooting for a few more hours: With the gague cluster installed, the dash harness connector hooked in, and the key on, I started probing several of the wires to see what kind of voltage they were getting. Several wires literally danced around on the voltmeter, showing anywhere from 8-12 volts and changing very rapidly. This included the fuel sending unit wire, the temperature sending unit wire, and several other wires. Seeing this, I tested the constant voltage regulator. Keeping the key on and everything hooked up, I probed across both pins of the CVR with my multimeter, and found that the voltage there jumped around sporadically also.
 
IIRC that is normal because it isn't a solid state (whatever it's called) CVR. There is a thread on the VMF about building one that is.

I'm trying to remember if the temp and fuel gauge are directly powered from the CVR (Midlife??). If they are, there is a little knob on the CVR that you can turn that will adjust your gauges properly. I'm not the best with 'lectwicity so I'd let Midlife confirm this stuff :ecit
 
"drummingrocks" said:
Thanks Dave, but we've verified that the engine is reaching operating temperature.

Just a couple things to add to the mix after troubleshooting for a few more hours: With the gague cluster installed, the dash harness connector hooked in, and the key on, I started probing several of the wires to see what kind of voltage they were getting. Several wires literally danced around on the voltmeter, showing anywhere from 8-12 volts and changing very rapidly. This included the fuel sending unit wire, the temperature sending unit wire, and several other wires. Seeing this, I tested the constant voltage regulator. Keeping the key on and everything hooked up, I probed across both pins of the CVR with my multimeter, and found that the voltage there jumped around sporadically also.
Click to expand...

Perfectly normal. My bet is that your fuel sending unit is incompatible with your gauges or vice-versa. Both should be set for 73 to 10 ohms.
 
I agree, it does seem like it could be a problem with the reproduction sending unit. I can't understand why both it and the original Ford unit would read 1/4 tank. I can see one unit being bad, but it's really strange that both would be doing the same thing, especially when the temperature gauge isn't working either.

I've been away from the car for a few hours now, and I've been wracking my brain nonstop. I still wonder if the entire assembly isn't searching for a ground. I was thinking that maybe the engine didn't have a ground strap; that would explain why a few of the sensors on the engine wouldn't function correctly, but it still leaves me at a loss about the fuel gauge.

Also, thanks for all of the help! I really appreciate it!
 
Also, can someone walk me through the procedure of testing the fuel sending unit? Specifically, where do I hook my multimeter to when checking for the recommended 73-10 ohms?
 
Yes, the ground strap from engine to firewall is definitely an issue!

How to test the sending unit: pull the fuel sending unit out of the tank, and put a multimeter on ohms across the post that the wire slips on and then the other line on the case. Swing the arm up (10 ohms) and the arm down (73 ohms).
 
Well, the car is semi-fixed, I guess. After two solid days of troubleshooting, I threw in the towel and put the dash back together. I'm not sure what exactly the problem was. Again:

1. Ground both sending unit wires, and both the fuel and temp gagues will read full
2. Use a test light at both sending unit wires and get a pulsing signal
3. Both turn signal indicators in the dash light up correctly
4. All dash lights operate correctly, including the seatbelt warning light


When the owner came and picked up the car, I recommended another local shop, one that specializes in automotive electronics. Now, get this: On the way home, he filled the gas tank up...AND THE GAGUE STARTED READING CORRECTLY! :ecit Well, that's one gague down, and one to go!


Thanks for all of the help, guys! :lol
 
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