Instrument Voltage Regulator Question

[Randalla]

I have a 1972 240Z that I'm having an issue with. This past year on a club run, I lost both the temperature and fuel gauges. Prior to losing them I noticed the readings were way off, and swinging slowly from one end to the other before both stopped working all together. I had a similar issue on my Datsun Roadster that I traced down to a faulty instrument voltage regulator, which steps voltage down from 12 volts to about 9 volts for those two functions. I'm wondering if that may be the same issue on my 240Z. I've seen photos of a box on the back side of a 1973 240Z that someone told me provided the same function. If true I'm wondering if that was only  on 1973 models, or earlier models as well. I'll need to find one or find a work around. Can anyone help?

[Yarb]

@SteveJ

[SteveJ]

I can't tell you anything about a box on the side as there was only one part number listed in the catalog for the 240Z, but there are other threads where @Captain Obvious and I have talked about the voltage regulator on the meters.

 

[Randalla]

Steve,

Thank you for weighing in on this problem. I realized this morning that I left out an important part of this issue. The box I mentioned is supposedly attached to the back of the speedometer. I neglected to mention where it was located. I haven't stood on my head yet to look under the dash to see if I can spot the box. Maybe someone on the site has a speedo in their parts stash they could snap a picture of. If this is indeed the culprit the next question is whether there is a replacement, or a work around. On my Roadster I was able to open up a small metal box attached to the steering column and replace the guts with a resistor and wire it in place. 

[Captain Obvious]

The little box on the back of the speedo does not contain the voltage regulator for the gauges. That box contains a "speed switch" that disables the throttle opener system below a certain speed (FSM says 13 mph). This is done with the help of a small set of contacts built into the speedometer that closes at low speed. Basically... They don't want the opener system to be actuated as you are coasting slowly to a stop.

So the voltage regulators for each gauge are built into each gauge. In other words, the temp/oil gauge has it's own built in regulator, and the fuel gauge has a different regulator built into it.

But getting to the root of your question.... If you lost both temp and fuel at the same time, it's probably something other than the regulators both failing at the same time. Are you sure you didn't lose temp and OIL (not fuel) at the same time?

[Randalla]

Just checked all the gauges again to verify. Fuel swings from empty to full (right) at start up and stays there till car is shut off (regardless of how much gas is in the tank). Temperature does not move from the full cold position (left). Everything else work as it should (ammeter, oil pressure, etc.). Thank you for your thoughts Captain Obvious. 

[Randalla]

I'm told that the two gauges share the same ground at the cigarette lighter. Could that possibly be the issue? 

[SteveJ]

4 minutes ago, Randalla said:

I'm told that the two gauges share the same ground at the cigarette lighter. Could that possibly be the issue? 

That would be easy to check. Plug a 12VDC accessory into the cigarette lighter. If it works, then it's unlikely to be a grounding issue.

[Randalla]

Why didn't I think of that?🙂 Thanks for the suggestion and quick check.

[Captain Obvious]

Those two gauges share the same ground connection, but so do a whole bunch of other things. I don't think that's going to be the issue.

I'm having a hard time coming up with a common denominator failure between the fuel and temp gauges that would cause the symptoms you're experiencing.

So if the accessory in the lighter doesn't yield any insights, then try this... Turn the key to ON and see what the gauges do. If the temp gauge stays at "cold", go out to the engine compartment, pull the single wire off the temperature sending unit (should be yellow/white) and connect that wire directly to engine ground. That should run the TEMP gauge all the way to HOT.

 

[Randalla]

Well, just checked and I have power at the lighter so at least it is grounded. Need to check further to see if there's a wire from the gauges to that same ground that may not be grounding (broken or disconnected wire). Would have thought the gauges would be grounded through the part of the dash structure itself, but maybe not.

[Yarb]

As old as these cars are, Have you ruled out just changing the gauge itself?