Hey all,

I've got a '79 SA, all stock. The temp and fuel gauges read low, dash lighting also seems low (been this way since I bought it last summer). If I ground the wire that goes to the temp sensor the gauge will go all the way to the top as it should. With the ignition on, that wire is at about 7v, measured from the temp sensor wire to the battery ground terminal.. From the diagram in my Haynes manual that voltage should be the output of the regulator in the cluster. Some research shows that 9v is a very common voltage for this circuitry in other cars. Does anyone know what the voltage should be? I suspect the regulator is bad and putting out a low voltage but I don't want to tear it apart without knowing what to replace it with. Thanks!

 

I would also be interested/appreciative if anyone could check their voltage on the temp sensor wire with the ignition on to see how it compares to my 7v reading. Maybe I'm misinterpreting the (very simplified) diagram. Thanks!

Further research shows the p/n of the regulator may be 8871-55-482, but the mighty Google comes up empty on that... :-(

 

I think 7V is what the cluster voltage regulator is designed to put out.

See the photo in this post, it shows 7V etched into the cluster's PCB:

https://www.rx7club.com/1st-gen-gene.../#post12600162

 

It appears that link is to a 1985 FB dash, much different than the picture in my 1979 workshop manual. I'm hoping someone with an SA and a voltmeter can measure the voltage on their temp sensor wire for comparison... (Sorry about the skew, don't yet have decent image software on my "new" PC...) I'm guessing the big metal box is the voltage regulator.

 

If you can identify this temp sensor wire and maybe take a couple pictures of how you are accomplishing your test, I'll see what I can do to copy and verify your voltage inquiry.

 

Thanks! I'm using a digital multimeter to test. With the ignition on I'm measuring from the yellow wire that connects to the temp sensor (below the oil filter on the engine block) to the battery ground. I get about 7v. I've attached photos of the testing.


Negative lead of DMM to battery ground

Positive lead of DMM to connector

The voltage i see

Wire on the sensor

 

I have a 79 SA dash and the regulator puts out 7V and the etching on the PCB is 7V.
I've run a few quick tests of the fuel and temperature gauges with the following results. Note that the gauges are thermal and slow to respond so I may not have waited long enough for them to settle. I also only have a 100 ohm potentiometer so I could not get the temperature gauge down to the lower peg. I was interested in the fuel gauge reading as I now have a Haltech ECU reading the stock fuel sender and wanted to fine tune the fuel level displayed on it. This validated the research I found as to the fuel sender resistance range.

Note that grounding the temperature sender wire will overrange the temperature gauge so that is only a quick test that validates the wiring.

Temperature:
55 ohm gauge resistance measured
100% (High peg) - 14 ohm simulated sensor input (1.4V at gauge terminal)
1 needle width up into the white band - 100 ohm sensor simulated input (4.5 v at gauge terminal)

Fuel
55 ohm gauge resistance measured
100% - 6.6 ohm simulated sensor input (0.74V at gauge terminal)
50% - 31 ohm simulated sensor input (2.5V at gauge terminal)
0% - 85 ohm simulated sensor input (4.27V at gauge terminal)

 

I had a chance to perform my own test today. My meter display was bouncing around a lot and I'm not confident I was getting an accurate reading. It did hold steady a couple times at 9-10 volts.

 

Thanks @Banzai , I'll be checking some of the other tests suggested in another thread but that's a great data point (of course, I've convinced myself that's the answer but I want to be very sure, only want to try to fix it once).

 

Thanks @anderml1955 I've got some other test points like that from another thread and I'll be looking at it hopefully this week.

 

Here is the back of my gauge cluster. I'd recommend pulling yours and loosen, clean and retighten all the nuts and screws. After 40+ years they can get corroded resulting in high resistance connections.

 

The other thing to check are the ground connections on the other end of the gauge cluster plugs.

 

All signs seem to point to 7V being correct for the SA cluster's voltage regulator. The symptoms you have are low fuel gauge, suspected low temp gauge, and dim dash lights. While you can quantify the fuel gauge issue, the temp and dash lights are more subjective. So if we set aside the idea that the voltage regulator is bad, then it might be just the fuel gauge. That was my experience in my FB. Here's a thread where I have comprehensively tested the fuel sender and the gauge readings, and ended up calibrating the fuel gauge. Perhaps this might help your situation:

https://www.rx7club.com/1st-generati.../#post12115915

 

None of mine work. Warning lights and turn signals work though.