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My 85 Mazda RX7 GSL Temperature/fuel gauges are not working - my car sat up for a few years & was not run much & they worked perfectly prior to this. The gas gauge moves from empty to just above the low on fuel mark. Any ideas? Thank you.
For sure check the sensors. Also, common to both of those and only those gauges is the voltage regulator in the dash cluster. Here's a clip form the wiring diagram for the inst cluster, red arrow points to what must be the voltage reg circuit. It's from this document from via Foxed.ca and can be found on page 18.
Thanks guys! Is the voltage regulator in the dash cluster the same as the voltage regulator that is on the left fender & if so I cannot seem to find one? Any ideas wher I may be able to get one? If not, I assume I need to take off the dash & find the voltage regulator you are referring too. Where would I find this for purchase? Thanks.
Do make sure you test the sensors first. It's easier than getting at and testing the voltage reg for the instrument cluster.
The voltage regulator you mentioned is not the one for the fuel and temp gauges. The one I referenced in the diagram above is a part of the instrument cluster.
Sensor tests:
At that same website, foxed.ca there is a wealth of manuals. Get your factory service manual there and get familiar with the fuel sender and temp sensor. I only have experience with the fuel sender in my carbed 12A 83 and I've never needed to test my temp sensor so my guidAnce is based on that. 85 is probably similar.
The fuel sender works with a float that moves a potentiometer, and varies the resistance between 0 to about 110Ω. 0Ω is full and ~110Ω is empty. As the resistance drops, the little heating coils in the fuel gauge get warmer and the bimetallic strip gets deflected proportionally, moving the fuel indicator needle. So if your fuel sender is always high ohms or open circuit, the needle would never move. TL;DR if you don't want to pull the sender from the tank then just check the resistance of the sender with an ohm meter when you have just filled up and with a low tank and look for changing resistance. You could also fill the tank to 1/3 to 1/2, put the ohm meter on the sender and then try to slosh the fuel around by pushing the car and looking for a change in resistance.
You can also remove the sender from the tank and get really good results that way. Just be careful of the delicate screws that hold the tank sender assembly onto the tank. They are easily stripped. You can get at the sender by jacking up and removing the rear driver's side wheel.
I have a ton of relevant info in a thread I posted where my sender was working and the fuel gauge was misadjusted. It's not the same as your situation, but a lot of it applies. There are also a lot of threads for testing the fuel sender.
For the temp sensor, this forum probably has a lot of info to help testing that. I have no direct experience with it.
If after testing the sensors you get one or both gauges to work properly, then your instrument cluster voltage regulator is working.
Voltage regulator:
*** IMPORTANT: If you test on a bench with a power supply you are effectively bypassing the protective fuses when you supply voltage directly to the cluster. You could easily destroy a part if you put voltage in the wrong place. If you are not confident with electronics, testing, and soldering, then you should get help from a friend. ******
The instrument cluster voltage regulator takes the cars varying voltage of ~11 to to 13.5 VDC and outputs a solid 7V (at least it's 7V on my 83 according to the FSM, 85 is prob same but you would need to check). If the voltage regulator is 0V or too low you would not have enough voltage to excite the gauge's warming coils. If the VR is too high/intermittent then you would get false and/or varying readings. To test, you would need to remove the cluster, supply ~13.5 DC to the cluster. You would then test the voltage at the VR output and look for that 7V and solid DC voltage (not sure if the gauges run off full 12V or something less). Vary the input to the cluster from ~ 12 to 13.5 and make sure the VR output stays solid at 7V.
The thread above shows how to bench test an 83 cluster, and the 85 is prob similar but I am sure it's not exactly the same, so you would need to make sure of the wiring diagram and be careful where you apply voltage.
WRT replacing the VR, I don't see a part listed anywhere, but then I have not searched more than a quick Google. For interesting info about repairing it, here are some threads for similar situations on other older cars (like really old).
02-23-18, 09:51 PM
