Just got the car running again for the first time in 14 years, oil pressure gauge was not working (I think this has something to do with the ground that went to the clutch master cyl in the engine bay to the sensor?) , everything else seemed to be functioning. Was not able to test speedometer.

As part of ongoing repair process, I removed the dash to get access to the heater core for a repair.

Putting everything back in, plugged everything back in, power car on, and the coil spring that returns the speedometer gauge gets red hot, melted the gauge face, and several of the odometer numbers. Not super worried about that, I do have a second dash cluster for parts. I removed its face in case, and it is doing same thing, so I feel if I can fix it on the ruined one, it will work on the one that is still good. For both now, power on, the spring starts to glow red hot, so powered off.

Whatever is causing it is something between the white plug and the speedometer cable being plugged into the back.

Previous owner had some vampire clips off of a wire (yellow with blue stripe wire) that just led to nowhere. When I removed the dashboard there was also a grounding wire (gray in color) attached to one of the bolts that holds the steering column up, but it also terminated to nothing. Not sure if due to the age of the vhecle that something broke and I am missing something somewhere.

Not sure where to start looking at this. The wiring harness says the yellow with blue stripe goes to the coils if I am reading it correct? (scribe pointing at what previous owner had a wire that just ended not connected to anything)

I am not sure what the (me) and (f) mean after the wire colors on this image either

 

>>> I am not sure what the (me) and (f) mean after the wire colors on this image either

For the S5, (F) designates Front harness and (IC) designates Instrument Cluster harness. For your S4, (f) likely designates Front harness and (me) likely designates Meter harness.

There's a lot of electronics on your instrument cluster printed circuit board. Probably an electronic component just shorted out an fried your board. It might have also been caused by a corroded high resistance connection between harness and meter. After sitting for 14+ years. it's really hard to say.

 

i think that is correct, ME = Meter Harness, and F is the front harness. the YL (Yellow/Blue) wire is the tach signal,

power is at the top (always) and its the BY wire, and the ground (always at the bottom) is the B[me} next to your pointer, i think.

i would think fire = there is current that is not going to ground, and is instead going somewhere, um flammable?

 

Some clarification:

The cluster worked fine before I removed the dash, other then the oil pressure gauge, which I think was because I did not have the ground near the clutch master connected

smoke and fire was due to the burning plastic

What I am trying to figure out, after the dash was removed, I had a wire drop to the floor that was using the steering column as a ground point, but it is about 6 inches long, and if it was connected to something, I cannot find what. I feel that this is the reason for the cluster issues now, but i cannot find what it went to.

The only part that is getting red hot is the clock spring on the speedometer when power is on, and the speed cable is connected, as if it is part of the loop where electricity is flowing

 

>>> What I am trying to figure out, after the dash was removed, I had a wire drop to the floor that was using the steering column as a ground point, but it is about 6 inches long, and if it was connected to something, I cannot find what. I feel that this is the reason for the cluster issues now, but i cannot find what it went to.

This doesn't help much unless you provide either a photo and/or color the wire in question.

 

Check connectors C-02 and C-03. Is that ground present? I agree a picture of the fallen off ground point would be helpful.

 

Pictures attached:


Black Plug

White Plug

Previous owner green wire attached to Yello/Blue stripe wire, was not connected to anything

Ground wire that fell out of dash, was on steering column bolt, no idea where it went to.



clock spring that gets red hot, damage to plastic numbers

damage to gauge face

damage to gauge face

 

Yikes!

Would there have been any way that stray green wire could have contacted one of the screws on the back of the circuit board that held the gauge in? The only things in the speedo that have current are the lightbulb, the Reed switch for cruise, and the mileage sensor/switch. For current to flow through that spring like that, something would have to short 12v to the gauge body, and then the speedo cable would ground out in the transmission. Without the cable to ground, current would have to go somewhere else other than the spring. The guages really only fasten to the circuit board and plastic housing, so it would be really weird for a component failure to do something like that as they're all isolated in their own loops for the most part. Can you find any weird continuity with the gauge fastened to the circuit board?

 

No, I only reattached that wire for this picture to show how it was, the bare wire is not grounded to anything

 

is the battery negative cable routed correctly and in ok shape? how about the little ground on the back of the engine to the firewall?

 

You're absolutely correct. The yellow/blue (Y/L) wire runs from the trailing coils to the tach. I'm not sure why the previous owner butchered that wire. Maybe to install another tach? Was your tach working?

If you look at the back of the gauge cluster, you will see several Phillips screws (photo attached). Some of those screws are at source voltage, i.e., 12 VDC. I could be that one of those bare wires you show touched one of the screws and shorted something out. Just guessing here.




>>> "...oil pressure gauge was not working (I think this has something to do with the ground that went to the clutch master cyl in the engine bay to the sensor?)"

That's actually a condenser (or capacitor) p/n FB01-66-991B. It's NLA from Mazda. It's normally an open circuit for a DC signal. It's there to smooth out any rapidly varying voltage. Usually, capacitors fail open circuit but sometimes they will fail closed circuit. If you suspect that's the problem, then I would suggest disconnecting it and see if the gauge responds. IIRC, Rockauto sells similar condensers for the FD, but they have two wires coming off them. So, you would need to modify it to fit your FC.

 

After taking a second look, the FD condenser looks like it might be a pretty good replacement for the FC condenser (https://www.rockauto.com/en/moreinfo...k6CQTe7fNpGtQD).

 

This is what the rear of the dash cluster looks like, I have been trying to find any pictures to make sure that all grounds match others

 

if power is coming through the speedo cable, that is suggesting a bad ground at the engine

 

Alright, so wanted to give an update on everything I have done so far.

Followed the grounding directions at this link: https://www.aaroncake.net/rx-7/grounding.htm

Looks like it fixed the majority of the issues I was having when it comes to the dash, got all except one gauge working properly now, the only problem I am having is the oil pressure gauge is pegging to max, which this link is saying is bad, only realized what was happening when the first one went up in smoke as well. https://www.rx7club.com/2nd-generati...ed-out-845554/

Unplugged from sensor, it drops to 0 as expected, link states that a bad ground to the clutch master will fix it, but mine does not have this, so trying to figure out where this was supposed to be and run now.

If anyone has pictures of this ground around the clutch master, would love to see it

 

so the oil pressure sender wire is usually ok, it goes from the little bundle to the sender. there is another wire which goes to the capacitor and is usually bolted to the slave cylinder, and i've seen a couple of these where the wire touches the engine/trans and shorts out, which causes the gauge to peg. have a look at the wire (i think its just black) and route or insulate it away from stuff that is metal. there is a connector nearby too, white 1 pin

 

Alright, so closure for this thread. Got another oil pressure sensor, seems the previous one was bad. Now just have to figure out its output and make the gauge match, as am sure that the needles are off.

Am using this link, and this line to calibrate the sensor to the needle: https://grannys.tripod.com/rx7info.html