Zero10

I am wondering if anybody else has encountered this and been able to resolve it. I searched and found one past thread but there was no solution posted and I haven't been able to find much else when it comes to similar problems.

Here is the issue:
The car is a 1987 RX-7 GX, with 154,000km on the odometer, I have only owned it for a little over 1 week.
Sometimes the speedometer will read 3x my real speed. That is, at 50 km/hr it will read 150km/hr. However, sometimes when I stop and start moving again it will work correctly until the next time I stop, then it may or may not work correctly. If it is over-reading sometimes it will wobble, so when I am going 50 it may swing from ~130-170km/hr, when it does this I know that the next time I stop it will work correctly.

Initially it read far higher than 3x the real speed, I could peg the speedometer before 3500RPM in first gear, but over the last 200km (all I have driven since buying the car) it has gotten better, and now occasionally reads the correct speed.

Does this sound like a grounding problem with the instrument cluster?
I would have suspected it is a mechanical problem with the speedometer, but the fact that I can simply stop and it will work properly makes me think it must be something else.
I would appreciate any suggestions for repairing my speedometer. I don't know anybody local that has a known working one and I would prefer to repair it than to pay for another used one, especially since I don't know the root cause of the issue.

Zero10

Did I do something wrong posting this thread?... only 7 views and no responses

Akagis_white_comet

iTrader: (4)

Hmm, 3x faster than any other FC...are you Char Aznable? Couidn't resist lol

It sounds like an electrical issue in the cluster. Are any of the other gauges affected too? Pretty sure that all of the gauges draw power and ground from the same source. You should download the FSM (check the Second Generation FAQ for links) to doublecheck the circuit diagrams as I may be incorrect about it.

hope that helps

Zero10

Every other gauge appears to work correctly (or at least provides believable numbers and can fool me). As far as I know my car has a cable operated speedometer, but I think there is still an electrical portion of it that moves the needle itself. Another member recommended that I check both ends of the cable for being properly tightened, but I am completely lost on how that may cause it to read 3x the real speed, and why it would misbehave for as long as I am moving, then occasionally correct itself when I stop. I guess I will probably end up pulling the instrument cluster and poking around blindly if it isn't something obvious like a loose speedometer cable

Riggs

something that inconsistent and with that wide of a margin for error has to be electric, as far as i can tell.

Teh 1nternet Perv

I have the same problem sometimes but I have been told it has something to do with the transmission. I haven't really looked into it as it barely ever happens but it still bothers me.

Zero10

I am finally going to get around to looking into this. I had my old project car shell sitting in my garage until monday morning when I finally got it hauled away. Now that I have room to pull the rx-7 inside to get underneath it I am going to check the speedometer cable at both ends this weekend and hopefully it will yield something. I will check all grounds that I find along the way as well. I'll let you guys know what I find out

vts4fc

My speedo also seems to be off in my 87 base. Not 3 times the actual speed, but at higher speeds it just seems off. Speedo said 90, definitely didn't feel like 90, more like 70ish. Keeping up with other cars on the interstate that were definitely not going 90. It only seems off above 60 though. Any ideas?

Zero10

I tried to get to it this past weekend unfortunately my exhaust install and shifter bushing swap ran long and I didn't have time to tear the cluster out.

I drove the car to work today and noticed something interesting. The mis-reading of the speedometer depends on the temperature of the inside of the car. When it is cold out the speedometer will wrap all the way around before 3500RPM in 1st gear (below 5*C) between 5-10*C it will read about 3x the real speed and when it is nice and warm, above 10*C it reads the right speed at least half the time.

Also, the odometer is counting properly so I don't think the problem could possibly be on the transmission end. Hopefully I'll get to the cluster in the next week or so and I'll try to fill you guys in on what I find.

Zero10

I need to replace the seals around the speedometer cable where it enters the transmission, and both axle seals on the rear diff and I am wondering if they can be removed without losing much (if any) gear / transmission oil?

I just picked this car up and both the transmission and diff are very low on oil due to these leaks. I picked up a couple bottles of redline mt-90 and heavy shockproof to put in the transmission and diff, however I can't get new seals for at least a week or two. Rather than continue driving this car with low diff and transmission oil I'd like to change the fluids now, but I only have roughly 0.5L of each fluid to spare to top up with after changing them. Am I going to lose more than this changing the seals? If so I'll have to just top them up with something else for now and wait, but I'd prefer to just get the fluid change over with if it is possible.

Also, what seals are required for the speedometer cable? I see there is an O ring and a seal, do I need them both?

Aaron Cake

iTrader: (1)

What year is your car?

Since the speedometer drive gear is on the bottom, loads of oil will come out.

As for the diff seals, it sort of depends on the angle of the car and how overfilled the diff is.

Buy why does it matter? Drain through a cloth into a bucket, do the work, then refill and top off as necessary. 5L is quite enough though as the transmission takes about 2.5 quarts and the diff about 2 (depending on year..1st gens take more).

Zero10

The car is an 87 GX, it only matters because I was hoping to get the old fluids out of there quite soon and I can't get the new seals for a couple of weeks. It would suck to change the diff and transmission oil then have it all leak out while replacing these seals. I guess I'll just have to clean the area really well and re-capture the fluid.

Also, I would only have 0.5L of spare fluid for each the transmission and diff, not 5L.


Regarding the speedometer cable question, do I need both the seal and the O-ring?

Sgtblue

iTrader: (16)

FWIW, this is a "how-to" on changing the differential side seals on an FD. https://www.rx7club.com/showthread.p...ighlight=seals
I'm not that familiar with the FC, so I don't know how much will translate usefully. But keeping the car level, very little gear oil was lost.

Zero10

I finally got around to looking into this, here is what I found:
The speedometer cable at the transmission end was leaking badly, both the O-ring and the internal seal had failed. I replaced both of these.
Checking the other end of the cable showed that a significant amount of grease had escaped from the drive mechanism and was dribbling down the back of the cluster.
Opening the cluster and removing the speedometer unit showed that quite a bit of grease had flung around inside of it, that it had dripped down onto the light tunnels for the front lighting and that it had leaked inside of the rotating assembly (the copper front plate and magnetic back plate).
When I spun the back plate by hand it produced a noticeable amount of drag on the copper front plate because of the grease that had leaked in between the 2 assemblies. I separated them and cleaned everything up as best as I can, but I won't be able to install the cluster until at least tonight.

That said, I have NO IDEA if I have even done the right things to fix this issue. Knowing that the alternative was to replace the cluster I figured this is worth a shot, but I really know almost nothing about the internals of how this speedometer works.

As far as I can tell there is a magnet in the back plate that triggers a reed switch mounted behind it, but that reed switch seems to control something outside of the odometer assembly.

Also, I noticed a third digits assembly mounted above the odometer. It has a lobed wheel on the end of it that actuates a switch which appears to be wired to something outside of the speedometer assembly.

How is the needle influenced? The copper plate doesn't seem to have any windings or other components to it, does the magnet just induce a current within the disc, or have I lost some crucial component here?

Also, what does the magnetic reed switch and lobed wheel control?

Zero10

It would appear there isn't a lot of knowledge about the speedometer/odometer internals around here to be found, so I'll share what I have discovered:

1) The 3rd set of counter wheels is the 20,000 mile counter. Apparently it is completely useless after 20k miles.
2) The induced current in the copper disc is enough for the speedometer to work properly.
3) The grease sitting in the other plate/magnet assembly that the copper plate slides into WAS the problem. After cleaning it out the speedometer works perfectly every time.
The magnetic reed switch controls something completely different. Unrelated to the speedometer, but I have no idea what

Zero10

I have completed the job and here are some of the things I learned:
If your differential is whining, check it IMMEDIATELY. I drove mine 200km since I bought it, the diff has been whining loudly the entire time. On draining the diff I found one tooth from something sheared off and stuck to the magnet in the drain plug, a massive amount of rough sand-sized metal flakes suspended in the oil, and only about 1.5 CUPS of oil left in the diff, not 1.5L, 1.5 cups, and it was more silver than golden.

Removing the axle seals is incredibly easy if you use a flathead screwdriver, just be very careful not to scratch the housing. I put a nick in the housing and I had to sand it with 800 grit sandpaper to get it nice and smooth again. I took the lip off and applied a bit of black gasket sealer around the nicked area to make sure it stays sealed.

When you pry the stub axles out of the diff it is easy to bend the little flange that meets the oil seal, make sure you bend this back before pounding the stub axles back in.

The speedometer seal is on the bottom of the transmission and it will empty almost all of the oil out of the transmission when you remove the cable.

The inner speedometer seal comes out very easy using a 1.5" #4 sheet metal screw. If you place it perfectly centered on the seal it won't nick the inner or outer edges and will pull the seal straight out. It puts a tiny nick in the lower flat surface but as that isn't a sealing surface it doesn't matter at all. My speedometer seal was so worn gear oil had creeped up the cable and was pouring down the back of the instrument cluster.

The inner seal seals the cable to the housing, if this fails some oil may leak externally but most likely it will leak up the cable and drain out at the cluster. The O-ring seals the housing to the transmission, if this fails it will leak oil externally and on the outside of the cable at the transmission. Both of mine were completely shot.

The fill plug on an open diff is 23mm, not 24mm, and with the right socket it comes off without rounding. Use the right socket.

Redline's bottles can take a surprising amount of pressure. I was using the hose-through-the-lid method of filling the diff and transmission where you have to pressurize the bottle. The guide I read said not to exceed 2psi on the bottle in case it bursts. This worked okay for the mt-90 (although it was a little slow), but when it came to the heavy shockproof it just barely crawled up the hose. After 20 min I had emptied almost none of it into the diff. Out of frustration I started increasing the pressure, I eased it up to 10psi, it started moving slowly, then I tried to reposition myself and accidentally grabbed the trigger on my blow gun, it ran the bottle right up 30psi before I could react. The bottle puffed up like a balloon but it didn't explode, so I finished filling the diff at 30psi. It still took almost 45 minutes to push ~1.5L of heavy shockproof up the 3/8" hose at 30psi though. This would probably be easier on a hot day, it was quite cool in my garage the night I started working on this, the fluid was only around 5*C.


Even with all the missing metal in the diff redline's heavy shockproof quieted it right down. It is now completely silent. I guess I'll just run it for a while and see what happens. This stuff is thick like tar so I am not surprised.

Redline's mt-90 made shifting extremely smooth and easy in my transmission. The transmission previously ground going into every gear and the synchro's felt completely worn out. It turns out the PO put ATF in the transmission, changing that to mt-90 made the transmission feel like new.

gracer7-rx7

iTrader: (16)

merged your two threads. please don't create new threads when you one already out there. thanks.