RPM problem
 

Hey guys, I'm having a problem with my rpm gauge. It's a 95 rx7, and when the car is first started up the gauge jumps all over the place, from 850 - 6000 rpm's. It will continue to do that for about 30 mins, then it'll settle down and act normal. I'm assuming it's an electrical problem, but I was just wondering if anyone else has had a problem like this. If anyone has any ideas or thoughts please let me know, it would be greatly appreciated.

Weezl

Some folks have run across this being poor quality solder joints on the backside of the instrument cluster. I would try pulling the instrument cluster and checking that all the joints are shiney silver and have wire connected to them (check this by wiggling the tach and see if there seems to be any wire connections from the backside of the tach that aren't "captured" in solder).

Happy hunting!

Hi spurvo, thanks for your help. I just bought the car three months ago, and ofcourse THIS had to happen. I've visited this site before, and I think it's awesome. But, I was talking to another guy who said it might be a "grounding" problem....would that tie into your suggestion or is "grounding" totally different.
Thanks again.

Well, the "grounding" issue is vast with respect to this car. My suggestions are specific to the backside of the gauge cluster. The typical refferent of grounding issues has to deal with the insanely poor level of ground wires and straps used throughout the car. This seems to effect the ability for the ECU to track changes in the various sensors correctly, leading to performance issues. One issue that has been occaisonally resolved through improved grounding is the so called 3K hesitation, wherein the car has a serious stutter at 3000 rpm or so under moderate acceleration conditions. This has actually been tracked to the timing of the secondary and primary injectors during transition between them (occurs at 3000 rpm). Improved grounding may simply be helping the ECU control the injectors better, or provide more current for more positive control, or the alignment of the planets! I won't say grounding ISN'T your issue, but I'd be surprised, UNLESS it is the sheilding on the wires for the ignition coils? Really stretching here, though. By the way, one neat thing you can try is to hook a separate tach to the signal at the diagnostic connector (engine bay, near the battery). You want to measure between the "IG-" terminal and ground, I believe. See if it's jumping around the same as the dash tach. If so, then it's not the cluster unit. If the diagnostic output is NOT jumping around, then I'd totally suspect the cluster gauge. At that point it's either the gauge itself, the connections like I mentioned above, or possibly the speedo cpu, which is where the signal for the tach comes into from the ECU (yellow wire with blue strip, I believe).

Hope any of this helps!

Good advice spurvo right on the money..... I had a problem much like yours the tach jumped to no particular rythm it had a tendancy to just go about doing its own thing on occasion it would jump all the way too 9000 rpm and then settle and start its little dance whenever it wanted..... i took the guage cluster out if you have the mazda repair manual it makes it much easier and finally tracked the problem to a loose solder pin directly off the tach to the circuit board heated it up soldered it back no problem at all.

****if you plan on taking the guage cluster out I would recommend also removing the steering wheel it makes this job much easier.

I'm one of those whose tach suffered from bad solder joints. I successfully repaired mine and documented in a DIY repair procedure. It’s in pdf format and the file is about 700K because of all the pictures.

Send me a PM or e-mail if you'd like a copy.

Thank you so much guys, I'll give it a try. Just one more quick question, how did you guys become so informed with this stuff. Did you just pick up a Mazda repair manual, or do you guys work in garages. I'm new to the whole "rotary" world, and I'm eager to learn.
Thanks again for taking the time and for all your info.

Started with reading the forum incessantly. Then reading the FSM. Then getting the parts lists to see all the parts the FSM DOESN"T tell you about. Then tearing apart the car and putting it back together.

But the forum, especially some of the older posts, REALLY have a lot of info. Almost inevitably, the problem you are having has been seen and dealt with before, just gotta find where.

Lastly, cars are cars. Once you learn a system, almost all other cars are just variations on a theme. Old Chrystler's have their disc brake pistons pushed in with a C-clamp, ours you have to screw in. But you still have to get the piston in in order to get the caliper around the disc with new shoes, and you still have to bleed the system, and it's still best to do the "down".. "up" with a friend at the pedal, and.... just a fer instance...