Hello Gurus! I am having the dreaded Tachometer issue that so many people complain about. Its broken more than working, so id say it intermittently works haha. The gauge will sit at 0 most of the time, and then work properly maybe about 10% of the time, which really sucks. Speedometer and all other gauges work properly. (I have a 1992 JDM FD3S - most circuits and cluster board appear to be the same as USDM) Ive been having this problem for a couple years now, but I havent had time to really look at it in depth till now. I have done a bunch and searches and have read a lot of threads on this issue. Ive mainly worked off of this thread: https://www.rx7club.com/3rd-generati...board-1118446/ Im new to fixing circuit boards and electronic repairs so the main reason im reaching out is to confirm I am on the right path before going any further as I have already done a fair bit of trouble shooting. Here is what I have done so far:

1) - checked for continuity from the tach signal wire from the ecm to the cluster (yellow/blue wire) it was good (almost 0 ohms)
2) - took apart the cluster and checked for any obvious faults, found a bad C3 capacitor on the speedo board. I then replaced C1, C2, and C4 as they were looking not great as well. I cleaned up the leaked electrolyte and re-soldered the zener diodes ZD3, ZD4, ZD5, ZD7 just to be on the safe side as the solder didnt look so great.
3) - with the hood off of the cluster (which I broke removing anyways due to the fragile sun soaked plastic) I was able to test the power and ground from the screws that go into the tachometer following the first picture in this thread: https://www.rx7club.com/interior-ext...1055763/page2/ (I have a good quality MAC led test light) so hooking up the test light between the 12V and ground screws it showed good power/ground. With the car running, I was then able to test from the 12V screw to the signal circuit screw on the back of the tach, the led in the test light flashes. I was able to test this when the tach was working and not working, with the same result. (note: as I increased the rpms when the tach was both working and faulted, the led reacted in the same manor, with the flashing increasing as rpm's were raised)

So heres my main question. Please correct me if im wrong, but from what I've read and understood about the tach system, is that It gets the signal through the speedometer circuit board which processes the information through a chip and sends it out to the tach through the ribbon circuit board on the back of the cluster. Since I have a good power/ground there and the test light led flashes when the car is running and increases in intensity as rpms are raised, I want to say the ignition system/ecm/wiring/speedo circuit board and everything up till the actual tach in the cluster are in good shape as there is clearly an rpm signal/power/ground being sent up to the points I am testing on the back of the cluster (same results when the tach is working and not working). Seeing as how those 3 screws go directly into the tachometer, I want to say that the tachometer/board itself is at fault, possibly a bad solder joint on the tach circuit board somewhere? are there any obvious things i could be missing? Is my thought process correct in concluding the tach itself is the only component that could be at fault from the testing that was performed? I just want to be sure before I rip apart the cluster again and go after the tach itself, as I am very hesitant to take off the needle as it looks like quite a tedious job.

Any information/tips would be much appreciated as I just want my tach to work again. Thank you in advance

 

You might be over complicating this. Many electronic issues in these cars are due to bad solder joints and don't require knowing how the circuit functions to correct.

I used this guide a while back to correct my tach and it's been fine since. Taking the needle off isn't so bad if you follow this guide... https://drive.google.com/file/d/0Bzh...ew?usp=sharing

Oh, and don't use pliers or forceps to grip the needle. All you need is to grip with fingertips on either side of the hub.

 

PHEn,

It's good to hear that you were able to repair some of the components from your speedo board. A couple of other members had positive results with resoldering the IC chip on the tach. The link in alexdimen's post will help you address that issue.

 

Thanks alexdimen and Gen2n3, i'll try it out and see what happens. Fingers crossed

 

What components does the tachometer rely on for signal? I heard the CAS and trailing coil. Want to verify those before pulling the cluster.

 

Narfle, I was thinking the same. I wanted to verify if the signal was actually getting to the cluster or not before yanking it out again. Thats why i tested the power/ground/signal at the back of the cluster. There is a yellow/blue wire coming from the ECU pin 2B (https://www.innovatemotorsports.com/...Mazda_RX-7.php). If you want to tap into that wire and see if its getting signal (use an led with a resistor in line, or an led test lamp, and hook the other side to battery positive) then everything up to the ECU and the ECU itself should be fine. I havent done this myself and cannot verify this would work but it seems like a good place to start? someone please correct me if im wrong.

(also, have not had a chance to pull mine back out to fix it yet, i'll update with any findings i have when i get around to actually doing it)

 

Narfle & PHEn,

The CAS & Trailing Coil is a configuration used to setup an external tach.

According to the schematics in the WDM (wiring diagram manual), the ECU sends a TAM signal from ECU Pin 2B to the Instrument Cluster Flex Print via connector C1-01-3 Series, Pin 3F. This signal is then sent to the Speedo Board at CON1 Pin 10. This TAM signal must be conditioned by the speedo board at IC4. IC4 will then output a Tach Signal to the Tachometer. In order to better understand the Instrument Cluster flex print connection, refer to this thread: Instrument Cluster Thread Post 17

Ultimately, re-flowing the solder to IC1 on the tachometer should restore a bouncing tach needle. Others had great success (and sheer luck on one) with this repair. You could also read up on a similar repair to the tach board on Troubleshooting the FD Speedo Pg4 thread.

 

Do you happen to have a pic of IC1? I'm going to try to re-flow the solder on mine, have a bouncing/dead tach every once in a while. It works again after I smack my gauge hood...(trying not to smash it into pieces).

 

Billy,

The info you seek can be found in the link in Post 7 (the same one you quoted me on). Reflowing the solder should fix your problem. This advice helped another member fix his bouncing tach a couple of days ago.

Please report back on your repair. If you have a problem then take pictures and don't hesitate to post an update.

 

Ok, I found IC1. I got to the point where I ran out of time, and had to put everything back together (this car can't have down time since I'm sharing the driveway with another car). I was afraid to pull the needle it without breaking it. So I just pull the needle? I've uploaded pics of the instrument cluster and IC1 in case anyone needs to see it for future reference.

 

Thread and reply 2 came just in time as I was tackling this exact problem. Had a tachometer that would bounce / stay at 0 / occasionally work. All my other gauges and odometer were working. Following the posted document and Gen2n3's advice, IC1 was resoldered, and the tachometer works like a charm.

Had some drama with my fuel gauge needle, that can just be pushed or pulled to change its seating, in case anyone else misaligns theirs like I did.

@Billy7 You just pull it. I'm in the same boat as you, the tach needle was flexing pretty badly but it slid out of the rod before I got to breaking it, just took some consistent effort. As Alexdimen said, you can just use your fingers, it slides right out.
@Gen2n3 Was very helpful in this process, thanks!

 

Thanks, I will attempt this the next time I have a few hours to myself... Might as well change to LED bulbs on the way.

 

Ok, before I continue, please have a look at this. Am I reflowing the solder on the top of the IC or bottom? Does everything else look good?


I'm documenting this as a YouTube video for future reference...

 

Billy,

You always work from the backside of the PCB. Search for my general soldering thread for tips and tricks that will aid in your repair. Use flux (for electronics and not plumbing) and solder wick to help draw solder away from the joint. You may not have to remove all of the solder. Once you get most of it out then you can add new solder to each joint. Again, make sure you use flux to aid in the flow of solder to each joint. The rest of the board appears to look good.


Remove/replace solder from the rear of the PCB. The highlighted sections are the pins to IC1.

 

Wow I didn't realize so many people were having the same issue, thanks for everyone's help, especially Gen2n3! Ive managed to take apart the tach and take the needle off, I found that one of the C1's legs had corrosion on it looked in bad shape like it had lost the electrolyte, so i figured that was the problem. I replaced it and put it all back together, sadly I get the same result, dead tach. So its back to the drawing board for me. Gen2n3, I've noticed that everyone else's C5 cap looks green where as mine is brown in color, do you think mine overheated? here are some pics (sorry for the terrible quality, I only had my cell on me, I'll try to find a better camera if you cant tell much from these pics). Does anything else look bad on the circuit board? I guess my next step would be to replace C5 and re-flow the solder on IC1. I was just trying to avoid touching IC1 because it seems quite delicate.

 

PHEn,

There is no need to replace C5. The discoloration is cosmetic. When you replaced C1, did you observe the proper polarity? The negative lead of C1 should be on the right. The silver band on the capacitor also indicates the negative lead.

You should be able to re-flow the solder to all of IC1's pins. @Jesturr had positive results from following advice previously discussed. Again, let us know how your repairs go!

 

Good news! I do not have the best soldering skills, but I applied flux (too much) and some solder, didn't remove any old solder though. Placed everything back and no more bouncing/dead tach! The only thing was re-calibrating the needle...I thought I broke it at first but I did it ghetto style (hope I didn't break anything). It was reading 2k at 1100 idle on the PFC, so I reved and kept it at 2.5k on the PFC, pulled the needle and placed it at 2,5k on the tach. Everything is reading perfectly now. I will hopefully have a full YouTube video of my whole Tach repair process by the end of the week. Thanks for all the help Gen2n3!

P.S. I did not know how to clean up extra flux so I tried to heat it up with the soldering gun hoping it would evaporate. It was still there so I left it alone.

 

Billy,

Use rubbing alcohol to remove the excess flux. I have a general soldering advice thread that covers a lot of the basic stuff. Please search for soldering tips with my username.

I am unable to link it because I'm using the GT app.

 

Doh, I knew I read that somewhere...but since the car is back together, I'll clean it up the "next time" I get around to it. Thanks!

 

Billy,

I strongly encourage you to reconsider cleaning up the flux sooner rather than later.

It will eat up the pcb & components when left untouched.

I'd hate to hear about damage to your tach after you carefully repaired it!

 

Oh man, I thought I read somewhere that it protects it, looks like I gotta take it apart next weekend...

 

Gen2n3,

Yes, I made sure the C1 was installed properly. I took the car for a drive today and the tach intermittently worked. I'll re-flow the solder on IC1 and report back.

 

I must have used too much alcohol and the tach didn't work. Then I found out using a toothpick was easier than using a q-tip. Got rid of lots of flux, then re-flowed more solder (on all components near the IC1 also) and everything is back to normal after calibrating the needle. Thanks again Gen2n3!

 

PHEn & Billy,

Thank you for the positive feedback. I'm glad to help. I look forward to hearing about your success with re-soldering IC1.

Billy, how much flux did you put on? It doesn't have to be a big blob. Dependent upon paste or liquid type, apply flux to the eyelet(s)...a little goes a long way. Remember, it acts as a catalyst for solder to flow. It would even defy gravity (allow solder to flow up a vertical wire).

You're welcome to hit that "Like" button on any advice given. Man, I sound like a YouTuber asking for "Likes"! LOL

 

I like likes.