Hazard15301

iTrader: (5)

Hey guys, My tach stopped working yesterday. I searched, but the only threads I found were when the tach and odometer stopped working as a group. In my car, the absolute only thing not working is the tach. I have all other gauges, lights, functions, etc. I have verified that both ends of the speedo cable are plugged in (not sure if that would affect it or not), all fuses in the car and under the hood are good, nothing simple that I can diagnose, except for one thing. I dug around in the harness by the leading coil pack and found a single green wire taped to the outside, with a bare end. It's nothing I did, and I always assumed it was either something factory or something the PO did, but everything worked so I didn't question it. I didn't remember that wire being bare and just hanging there, but I know it didn't go to the bullet connector on the coil either, like you would assume it would. I can't find the other end of the wire either, if it was ever even there.

I'll tear the dash apart and trace the wire when I get home, but wanted to see if anyone has ever had a similar situation, or have an alternate fix for the tach that I could use.

Car is an S4 n/a, thanks in advance.

*edit

Also, some possibly relevant info I forgot, the tach bounces when cranking, and when it catches, jumps up to about 1k before bottoming out.

MIDNFauciUSN

iTrader: (8)

The speedo has nothing to do with the tach.

1. Check the METER fuse under the dash. EDIT... YOU DID THIS ALREADY.
2. The tach signal comes from the trailing coil... specifically the Y/L wire. That goes straight to the tach. Make sure the trailing coil has power... and while you're at it, check for spark.

That gives you something to do for a few minutes.

Your "alternate fix" would be... run a wire from the Y/L wire coming out of the coil, to pin 10 on the connector behind the instrument cluster (ME-02).

In any case, figure out what the real issue is. You may not notice a trailing coil going/gone bad... so definitely check for spark.

Hazard15301

iTrader: (5)

I knew this, but I checked it to see if it had pulled out due to snagging the cable under the car and possibly caused other damage. Obviously this wasn't the case.
Also I did check both the interior and under hood fuses, and all are good.

I assumed the tach signal came from the leading coil, because there is a bullet connector there (which I figured was there for diagnostic purposes), I didn't look anywhere near the trailing coil so that'll be a good start.

Also didn't notice any driveability issues on my way to work, so I presume I have spark, but I'll obviously verify this. Thanks for the info, appreciate the quick response.

MazdaMike02

iTrader: (4)

AFAIK the signal comes from the trailing coils...y/b wire. Check to see if its shorted or open, what your describing sounds like an issue with the wiring. And FCs are very typical for electrical problems.

MIDNFauciUSN

iTrader: (8)

No problem brotha!

What I was saying about the trailing coil is that it can be dead, and you probably wouldn't notice... the engine will run fine on the leading spark by itself.

If you're getting spark, and the wire is connected at both ends... *I* would suspect a jacked up tach (or a Y/L wire that is shorting out somewhere). I'd bust open the instrument cluster, look for any obviously bad (charred) electrical components and replace them - if there are any.

It may be worth it to take a soldering iron and heat each of the soldered connections on the PCB... sometimes it works wonders.

Good luck

MIDNFauciUSN

iTrader: (8)

Its not a might... unless someone screwed with it previously. Since you mentioned it... the connector from the coil is F-35, the 4 pin one.

Hazard15301

iTrader: (5)

Well I'll report back with what I find. I have a feeling that green wire I mentioned has nothing to do with the situation. It was taped onto the leading coil harness, so I assumed it had something to do with it, but it seems not.

I'm starting to suspect the trailing coil myself, as I haven't done anything under the hood or behind the cluster to potentially break anything.

*edit

1000th post

misterstyx69

iTrader: (142)

clean the grounds to your trailing coil and try it again.

satch

iTrader: (1)

There are bullet connectors at both coils. Jumper them together and the tach will use the lead coil signal instead. If the tach works then you know the issue is not the tach itself. If the tach continues not to work then there's a decent chance the problem might be isolated to the tach. This assumes the wiring to the tach providing the signal is good to go.

Hazard15301

iTrader: (5)

Thanks to you guys who pointed me in the direction of the trailing coils. I was gonna pull the cluster, but I had a spare set of coils in the garage so I swapped the trailing pack and voila! Tach went back to normal.

*edit never mind. Stopped working after I shut it down and started it back up. Dammit

satch

iTrader: (1)

Low voltage output at the alternator can turn off the trailing coil I have read. And you can try pulling the EGI INJ fuse from its slot and placing it back in.

Hazard15301

iTrader: (5)

I was trying to verify that before anyone could ask, checked voltage at the 2 prong plug for the coil and got 14v on each (B/Y). I guess both coils could be bad but what are the odds right (pretty good obviously).

Here's what's strange... If I totally disconnect and unbolt the coil, the reconnect and bolt it down, the tach works fine and I get spark, but if I shut the car off and restart it once or twice, it cuts out again.

satch

iTrader: (1)

I was speaking of the voltage output of the alternator while idling. And 14 volts seems a bit high as it should read 12 volts I thought at the B/Y wires. And have you tried to use the lead col to run the tach?

Hazard15301

iTrader: (5)

Well it's usually 14v for the first 20-30 seconds then13v at idle, I just exaggerated slightly to show that I don't have any voltage issues. Just took the brackets apart and took a wire wheel to them and applied dielectric grease, no change. Guess I'll hunt down another coil and go from there. I forgot about jumping the bullet connectors. Guess I'll give that a shot.

satch

iTrader: (1)

The Yellow/Blue wire from the Front harness of the trailing coil runs to the tach. Before it reaches the tach it mates w/the Y/L wire from the Meter harness at connector FME-01. Perhaps you might want to check this connection for it could be an iffy one.

Hazard15301

iTrader: (5)

Will do.

Jumping between the two bullets gave me tach but no spark.

So I would say it's pointing to bad coils eh?

satch

iTrader: (1)

Or igniter, or poor connection at the ECU, or a wire shorting out between the coil and ECU (job for an ohm test), or poor CAS connection perhaps?

And if the coil is not sparking, trailing, then it should not be related to FME-01.