I went hunting for the cause of my code 20, and after checking the ECU, OMP and connectors, it went away (dirty connector?), but now my tach is dead.

S5 TII. All the other gauges work. Meter fuse is good. I jumped the trailing coil bullet connector to the leading one as suggested by another thread, but it doesn't fix the tach. Both trailing coils measure at 0.8 ohms. The only other thing I did since the tach was working last was replace the positive battery cable, but as far as I can tell, I didn't mess with any wires near the tach signal routing when doing that.

What else can I check? Am I left with ripping out the instrument cluster?

I'd really like to get the tach working for a short road trip to see friends this weekend.

Thanks

 

The Yellow/Blue wire from the trailing coil is part of the Front harness. This harness mates w/the Instrumen Cluster harness. The mating point is referred to as connector X-21. The Y/L wire from the Front harness mates with a similar colored wire on the Cluster harness. This connector is basically below and behind the gauge cluster. Page 47 of the wiring diagram indicates this position. You want to make sure these wires are connected properly and you would want to take a voltage reading w/the engine running on this wire first at the Front side of the connector and then compare what reading you get to the other side of the connector.

 

Also as a quick check be sure that your trailing coils are actually sparking. If the trailing ignition isn't working then it won't send a signal to the tach.

 

Ah, found the bugger. Looks like the wire comes through the firewall, to X-20 (looks like a 20x2 green connector above the hood release), to X-21 (another 20x2 connector on the stereo side of the steering column), then to the instrument cluster connector. I'll check the voltages tomorrow as you suggest at both sides of X-20 and X-21.

I'm thinking jumping the bullet connectors between both coils as I mentioned would rule that out, but it wouldn't be a bad idea to check anyways. Did you have a particular method in mind?

 

X-21 is the relevant connector and not X-20. The wire that the trailing coil uses to run to the tach is Yellow/Blue which is found in X-21. X-20 does not house this wire as it houses a Blue/Yellow wire which is not going to help you w/this problem. Might look the same from a far but that's where the silmilarities end.

 

check teh coils first before you do anything. thats the main culprit.

 

That may be. Parts of the harness were obscured. Didn't get around to checking though. Tach ended up working again this morning, but the boost gauge went back to non-functional. Ah well, I'd rather have the tach back and rely on my aftermarket boost gauge then have the tach broken but the stock gauge "working" (it doesn't seem to like high altitude). All I did was remove X-21 from its mount point to get a better look at it, then put it back. Meh, I'm happy now. Tach works, and Code 20 hasn't come back, so I can go back to fixing other things.

Seriously, is something not clear about my original posting? I'm not trying to be sarcastic or anything, but I've already checked the resistance of both trailing coils (in spec) and bypassed them by using the leading coils (see OP) to no effect. Is there something else I'm missing? Doesn't matter to me now that the tach is working again, but it would be good info for the future if it dies again.

 

You want to be careful about code #20 for it will lead to ruins if the problem persists. The Boost gauge and the OMP appear to be related somewhat based on the S5 wiring diagram. Also, the two sources I have for the S5 wiring diagrams are both wrong w/respect to the gauge wiring as it shows no Boost gauge on either for the Turbo wiring. Find An S5 wiring diagram that is correct and I should be able to give you some more advice perhaps.

And you likely had a faulty connection at X-21 which helped to fix the tach and perhaps the wiring for the Boost gauge runs through the same connector.

 

I'm keeping an eye on the code 20. Its acting like the connector just needed to be cleaned. Probably the same thing for the tach.

I've got an actual hard copy FSM at home, I'll see what it says about boost gauge wiring.

 

Digging this back up, because I hit something weird and I'm looking for some additional help.

Shortly after the last update, the tach issue went away on its own. Since I had other issues to deal with, I was mostly just grateful and didn't ponder on it much.

On Saturday, I went to an autocross event. Raced in the morning. When I started the car in the late afternoon to go home, the tach was dead. Did not move at all from 0 for the entire hour drive home.

Last night, I decided to take a look at it. I started off by starting the car, since it had sat for about 48 hours, and I wanted to make sure it still had a tach issue. sure enough, it did. So I pulled the ECU codes to see if perhaps there was anything stored that wasn't triggering a CEL. No codes. I jumped the two bullet connectors to bypass the trailing coil, and the tach started working again. Seems like the trailing coil might be dead.

Whats weird though, is what happened next. To confirm a dead coil, I removed the jumper wire between the bullet connectors and started the car. I expected the tach to be dead again. Instead it continued working. Just to be sure, I turned off the car, let it sit a minute or two, and started it again. Tach was still working.

So, while I'm grateful that the tach is working again, I'm really puzzled as I can't explain why its fixed, other than I somehow "jump started" the trailing coil with the leading one. I'm concerned that the tach will act up again in the future, but I don't have any idea of what is actually wrong (seems impossible to diagnose now that the issue has gone away).

Any suggestions?

 

use a timing light to see if the trailing plugs are firing, if they aren't it will explain the tach issue.

if they're not firing it is usually a faulty trailing coil, but could be a bad connection to the coil, or bad 12v feed off the black/yellow wire or issue with the coil trigger wires leading up to the ECU. try swapping the coil versus doing a simple ohm test, there's more to the coil than primary resistance and the ignitor is nearly impossible to test accurately.

fouled trailing spark plugs can also lead to an inoperative tach, since the coil cannot fire the plugs and generate an RPM signal. this is usually the case where the tach is dead until the car is fully warmed up and plugs get a chance to self clean, slightly(the trailing plugs are very obscured in the engine).

 

The tach currently works (as of last night, haven't checked today) running in the factory configuration (just off the trailing coil), so I assume the trailing plugs are firing. Also, the trailing plugs were replaced 5000 miles ago, so I really hope they are not fouled.

These are all good suggestions, but it seems like they won't have any effect right now as the issue is currently not manifesting, or do you think they still have value?

 

wait until it crops up again and try swapping the trailing coil instead of jumpering the tach wire.

 

Ah, got it. Thanks

 

Just curious since it was brought up, why exactly is the trailing plugs obscured so much from the combustion chamber?

 

Finally hit the issue again. Unfortunately it was during a road trip this past weekend. Luckily a friend in the convoy had a basic timing light so we pulled over for 10 minutes and tested the trailing coil. Verified the trailing coil was firing both trailing plugs. About two hours later, the tach started working again.

This week, assuming I find time, I'm going to pull the cluster and see if the wiring/connectors look suspect.