Another tach/economy gauge not working :( wiring is correct! (C104)

[six2midnight]

Ok, I have been trying to figure this out the last couple days, and been tracing all the wires in the ETM. Let me give you a background:

-Early model eta (1985)
-I have traced wires from x20 to black and yellow/white wire on c104 and shows continuity.
-I have tracked from x6000 dme plug to x20 and c104 and all shows continuity.
-I have traced c104 to blue plug on gauge cluster (pin 7 and 11) and shows continuity.

When everything is hooked up, I don't get any signal for economy or tach.

The only thing I cant do is check for a signal from dme to blue plug in back of cluster because its a square wave.

I have tested 3 clusters (two 325i and my old eta) I am not 100% sure on the working condition of the 325i clusters, but my eta one worked 100%. I know the coding on the eta cluster is wrong but the tach should still show something?

Could there be a bad ground? could all 3 clusters be crap? can I check signal from dme without an oscilloscope?

HELPPPP thanks!

[six2midnight]

Well did some more troubleshooting and It is not the cluster. I swapped a known working cluster into the car and the tach still does not work.

Im really out of ideas. I traced X20 pin 20 to DME pin 36 to C104 black wire, to blue connector on cluster black wire.

I then took the AC voltage of the signal (I know its a square wave) but read I should be getting between 1-6V depending on pedal travel. I saw about 1.9V at idle and redline it only got up to about 3V.

The S52 should be grabbing the tach signal from the crankshaft pos sensor, and since the car runs fine, the DME should be getting a signal. Maybe not outputting a signal for some reason?

[six2midnight]

any ideas?

[Appleseason]

any ideas?

An idea would be to check for continuity for whatever wire color the tach is.

Does everything else on the gauge work?
Did you do your own adapter plug?

[six2midnight]

I traced X20 pin 20 to DME pin 36 to C104 black wire, to blue connector on cluster black wire.?

I checked continuity like a million times. I did not make the adapter, but i verified the adapter as well. all checks out.

Its like the DME is not outputting the signal to tach?

[Appleseason]

I checked continuity like a million times. I did not make the adapter, but i verified the adapter as well. all checks out.

Its like the DME is not outputting the signal to tach?

Now this might be a stretch, but a rare problem is if the headunit is wired wrong, it screws with random gauges, it's actually what I'm dealing with right now. It might be worth investigating.

Honestly it's hard to tell since we can't see it, I don't want to give advice about things you already know.

I'll give you an update whenever I find a fix for a similar problem that I have. But at least you know it is carside.

[six2midnight]

Yeah i do have a headunit, but the headunit and all the gauges were working before the swap, so i dont expect there to be a wiring issue with the head unit.

ive heard of that problem before too actually. may be worth pulling the head unit out and see if there is a pinched wire or something. Not a terrible idea. ill try anything at this point.

[TheRob]

Is the connection at the DME good? No corrosion or pins that have potentially been damaged in the past? It sounds like your wiring is spot on from what you describe so its hard to say what the problem could be. Do all the other gauges work properly?

[e30user]

Do your power windows work while the car is running? When I first dealt with my swap, some electronics would stop working once the car switches from accessory to started. It turned out I had a missing ground connection, I believe from the starter. I remember the power windows were affected. Honestly, I can't remember if the tach was also involved. This might be irrelevant. It's been too long ago for me.

[Mwishlist]

I had the same issue, checked everything a million times, turned out it was a bad DME. The DME had been water damaged at some point and even though it ran the car fine, it would not output a tach signal. I actually tracked it down and reflowed the solder in the DME and voila, tach started working like a charm. I replaced the DME anyway with a clean one, but I verified that was the issue.

[six2midnight]

I had the same issue, checked everything a million times, turned out it was a bad DME. The DME had been water damaged at some point and even though it ran the car fine, it would not output a tach signal. I actually tracked it down and reflowed the solder in the DME and voila, tach started working like a charm. I replaced the DME anyway with a clean one, but I verified that was the issue.

Wow, great post. Thanks for the input. Yeah, I am still struggling with this issue. I have traced everything an million times and since the s52 gets the signal from the crankshaft sensor, sends to dme and dme outputs to tach, its a pretty simple system. Not really any way to screw it up.

While I drive I have seen the tach come in for a second then go out, which sounds like a short, but I have the wires soldered and there is no way. I have run continuity checks a million times.

My next thought, was a bad dme. I believe the DME has a transistor that coverts the signal from the crankshaft sensor and spits out the right signal for the tach. I have to believe that the output signal is damaged. I have a new computer coming this week. I will follow up if it works. I really hope it does. Very very annoying

[six2midnight]

To come full circle, in case someone looks for this issue in the future. My problem was a bad dme. The output signal from the dme was bad causing my tach not to work. swapped in new dme and everything works perfect.

[sweet3]

Excellent, glad you figured it out eventually

[Mwishlist]

Man, glad you found it. Yeah, stumped me for freaking ever; couldn't believe it was the DME in my case since everything else worked.