No Tach output

[RIPDOTCOM]

I got the lights figured out in the cluster so i am moving on to the next one. I have no Tach Signal. Hooked up a Tach/Dwell meter to the black wire on the rear of the white connector on the cluster and have no tach signal. Pulled the DME and probed the black wires that come out of there and no tach signal, probed pin 1 on the obd connector and no tach signal. I am thinking that the tach driver is fried on the DME. I can however get a tach signal from the cps but it is obviously incorrect as I understand the dme doubles the output signal of the cps i.e. 350RPM from the cps=700Rpm from tach output. Before I spend a load of cash on a siemens DME from overseas, is there any other location to attempt to pull tach signal from? Is it possible that it is grounded out and not giving feedback?

[benemorius]

You're making it really hard for anyone to help you by continually starting new threads and leaving important information behind in the old ones.

Were all the cluster lights really bad? What voltage is the alternator putting off when the engine is running? You seem to have some serious issues going on and trying to patch a tach signal over to the gauge cluster may or may not be the best idea in the long run.

The dme outputs the only tach signal. The crank and cam position sensors are the only other two places that would have a decent chance of being useful. I don't know whether cps means crank or cam to you, but the cams only turn at half the speed of the crank so it's the crank you want if you're going to try that. I don't know whether the signal from the sensor itself would work for the tach or not. The tach signal is usually a nice clean square wave. The crank sensor is going to output more of a funky sine wave with one increased amplitude peak per rotation.

[RIPDOTCOM]

Thank, sorry for posting multiple threads, I just thought it would actually be easier to diagnose one thing at a time as the lights must not have been related. i replaced the bulbs and the lights work great. Now I am moving on to the tach then the temp gauge. I had simply assumed they were all reladed initially but couldnt confirm if the lights had initially went out when everything else did or after I started poking around, obviously it was the latter. Furthermore, i guess I can only get a cam position signal which leads me to believe that I am either not getitng a crank signal (would the engine even run?) to the DME. I can definitely confirm that I am not getting a tach output from either pin 1 on the obd port or the tach output black wire from the dme. I will check alternator voltage today and see if that is an issue, I can also assure you that I will stick to this thread till the tach problem is resolved. Thanks again.


You're making it really hard for anyone to help you by continually starting new threads and leaving important information behind in the old ones.

Were all the cluster lights really bad? What voltage is the alternator putting off when the engine is running? You seem to have some serious issues going on and trying to patch a tach signal over to the gauge cluster may or may not be the best idea in the long run.

The dme outputs the only tach signal. The crank and cam position sensors are the only other two places that would have a decent chance of being useful. I don't know whether cps means crank or cam to you, but the cams only turn at half the speed of the crank so it's the crank you want if you're going to try that. I don't know whether the signal from the sensor itself would work for the tach or not. The tach signal is usually a nice clean square wave. The crank sensor is going to output more of a funky sine wave with one increased amplitude peak per rotation.

[RIPDOTCOM]

Getting 13.34v @ the alternator. I found an accurate tach signal with an aftermarket tach, it appears to be pin 27 on the DME, according to the bentley it is a pulsed ground open signal but I could be mistaken, i need to go get some wire and try to splice the signal into the back of the cluster, hopefully the tach will fire up and i can move on to the next thing.

[RIPDOTCOM]

Hell, since I have no tach output on pin 1 of the obd I wonder if I can simply run it there, anyone know if the pin 1 on the obd connector connects to the actual tach?

[benemorius]

Sorry I didn't mean to be rude about the multiple threads. I only meant that getting accurate help can be delayed when the history behind the problem is unknown. For instance, I asked about the alternator output because it sounded a bit like a high voltage spike may have taken out all the instrument cluster bulbs, and something like that can open up a whole new can of worms with regard to fried electronics. Then again, I also didn't realize that you had been doing much poking around in there, which of course is also a pretty good explanation for the bulbs.

I still don't fully understand what happened leading up to this so it might be a good idea to tell the whole story if you're interested in really fixing the whole problem. It's just that this isn't exactly the sort of thing to randomly happen one day, so I would really expect to find a cause somewhere that lead up to it. Of course, if you're content to just circumvent the problem and just get stuff working again then that's your choice to make.

Anyway, I don't seem to have a wiring diagram for your particular engine so I can't tell what pin 27 is. The only functions I see assigned to that pin on any engine are either dme relay or icv control, and needless to say you're in big trouble if either of those are outputting a tach signal. Could you verify it's purpose?

Are you saying that your crank position sensor isn't outputting anything? If so, that's trouble. Technically the dme would still be capable of running the engine without it, but it depends on whether it was designed to be able to and I don't know the answer to that. If your crank sensor is bad then that could certainly leave you without a tach signal, but once again it's hard to suggest a bad crank sensor without really knowing what else is going on.

Yes, the tach pin in the diagnostic port does run to the tach in the gauge cluster. However, it also runs to the dme, so you may or may not be able to input a tach signal to the diagnostic port. This is due to the unknown (to me) design of the tach output from the dme. If it isn't outputting a valid signal, it is likely to be holding the output either high or low. And if the output is fried then it it is likely to be doing so with a pretty low impedance. Either way, it will all depend on the input/output impedance of the dme, tach, and whatever you try to feed in to the diagnostic port. Due to that unknown, I wouldn't recommend connecting anything to the port without a current limiting resistor unless you're willing to fry whatever you connect to it.

[RIPDOTCOM]

Thanks again. The entire problem started when i held the cruise control to accellerate for 4-5 seconds, it did not work. on the 5th second the temp and tach went to zero. I cannot confirm if this took the dash lights out since it was daylight and I did not realize the lights were out until later on after troubleshooting and "poking around". I guess my concern is that if I am simply not getting a tach signal output from the DME then it would obviously make sense that neither the tach or the pin 1 on the diagnostic connector would work. This leads me to believe that I have fried the tach output of the DME. unfortunately a siemens DME is a real expensive baby and I would hate to spend the bucks on one if the pproblem is elsewhere. I am pretty certain that it is not the cluster as I swapped another cluster from my 325ic in it and the same exact symptoms occured. Also, another symptom is when i drive the car to normal temperature then disconnect/reconnect the battery, the temp gauge is accurate until the car cools down then it never goes back up unless I reset the battery. Thats about all i can think of. The only other thing that may or may not be related is recently (seems like the same time) my driver side window wont go up automatically when i touch the button (passenger side does) i have to hold the button all the way up. I am starting to think the problem is going to be associated with connector X188 and the BW/WB wire that connects from there to the cluster. I do have continuity there but do not know where the BW/WB wires go from the X188 connector.

[RIPDOTCOM]

Well hells bells, I got up enough nerve to jump the wire on the DME that was giving me tach signal to pin 1 on the diagnostic connector and not only does my tach work but the temp gauge is going back up without a battery reset. Window still doesnt go up automatically but that was the least of my problem. After 10 minutes the temp gauge is at dead center where it always was warm before the problems. I am going to disconnect the wire for now, make a 5A fuse wire and put it inline in case this happens again I will not fry my DME. Thanks for all the help, as long as smoke doesnt start coming from the DME bay I think it is wrapped up.

Thanks to chancebanning and benemorius for helping me get this thing back to spec.

[benemorius]

That's most bizarre. Out of curiosity, what exactly is pin 27 on whatever diagram you're using?

As for the window, it probably just needs to be reinitialized. Roll it all the way down, then hold down the button to roll it up. Wait until it stops at the top, then count another ten seconds. Release the button only after those ten seconds. See if it works after that.

[RIPDOTCOM]

The pin on the DME is Pulsed ground. Thanks for the info on the window, will try it tonite.