can't seem to fix tach

[duboy]

Since i got my 318is, the tach/mpg has been inoperable. The PO had a shop replace the SI batteries the day before i picked her up, but it only fixed the issue for a day. Since i've been driving the car, once in a while the tach/mpg guages will start working, but not for long (seemingly when it is warm out). So i thought my SI board or tach itself was faulty, but apparently i've missed something.

i got a second fully rebuilt cluster that i was going to jumble some guages around to get the look i wanted in the 318is and the M3.

So far i have done the following with no good result, each time the tach/mpg still does not function but everything else does:
- put in the second SI board with the original tach in the original cluster
- put in the original SI board with the second tach in the original cluster
- put in the second SI board with the second tach in the original cluster
- put the fully rebuilt cluster in

Coding plugs are for 4-cyl. Where is the sensor that measures RPM anyway? any other ideas?

thanks for the help.

[jlevie]

The signal for the tach originates within the DME. If your scond cluster works properly in another car, the the problem lies in the wiring or DME.

[duboy]

The signal for the tach originates within the DME. If your scond cluster works properly in another car, the the problem lies in the wiring or DME.

i'm guessing that could be it...since the MPG gauge also works only when the tach is working (this is assuming that MPG gauge gets signal from the DME as well) i hope it's not the wiring...at least the DME can be swapped pretty easy.

[87' 325]

this may be a noob/simple question but where does the DME get the signal from for the tach? Could that sensor be bad possbily?

[jlevie]

The tach signal is generated by the DME based on data from the crank position sensor. Since the crank position sensor signal is also used by the DME to control the fuel pump, spark, and injectors, if that sensor was bad the engine won't run.

As an FYI: the signal for the econometer is derived from the injector firing pulses. Basically is it a measure of injector on time.

[N8857E]

So far i have done the following with no good result, each time the tach/mpg still does not function but everything else does:
- put in the second SI board with the original tach in the original cluster
- put in the original SI board with the second tach in the original cluster
- put in the second SI board with the second tach in the original cluster
- put the fully rebuilt cluster in

Check fuse 21 in the engine fusebox. May be a simple fix.

[duboy]

Check fuse 21 in the engine fusebox. May be a simple fix.

Everything on fuse 21 (and 10) works, except tach/mpg, however i will check it again anyways. for now i've put everything back together. I'll do more troubleshooting maybe next weekend, try my cluster in the M3 and see how it fares.

[JoeyBones]

ok...i'm an e30 noob... I just got a 318is and also the tach/mpg isn't working. I'm not familiar with these cars. Obviously i'm going to check the fuses first, but i'm not familiar with the lingo...

SI board?
DME?

sorry for my ignorance

[duboy]

ok...i'm an e30 noob... I just got a 318is and also the tach/mpg isn't working. I'm not familiar with these cars. Obviously i'm going to check the fuses first, but i'm not familiar with the lingo...

SI board?
DME?

sorry for my ignorance

SI board = service interval board. controls those green/yellow/red light strip at the bottom and the whole inspection telltale thing. has batteries that take dumps over time and throws the whole cluster off.

DME = engine management controller

many many info's abound about both topics, search and you'll find way more in depth info.

fuses are good to start with, i hope it's just your fuses :-) however keep in mind that both fuse 10 and 21 have multiple things running on them, so if a fuse was out, there would be many other things gone, like cluster lights, reverse lights etc.

[jlevie]

SI board = Service Indicator board in the cluster
DME = Digital Motor Electronics (the engine computer, aka ECU)

[JoeyBones]

Well thats where i'll start... I'll go about replacing batteries after that I suppose. Only light I really care about is the check engine...i would eventually like to make a custom cluster with autometer units. (it's going to mostly be a weekend track whore that I can drive on the street with no problems)

[agile30]

my old car. my speedo was working then one day went out. i traced it to the diff wire being cut some how. check that

[gearheadE30]

^^and that helps how? This is tach and econ gauge, not the speedo.

DMEs are like $40-$60 on ebay or the forums if you look around for a little while. That would be a pretty easy way to make sure that isn't the issue. They are 175-code ecu's, and I think the whole number is 261 200 175, but I can't remember for sure.

[duboy]

^^and that helps how? This is tach and econ gauge, not the speedo.

DMEs are like $40-$60 on ebay or the forums if you look around for a little while. That would be a pretty easy way to make sure that isn't the issue. They are 175-code ecu's, and I think the whole number is 261 200 175, but I can't remember for sure.

yeah, i got a 175 ECU coming to me in a couple days, this weekend i'll give it a shot and see. i'll update thread with my findings.

[agile30]

yea. sorry anout that. no idea what i was thinking.