e30 tach with s50 engine

[BingM3]

Yeah,

that idea already crossed my mine. I looked it up, and I believe that the coding plug won't fit in an e30 tach.

It will fit, but you have to remove the plastic black cover

[X2theZ3]

Hmm weird. My tach says I idle is at 600 rpms.

i idle around there also.

[TWRViper]

Hmm weird. My tach says I idle is at 600 rpms.

[equate975]

I have an e30 m3 cluster with a 325i encoder plug. Tach says I idle at 1,000 rpms, fuel cut off will come in around 7200-7500 rpm says the tach. I dont know where the rpms are really at, at the same time I dont think I really care.

[TWRViper]

Ben, what you said makes sense. But I seem to be hitting revlimit at ~6800 in 1st and ~6500 in other gears.

Are the electronics that control the motor and the motor for the tach interchangable (the same) between the e30 m3 and the e30 325i?

Tom

[LINUS]

Yes - you need a 6 cyl code plug, but beyond that you should be accurate with your setup since you have the same swept area as a 325 and are using the total swept area to indicate rpms (via your 325 paper face).
The inherent problem with using a e30M3 cluster is obviously the code plug, but then once you take care of that you will be reading high rpm's since a factory M3 tach registers to 8k in the same swept area a 325 can only indicate 7k. So you will be off something like ~12.5% because of the math involved with total swept area by needle and the difference between 7K being maxed out and 8K being the max.
The needle should read accurate in your setup - mine won't read correct (I'll be reading ~12.5% high on rpm's) until I either do what you did and swap faces - or what I plan to do is to get the signal to the tach decreased by roughly 12% so I read lower and slightly closer to correct.

I haven't researched it fully, but I'm thinking a ~12% increase in ohm resistance on the voltage going to the tach will solve the problem.
DISCLAIMER :
**I haven't even looked at the cluster yet, so there may be no basis that the last sentence is true or even possible, but it's my starting ground. So don't quote me on that yet.**

---Ben

[X2theZ3]

Has anyone tried like an e28 or e34 m5 plug? That would be a coded plug for a 6 cyl and a 7k redline on an 8k tach would it not? That plug if it fits should make the e30 m3 tach accurate. Right?

Yeah,

that idea already crossed my mine. I looked it up, and I believe that the coding plug won't fit in an e30 tach.

[Van Westervelt]

Has anyone tried like an e28 or e34 m5 plug? That would be a coded plug for a 6 cyl and a 7k redline on an 8k tach would it not? That plug if it fits should make the e30 m3 tach accurate. Right?

[X2theZ3]

Yes,

the problem here (with the e30 m3 cluster) lies in the different faceplates. Swapping the coding plugs help a bunch but doesnt make the tach completely accurate.

[Rev Engineer]

When you say redline at 7200, do you mean rev limiter? My Mazda MX6 has a redline at 7000 but the revlimiter doesn't kick in until ~7500. (I try not to take it up that high)

I ment to say revlimit.

[Rev Engineer]

Coding plug alone does not necessarily work. I tried a 6 cyl coding plug from a 325e and the tach was way off. I've heard people comment that the 325i coding plug in the e30 m3 cluster works ok, but it is still off. I have to think that the tach units between the 325i and e30 m3 are the same, but the faceplates are different.

FYI - The tach in the late model e30's use some type of quadrature pwm. The entire tach range is divided into four quadrants/coils and a chip on the SI board modulates one of the four tach coils at a time. ie - one coil is pwm'ed up to say 2000rpm and after that a second coil starts being pwm'ed up to another rpm and so on. I looked at this as a possibility to correct the tach, but it would extremely difficult to modify the signal between the SI board and the tach. The same also applies to the late model coding plug. It would also be extremely difficulty to modify or reverse engineer the coding plug. Best bet for us e30 m3 folks to have an accurate tach is to modify the signal from the engine to the cluster.

I should comment that our tachs are not necessarily accurate. I've been testing mine on the bench and found it to be a few hundred rpms off between ~1500 and 4000. At around 5k it's very accurate and falls back off a couple hundred rpms around 8k. I suspect every tach is a little different.

[TWRViper]

Plug from a car with right amount of cylinders will give right tach reading. A plug from a car with same cc/m injectors will most likely to give right MPG reading.

But I have a e30 m3 cluster with a 325i 7k faceplate and a 325i codeplug. 7k on the e30 m3 faceplate and the 325i faceplate are slightly different. If I remember correctly I think that 8k on the e30 m3 faceplate is in the same location as 7k on the e30 325i faceplate.

So I'm trying to figure if I need to swap back to the 8k m3 tach faceplate?

Thanks

[Rocla]

Plug from a car with right amount of cylinders will give right tach reading. A plug from a car with same cc/m injectors will most likely to give right MPG reading.

[TWRViper]

Stock s50 redline is 7200 in 1st gear, but otherwise 6500.

When you say redline at 7200, do you mean rev limiter? My Mazda MX6 has a redline at 7000 but the revlimiter doesn't kick in until ~7500. (I try not to take it up that high)

[Rev Engineer]

Are the 318i code plugs interchangible with the e30 m3 code plugs?

I imagine they are not. I believe the coding plugs sends out a blurp of data about the tach range and the number of cylinders- among other things. If the tach range of the 318i is the same as the e30 m3 than it proabbly will work, but I think they are different.