OBD1 M52 - Tach "dies" at 6k RPM

[slums]

Car has a M52 swap with M50 wiring, OBD1 with chipped 413 red label DME.

Just replaced the SI board with Programa's battery-less board (it's pretty sweet by the way!) thinking this was the cause of my tach's weird behavior.

No change - same symptoms - as I climb revs all is normal until hitting 6000rpm on the tach, at which point the needle drops down to 0 as if suddenly the gauge is dead. If I let off the throttle the needle will jump back up again.

PO had wired in a shift light which is tapping into the tach signal wire from DME. Shift light doesn't come on when it's meant to, but instead will randomly flash on/off when cruising.

Also, wiring harness is new. CPS is new.

Lastly, the tach seems to be reading a bit high. It's an old Motometer unit before they used the chip to dictate if it was a 4 or 6 cylinder. There's a plastic plug that clips into the back of the tach which is from a 6.

Any suggestions on what this could be?

[jayuuey]

Did you remove the shift light? This sounds like a bad installation/poor wiring.

[TeXJ]

OP, what chip are you using?

[slums]

It's a Midnight Tuning chip labelled "Custom". They're a pretty reputable BMW tuner out of the Ottawa region. I imagine they dyno'd the car at one point since they have off the shelf chips available for M50 manifold swap.

I will try to cut out the shift light.

Meanwhile I had an aftermarket tach hanging around that I plan to wire up to see if the signal is clean or if it's actual cluster/tach that's faulty

[thedentonmare]

Following up on this, since I just went through some tach problems myself.

There is no engine speed/rev sensor to be concerned about - the signal apparently comes from ECU, so you can eliminate pretty much anything not having to do with the cluster itself unless you're having driveability issues.

Check the 7.5 amp fuses in spots 10, 12, and 21 and replace them with new ones. The fuses can still be good but with copper oxidation and micro cracks, they can cause a voltage drop or intermittent failure, better to just replace. I've never read up on that nifty SI board replacement you did without batteries - sounds cool.

Since you were having issues prior to, and still have em now the same, it has to be something else.

The correct plug for the 6 cyl will certainly be important.

My issue ended up being a loose tach needle itself! It was registering lower than normal (0 revs for idle, around 2,000 for 3,000 actual etc.) And was acting funny/floaty in general. I feared the worst with some nightmare electrical issue but it was the physical needle not being pressed fully onto the drive pin.

You should be able to use a fork to pull the needle off and re-calibrate it 5mm off the rest pin ( to the right) to set it back to correct. Check the tightness at any rate to ensure this isn't your issue.

[Gregs///M]

I would log the signal going to the tach and see what happens at 6k. If you see your signal, going into the cluster, lose signal at 6k, the problem is not cluster related. If your signal maintains while the tach fails, the cluster may be at fault.

The SI board powers and controls the tach so if the problem is indeed in the cluster, I would suspect the tach or main circuit board but definitely suspect the tach first. The issue with going with the Programa batteryless SI board is that, although its a great product, there are still a bunch of circuits in the tach itself that can fail. Basically, if you replace only the SI board on an early model cluster, half of the SI board is located in the tach itself and therefore, you are essentially only replacing half of the SI board.

Unlike the late model, which the SI board is all contained within one circuit board, the tach and MPG gauges are simply just motors with no aux circuits located on that assembly.

In the end, there is no modern replacement tach and using another used tach may have the same issues. This is why the only way to have a reliable and accurate gauge cluster in the E30 or any vintage BMW is to have the entire cluster refurbished. Most people have no idea that on the early model cars, half of the SI board is on a PCB and half is on the tach assembly.

If you want your 7k tach to be configured to your 6cyl engine, you will need the 325i coding plug.