Tachometer and MPG not working / Questions

[Norm]

Hi All,

There is a post further below titled "tachometer/mpg not working.....please help 86 528e". Well I have been having the same problem. Dave, at the bottom of that thread, says something about a rectangular plug.

I was looking for the plug he is talking about but it is a bit difficult to know exactly which he is talking about without a picture. In my 88 535is E28, in the glove box there are two "Black Boxes".

They are mounted more or less on top of each other. The top box has its connector in the front (facing me) aligned with the lateral axis of the car, i.e., side to side. The bottom box has its connector inboard and aligned with the longitudonal axis of the car, i.e., front to back.

I believe the bottom box is the ECU/DME, and the top box I have no idea. I disconected the top one while the engine ran and there was no change. Could this be the rectangular plug he is talking about? Does this top box control the MPG and Tach? If it is this could be a break through!

Before anybody says anything, I have already checked my SI board and batteries. They are good.

[RonW]

Bottom box is the ECU.

If the OBC reports MPG correctly after you reset that function (it reports average MPG, not instantaneous), then the problem is on the instrument cluster circuit board, regardless of whether its batteries are good.

The ECU supplies the tach signal and the fuel consumption signal.
The sender on the differential supplies the speed signal. If your speedometer works, then this signal is getting to the circuit board.

The circuit board creates the MPG signal to the tach from the fuel consumption and speed signals.

For what it's worth, the only time I've seen a gauge problem that turned out to be the gauge was with the odometer. The gears failed (which is a common problem) and the small circuit board behind the speedometer failed (which is less common). All other gauge problems have turned out to be elsewhere, usually the instrument cluster circuit board.1986 535i
1987 535is

[Norm]

of these components were interconnected like that. Sufice to say that it looks like I might have to be looking for a new or good used SI board. Do you know where I can find the cheapest deal on one (either used or refurbished)? Maybe I can send my own somewhere to have it repaired. Might you know of a place like this?

Thanks again,
Norman

[DB35]

Follow the ECU wire bundle to the firewall. Just before the firewall near the rear of the glovebox will be a small (1 1/2 x 1/4 x 1 1/4) white rectangular connecting plug. The plug will have 3 wires. Black (RPM), white (fuel rate) and Green (ECU code). Your colors may be different. There may be a couple of other single wire connectors outside the ECU wire bundle. Look for the 3 wire next to the firewall on the outside of the ECU wire bundle. Yours is probably not connected. Hope this helps.

[RonW]

You have to know which variety of circuit board you have. BMW switched from the early style to the late style in early '86, so you likely have the late style. The easiest way to tell is to count the red SI lights in the center of the instrument cluster. If you have three red LEDs, you have the early style. If you have only one, you have the later style. Complete early cluster assemblies can be swapped with late assemblies (though you may have to monkey under the hood just a bit if you put an early cluster into a late care), but components within them cannot be swapped.

When I have a problem with my circuit boards, I attack them with a soldering iron. That's a time-consuming process, and not most people's cup of tea, but it gets them going again for a while.1986 535i
1987 535is

[Norm]

white plug which measures approximately as you said. It was plugged in and I didn't disturb it. Maybe I should try unplugging and replugging it. I've included a picture for you to see what I see. Is this the plug you are talking about?

<a href="http://picasaweb.google.com/Norman.Camamile/MyPictures/photo#5181897459434485410"><img src="http://lh5.google.com/Norman.Camamile/R-nMSNQXdqI/AAAAAAAAAOg/jU9c1WEo3Gs/s144/Glovebox%20005a.jpg" /></a>

[Norm]

[DB35]

Looks like the correct plug. If there was any deposits due to age disconnecting and reconnecting would help the restore some electrical lost. Since the plug is connected which sends the RPM/TAC signals to the dash, I would discount it as the source of your problem.

[Norm]

post means that the differential sender is wired into the system and working well. Then it seems like the instrument cluster itself is not working well. I guess we can conclude this without any other checks. Thanks for your help!

[ncamie]

when I disconnected this white plug. I thought that it should continue running but it just shut down. I tried restarting but got no fire. So I reconnected the plug and she fired right up. I was under the impression that some people have been running around with this plug disconnected, but from my experience there is no way that I could have ran my engine with the plug disconnected. Hmmm. Any other ideas before I go to Programa?

[DB35]

E28 had a bad TAC/RPM identical to yours. The plug was disconnected but it ran fine. He reconnected the plug and RPM/Fuel worked. I have no other ideas to help but have included the site to download the ETM. I went with the Programa board about 5 yrs ago.