odonnell

I'm preparing to build the wiring harness for my MS3X and am unsure of the function of the red 4-pin connector that's located right behind the firewall grommet.

I've identified 2 of the wires, 1 seems to be switched +12v from the O2 sensor heater and the other goes to DME pin 11:

http://www.arnnworx.com/images/dmeplug1.jpg

I understand that the DME sends out a processed injector duty signal for the economy gauge, I'm guessing it's that pin.

Still not understanding the other functions though. Anyone have any ideas?

Tom M'Guinn

Is this a 944 Turbo? If so...

Brown=ground for boost gauge
Red w/white stripe = boost gauge signal
Green w/ black stripe = tachometer signal
White w/ green stripe = fuel consumption indicator (on cars so equipped, or so says the manual)

You can drive the tach with the stand-alone's ignition signal, no problem. The boost gauge relies on a voltage signal from the KLR, which you won't have with a stand-alone. You'll need to make a driver like this if you want the factory gauge to work:

https://rennlist.com/forums/944-turb...e-control.html

Edit: I assume you are talking about the red plastic square connector that feeds the instrument cluster harness, although some of your description makes me question if we're talking about the same thing...

odonnell

Thanks! Yes, it's for my '86 951. I come from the world of early NA cars so I'm trying to get a handle on all the new wiring.

For the tach signal, I'm going to be feeding it the standard MS tachout wire like I did on my '83 which works perfectly. Interesting read about the boost gauge...I already have a nice aftermarket one, but after looking at your code, it looks like a GM 3 bar MAP sensor could directly supply a regulated 0-5v signal to the gauge, between vacuum (true zero) and 2 bar (relative to atmosphere). The sensor gets +5v regulated via the ECU.
http://www.robietherobot.com/storm/mapsensor.htm

Table is somewhere near the bottom. Would be cool to have in addition to the real boost gauge.

Tom M'Guinn

You can't drive the factory boost gauge directly off a GM Map sensor. You'd need to add the resistors/cap per that thread (and perhaps a voltage follower first) to get the voltage in the right ballpark, and still the scale on the gauge wouldn't correlate to anything per se, though you could get a rough sense based on how far up the needle is swinging... The Arduino can dial it in so that the gauge reads accurately across it's sweep however you want -- for example, so that 1 on the gauge corresponds 1 BAR or boost, or 1 BAR absolute, or 10psi boost, whatever...

odonnell

Can you elaborate on the green/black wire for the tach signal? My buddy installed MS into his 951 this week and his tach isn't working. On my NA, the tach signal came from the green wire on the connector near the brake booster (the ground to the coil from the DME). Tach isn't working.... I suppose it's different on the 951? Just want to clarify that a tach signal going into that wire from a standalone is indeed the correct configuration.

Tom M'Guinn

Read all about it here:

https://rennlist.com/forums/944-turb...hometer-2.html

Is the stand-alone driving a single coil and distributor like the factory ignition? You need to feed the tach two ground pulses per revolution. Are you sure the tach is getting power after the stand-alone wiring changes?

odonnell

It's driving 4 LS1 coils in wasted spark, with the tach-out wire configured to match the output of a single coil firing 4x per cycle.

I'm not sure about the power to the tach. On my early NA, there was a black wire that connected to the dash harness just after the DME connector. Of course after removing the stock harness I had to recreate that power source. On the 951 harness, I can't seem to find a wire that powers the tach coming from the DME harness. Did I just miss it? If so, that's the issue.

Tom M'Guinn

You might try just testing the tach in situ using the bolt and ground trick in the other thread. If that gets the tach jumping, then I'd focus on the MS tach signal. If it doesn't, then I'd focus on getting power and the signal to the tach (assuming it was working fine before...).

odonnell

The tach is known to work. It looks like based on the wiring diagram, there's a black/red wire that supplies power to the tach (terminal C). Can anyone confirm if this power is delivered via the DME/KLR harness, or through the body harness? We can remove the cluster to do those tests if need-be, but I'm positive that it's just a matter of making the right connection. The next step is to move the MS tach wire over to the green/black wire on that red square connector.

Edit: no dice. 99% sure it's a matter of restoring +12v to the tach.

Tom M'Guinn

Yes, the black wire with red stripe supplies 12v to the tach. There are three edge connectors behind the cluster. Two sit horizontally, and one sits vertically on the tach side. Power to the tach is supplied via the black/red wire, which is the 4th wire down from the top on the vertical edge connector. It is part of the interior harness, and is connected inside the interior harness with a number of other gauge power wires (see "welding point' #10) on schematic Sheet 4 at I19. If all the other gauges are working, then it would seem like your tach must be getting power, absent some unique harness or cluster circuit board issue. Ground is on the 2nd wire from the top of that same connector, and joins a large number of other grounds ultimately going to MPII. It "looks" like the ! warning light would not have power if the tach doesn't, though I haven't confirmed that first hand. Can you get the ! to light up? I'd try the grounded bolt trick on the tach input (with ignition on) before tearing much apart. If the tach bounces, then you know it has power. If not, then still could be anything...

odonnell

Thanks. The "!" Is indeed working, so I guess we're back to the correct tach input...

When you say the tach bounces - would that be right at key-on? In my NA it moves slightly before I crank, as it gets switched 12v.

Tom M'Guinn

Not sure you saw the video. But here it is. The bolt is connected to ground. Power and ground are hooked up to the tach. By dragging the tach input over the bolt, you can make a crude ground pulse to test the tach.

https://www.youtube.com/watch?v=CL82MxwqXbI

odonnell

Yes I read the entire thread you linked and saw the video. I'm positive that nothing is wrong with the tach, it was perfect right before the standalone. There's no reason why it would have gone bad? We'll do more tests I guess.

Tom M'Guinn

Well, I suggested the test really as a way of making sure it was getting power. I does seem unlikely that the tach suddenly stopped working, although without knowing exactly what kind of voltage/signal the MS was sending it, there's always the (hopefully remote) chance you fried it.

Try the test -- it's super easy. Just stuff a wire into the green/black wire on the non-DME side of that red connector, then drag the other end of the wire across any grounded bolt thread. With the ignition on, the tach should respond like my video.

How sure are you that the MS signal is tach-ready? I just poked around on a MS site, and it shows various conditioning circuits to adapt the MS output to various kinds of tachs...

http://www.msextra.com/doc/ms1extra/...al.htm#layout3