DME Tach signal characteristics

[zooklm1]

@CarBot
I am trying to connect an Innovative Motorsports MTX-D boost gauge that has a shift indicator based on engine RPM. I have the boost gauge side set up but struggling to get the tach side set up. This is for a 1986 944 turbo. What are the DME signal output characteristics and are there specific settings for the MTX-D that should be used for the 944?

[CarBot]

I am trying to connect an Innovative Motorsports MTX-D boost gauge that has a shift indicator based on engine RPM. I have the boost gauge side set up but struggling to get the tach side set up. This is for a 1986 944 turbo. What are the DME signal output characteristics and are there specific settings for the MTX-D that should be used for the 944?

On an ’86 944 Turbo the easiest, cleanest RPM signal for an aftermarket gauge is the factory tach signal (TD) rather than trying to read anything “directly” off the DME internals.

The TD signal is a 0–12V square-wave style tach output (not a high-voltage coil negative signal like older cars). Frequency is proportional to RPM and it’s intended to drive the cluster tach, so most aftermarket RPM inputs that can accept a 12V tach signal will read it fine.

Where to grab it: at the DME harness you want the TD/tach output wire, or you can pick it up behind the instrument cluster at the tach input. I don’t want to guess a pin number or wire color without knowing which harness/diagram you’re looking at, because Porsche did some year-to-year changes and it’s easy to land on the wrong cavity. If you tell me whether you’re tapping at the DME connector under the passenger footwell or at the back of the cluster, I can point you to the usual spot people use on the 951.

MTX-D settings: set it for a conventional tach input (12V square wave / ECU tach / “tach signal” mode), not “coil” mode. Then set cylinder count to 4-cylinder and use whatever “pulses per rev” option gets you a correct reading; many devices default to something like 2 pulses/rev for a 4-cyl wasted spark, but the Porsche TD output doesn’t always match those generic assumptions. The quick way is to compare the MTX-D RPM to the factory tach at idle and at, say, 2500–3000 rpm and adjust the PPR/cyl setting until they match.

If you’re seeing no RPM at all, the common gotchas are a poor ground reference for the gauge, tapping a wire that’s actually the speed/reference sensor line (VR sine wave, not what the MTX-D wants), or having the MTX-D set to “coil negative” which it won’t like on the 951.

If you reply with where you’re tapping (DME plug vs cluster) and what the MTX-D menu options are for tach input (screenshots or exact wording), I can tell you which mode is the right one and what to try for the pulse/cylinder setting.

[zooklm1]

I am trying to connect an Innovative Motorsports MTX-D boost gauge that has a shift indicator based on engine RPM. I have the boost gauge side set up but struggling to get the tach side set up. This is for a 1986 944 turbo. What are the DME signal output characteristics and are there specific settings for the MTX-D that should be used for the 944?

On an ’86 944 Turbo the easiest, cleanest RPM signal for an aftermarket gauge is the factory tach signal (TD) rather than trying to read anything “directly” off the DME internals.

The TD signal is a 0–12V square-wave style tach output (not a high-voltage coil negative signal like older cars). Frequency is proportional to RPM and it’s intended to drive the cluster tach, so most aftermarket RPM inputs that can accept a 12V tach signal will read it fine.

Where to grab it: at the DME harness you want the TD/tach output wire, or you can pick it up behind the instrument cluster at the tach input. I don’t want to guess a pin number or wire color without knowing which harness/diagram you’re looking at, because Porsche did some year-to-year changes and it’s easy to land on the wrong cavity. If you tell me whether you’re tapping at the DME connector under the passenger footwell or at the back of the cluster, I can point you to the usual spot people use on the 951.

MTX-D settings: set it for a conventional tach input (12V square wave / ECU tach / “tach signal” mode), not “coil” mode. Then set cylinder count to 4-cylinder and use whatever “pulses per rev” option gets you a correct reading; many devices default to something like 2 pulses/rev for a 4-cyl wasted spark, but the Porsche TD output doesn’t always match those generic assumptions. The quick way is to compare the MTX-D RPM to the factory tach at idle and at, say, 2500–3000 rpm and adjust the PPR/cyl setting until they match.

If you’re seeing no RPM at all, the common gotchas are a poor ground reference for the gauge, tapping a wire that’s actually the speed/reference sensor line (VR sine wave, not what the MTX-D wants), or having the MTX-D set to “coil negative” which it won’t like on the 951.

If you reply with where you’re tapping (DME plug vs cluster) and what the MTX-D menu options are for tach input (screenshots or exact wording), I can tell you which mode is the right one and what to try for the pulse/cylinder setting.

The gauge has a default 0-5v input range. It also has a 4cyl/2pulses setting. Will these work for the 944? I am taking the signal from a diagnostic connector in the passenger foot well that connects to the DME tach output pin.

[CarBot]

The gauge has a default 0-5v input range. It also has a 4cyl/2pulses setting. Will these work for the 944? I am taking the signal from a diagnostic connector in the passenger foot well that connects to the DME tach output pin.

The 4cyl/2 pulses setting is the right place to start for the 951 tach output, but the 0–5V input range is usually not.

The DME TD/tach output on these cars is typically a 0–12V square wave meant to drive the factory tach. If the MTX-D is set up expecting only a 0–5V logic signal, it may not read it correctly (or it may clamp it internally and get flaky). If the MTX-D has an option for a 12V tach/ECU input (sometimes called “tach signal,” “ECU,” or “12V square”), use that instead of the 0–5V range.

Since you’re pulling it from the diagnostic connector in the passenger footwell, you’re on the right signal. Next things I’d check are that the MTX-D tach input shares a solid ground with the DME/chassis, and that the MTX-D is not in “coil negative” mode.

If you can, put a meter or scope on that TD pin while cranking/idle. You should see a pulsing signal that swings up near battery voltage. If you tell me the exact MTX-D tach input menu choices (the wording Innovative uses), I can tell you which one matches the Porsche TD output best.

[zooklm1]

The gauge has a default 0-5v input range. It also has a 4cyl/2pulses setting. Will these work for the 944? I am taking the signal from a diagnostic connector in the passenger foot well that connects to the DME tach output pin.

The 4cyl/2 pulses setting is the right place to start for the 951 tach output, but the 0–5V input range is usually not.

The DME TD/tach output on these cars is typically a 0–12V square wave meant to drive the factory tach. If the MTX-D is set up expecting only a 0–5V logic signal, it may not read it correctly (or it may clamp it internally and get flaky). If the MTX-D has an option for a 12V tach/ECU input (sometimes called “tach signal,” “ECU,” or “12V square”), use that instead of the 0–5V range.

Since you’re pulling it from the diagnostic connector in the passenger footwell, you’re on the right signal. Next things I’d check are that the MTX-D tach input shares a solid ground with the DME/chassis, and that the MTX-D is not in “coil negative” mode.

If you can, put a meter or scope on that TD pin while cranking/idle. You should see a pulsing signal that swings up near battery voltage. If you tell me the exact MTX-D tach input menu choices (the wording Innovative uses), I can tell you which one matches the Porsche TD output best.

I am running the gauge ground to the battery ground. I will check on the ability to change the input voltage and report back. If I can assign the voltage to 0-12, should I use 6500 rpm as the speed at 12v?

[CarBot]

I am running the gauge ground to the battery ground. I will check on the ability to change the input voltage and report back. If I can assign the voltage to 0-12, should I use 6500 rpm as the speed at 12v?

No, don’t map it like “12V equals 6500 rpm.” The TD/tach line isn’t an analog voltage that rises with RPM. It’s a pulsed square wave where the frequency changes with RPM, and the voltage just swings between low and about battery voltage.

So if the MTX-D has a setting for 0–12V, that’s just the input amplitude range/threshold it can accept, not a calibration point. You’d still set it up using the cylinder/pulses-per-rev option and then verify the displayed RPM matches the factory tach at idle and a midrange steady RPM.

Battery negative is fine for ground, but if you still get weird readings, try grounding the MTX-D to the same chassis point the DME uses (or at least a nearby clean chassis ground). Sometimes long ground runs to the battery can add noise or offset that confuses tach inputs.

If you post the exact MTX-D tach setup screens (the names of the options), I can tell you which mode to pick for a 951 TD signal.

[Tom]

I am running the gauge ground to the battery ground. I will check on the ability to change the input voltage and report back. If I can assign the voltage to 0-12, should I use 6500 rpm as the speed at 12v?

No, don’t map it like “12V equals 6500 rpm.” The TD/tach line isn’t an analog voltage that rises with RPM. It’s a pulsed square wave where the frequency changes with RPM, and the voltage just swings between low and about battery voltage.

So if the MTX-D has a setting for 0–12V, that’s just the input amplitude range/threshold it can accept, not a calibration point. You’d still set it up using the cylinder/pulses-per-rev option and then verify the displayed RPM matches the factory tach at idle and a midrange steady RPM.

Battery negative is fine for ground, but if you still get weird readings, try grounding the MTX-D to the same chassis point the DME uses (or at least a nearby clean chassis ground). Sometimes long ground runs to the battery can add noise or offset that confuses tach inputs.

If you post the exact MTX-D tach setup screens (the names of the options), I can tell you which mode to pick for a 951 TD signal.

Use the green wire with black stripe on pin 21 of the DME, which goes through the bright red 4-pin connector in the passenger footwell and is stubbed out in the 12-Pin unused Kostal connector in the same area. It's a 5 volt digital (cpu-friendly) square pulse that drives the factory tach (not 12 volts). It is derived from the actual ignition pulse, but not a high current pulse.

[CarBot]

Use the green wire with black stripe on pin 21 of the DME, which goes through the bright red 4-pin connector in the passenger footwell and is stubbed out in the 12-Pin unused Kostal connector in the same area. It's a 5 volt digital (cpu-friendly) square pulse that drives the factory tach (not 12 volts). It is derived from the actual ignition pulse, but not a high current pulse.

Good detail on where to pick it up in the passenger footwell, and that’s the right general signal to use for the MTX-D.

On the voltage level, I’m not fully confident it’s always a strict 5V signal on every 951. In practice I’ve seen the tach/TD line described and measured both ways depending on where you tap it and what’s loading it. The safe approach is to treat it as a digital square-wave tach signal and set the MTX-D to whatever “ECU/logic/tach signal” input mode it has, not an analog 0–12V mapping.

So to your earlier question, you still should not set “6500 rpm at 12V” (or at 5V). That’s not how this signal works. You want the MTX-D configured for frequency-based RPM with the correct cylinder/pulses-per-rev setting, then verify against the factory tach.

If the MTX-D only offers a 0–5V tach input range, that may actually be fine if your tap point is the DME TD output as you described. If it offers both 0–5V and 0–12V, I’d start with 0–5V when using the DME TD pin, then switch to 0–12V only if you confirm with a meter/scope that the high level is near battery voltage.

If you can share the exact MTX-D tach input options (the words in the menu), I can tell you which one matches a DME TD square-wave best.

[zooklm1]

Use the green wire with black stripe on pin 21 of the DME, which goes through the bright red 4-pin connector in the passenger footwell and is stubbed out in the 12-Pin unused Kostal connector in the same area. It's a 5 volt digital (cpu-friendly) square pulse that drives the factory tach (not 12 volts). It is derived from the actual ignition pulse, but not a high current pulse.

Good detail on where to pick it up in the passenger footwell, and that’s the right general signal to use for the MTX-D.

On the voltage level, I’m not fully confident it’s always a strict 5V signal on every 951. In practice I’ve seen the tach/TD line described and measured both ways depending on where you tap it and what’s loading it. The safe approach is to treat it as a digital square-wave tach signal and set the MTX-D to whatever “ECU/logic/tach signal” input mode it has, not an analog 0–12V mapping.

So to your earlier question, you still should not set “6500 rpm at 12V” (or at 5V). That’s not how this signal works. You want the MTX-D configured for frequency-based RPM with the correct cylinder/pulses-per-rev setting, then verify against the factory tach.

If the MTX-D only offers a 0–5V tach input range, that may actually be fine if your tap point is the DME TD output as you described. If it offers both 0–5V and 0–12V, I’d start with 0–5V when using the DME TD pin, then switch to 0–12V only if you confirm with a meter/scope that the high level is near battery voltage.

If you can share the exact MTX-D tach input options (the words in the menu), I can tell you which one matches a DME TD square-wave best.

The MTX-D programmer shows a pressure tab to set the display, overboost warning and altitude compensation; a RPM tab to set mode ( number of cylinders/ pulses and scale, either 0-10230 or 0-20460; and a display tab to set needle bar RPM CONFIG, single stage shift light, set the progressive shift light and set boost trace. There does not seem to be a way to change the incoming signal or range.

[CarBot]

The MTX-D programmer shows a pressure tab to set the display, overboost warning and altitude compensation; a RPM tab to set mode ( number of cylinders/ pulses and scale, either 0-10230 or 0-20460; and a display tab to set needle bar RPM CONFIG, single stage shift light, set the progressive shift light and set boost trace. There does not seem to be a way to change the incoming signal or range.

If the MTX-D doesn’t give you an input type/voltage selection, then it’s basically expecting a logic-level tach signal and doing RPM from frequency. That’s good news for the 951, because the DME TD output you’re tapping (pin 21, green/black, via that red 4-pin and the unused Kostal stub) is the right kind of square-wave signal.

In the MTX-D RPM tab, use 4-cylinder and 2 pulses as your starting point. The “scale” choice (0–10230 vs 0–20460) is just the display range/resolution, not the input range. For a 944 Turbo with a stock-ish rev limit, pick 0–10230.

Then verify calibration by comparing MTX-D RPM to the factory tach at idle and at a steady mid-RPM. If it reads about double, halve the pulses setting (if it allows). If it reads about half, double it. If it’s close but not perfect, that usually points to a noisy pickup or a grounding/reference issue rather than the wrong cylinder count.

Since you’re using the diagnostic/footwell TD stub, I’d also make sure you’re not sharing that wire with anything else that could load it, and I’d ground the MTX-D to a clean chassis ground in the passenger footwell area (same general ground reference as the DME) rather than running a long ground back to the battery. The battery ground can work, but it’s a common source of offset/noise on frequency inputs.

If after setting 4cyl/2 pulses you still get no RPM at all, the next step is to confirm the MTX-D is actually seeing pulses on that wire with a meter that has Hz/duty function, or a simple LED test light made for logic signals, or ideally a scope. That will tell you quickly whether it’s a wiring/signal issue or just a configuration mismatch.