Factory tach to micro squirt tach out

[Drscottsmith]

For you m/s guys…

I am trying to get my tach connected. I have the tach output wire from the MS and understand that it is to connect to the green/black in the red 4 pin in passenger footwell.

I think I read somewhere that a second wire needs to be connected, but not sure which one. I also saw something about the tach needing power, but the cluster does have power as other gauges work.

As an aside, on my test drive the other evening, I don’t believe the Speedo was working - does something send a pulse to that as well that I have removed?

Thanks all!

-scptt

[Tom]

There is a red/green wire in that same red 4-pin connector. That wire needs to be grounded for the boost gauge to work. I'm 80% sure the same wire is ground for the tach. No harm grounding it to check as it is designed to be grounded for the boost gauge. If that doesn't work, I'll look it up for you. By the way, our Speedobooster is an easy way to drive the boost gauge when using a stand-alone.

The speedometer is driven by a sensor in the transmission. It sends ground pulses to the speedometer. Can't really think of anything related to a stand-alone that would affect the speedometer itself...

[cda951]

MS tach output wire to factory green/black wire as you mentioned should work.

The 944/951 is a great car to convert to standalone engine management, because aside from all the usual benefits, any auxiliary systems such as A/C, cruise control, speedometer, ABS, etc will all work without modification (well, might have to add an A/C input to the MS for idle control) because they are independent from the factory DME system, which is not the case by the time CAN-bus inter-module communication became the norm in the later 1990s

I was talked into a nightmare project at my shop a few years ago, a 2001 BMW E38 740i with a GM crate LS and 6L80e auto transmission swap, the owner wanted to get the factory HVAC and A/C to work, along with the gauges if possible . . . . it ended up being far more work than I ever could have envisioned, the slightly later E39 and E46 BMWs use CAN for such functions and aftermarket converters are readily available to run the factory gauges and HVAC with an LS swap----E38 uses BMW's older K-bus protocol for such things, I simply could not satisfy the stock HVAC control module's requirements to merely supply +12V to the A/C compressor, even after wiring in the old BMW DME and simulating all the signals required for A/C compressor clutch activation per the factory manual . . . . I ended up installing an external circuit with evaporator core temperature feedback just for A/C compressor clutch control, all other factory HVAC functions were maintained, had to get HP Tuners and some aftermarket fan control harnesses to inform the GM ECU of A/C activation, and got the gauges working. UGH.

Back to 951-land, a curious aside is that ever since I converted my 951 to VEMS engine management and connected the factory KLR manifold reference hose to the built-in MAP sensor of the VEMS ECU, my factory boost gauge reads gauge pressure instead of absolute pressure. I have never bothered to investigate why this is . . . .

[Drscottsmith]

Ok - question based on responses you guys provided (thank you as well!)

So my understanding was that the Tach Out wire from MS went to the Green/Black wire in the 4-pin connector, which Tom verified and then added that the Green/Red wire needed grounding.

CDA - you mentioned that the Tach Out wire needed to go to the Green/Red wire.

Just wanted to double check. I can verify I did not get anything on the tach with TachOut connected to Green/Black and the Green/Red wire grounded.

Thanks!

[Tom]

So I looked it up to be sure. The Green wire with Black stripe carries the tach signal to the tach. It looks like the Green wire with Red stripe only supplies ground to the boost gauge, so should be irrelevant to the tach. Be sure you are connecting your tach signal to the side of the connector that runs to the gauges. The other side runs back to the DME if you still have the factory engine harness. Assuming that's ok, my guess is you need to check the configuration settings in you ECU for the TachOut signal. Do all the other gauges work (to rule out a grounding problem)?

[Drscottsmith]

Success! I had marked the wire wrong - was trying to use the boost gauge output from the arduino for the tach - all fixed now!

[cda951]

Ok - question based on responses you guys provided (thank you as well!)

So my understanding was that the Tach Out wire from MS went to the Green/Black wire in the 4-pin connector, which Tom verified and then added that the Green/Red wire needed grounding.

CDA - you mentioned that the Tach Out wire needed to go to the Green/Red wire.

Just wanted to double check. I can verify I did not get anything on the tach with TachOut connected to Green/Black and the Green/Red wire grounded.

Thanks!

Sorry, I edited my post, was going from memory about the stripe color of the factory tach signal wire, should be green/black as @Tom mentioned!