How To: Control factory gauges with MS3X (FC)
 

Hi everyone,

I've been busy bench testing my MS3X and preparing to wire it into my '88 TII.
One of the small details I hated about my MS2 setup was not having functioning temp and oil pressure gauges in the factory cluster.
Because the MS3X has generic 0-5V inputs, I am adding fuel, oil, and water-meth pressure transducers.
I want to use the oil pressure transducer and the CLT sensor to to power the factory gauges.
I'd like to use the stock boost gauge, but it's range is limited for anything but stock boost pressures.

I've read on MSextra.com that Generic PWMs can be used for controlling resistant-type stock gauges. Below are my results.
For my testing, I used spare MS3X board injector drivers. I'm honestly not sure what the current draw from one of the gauges is, but the single power wire for the whole cluster and instruments is only approx 20AWG.

Disclaimer: Perform at your own risk. These values and methods worked for me, but your experience may vary.


Tach
The tach is probably the easiest to set up. Run a spare output wire like TACH (pin 26) to the Blue/Yellow wire at the back of the cluster. Set up the tach settings in Basic Settings > Tacho Output. Turn Tacho output on, select your output (TACH in my case) and if your general settings for a rotary are correct, it should drive the stock tach perfectly.

Stock Coolant Temp (S4 values)
The stock coolant gauge uses a Yellow with White stripe wire that can be found at the back of the gauge cluster or under the hood if that wiring still exists.
I wired a spare injector wire directly to the cluster.
I used InjG and built a PWM table in Advanced Engine > Generic PWM Output A
I played around with the min and max duty cycles to get the best results. Here are what I found to work:

Freq: 100hz
Off: 0% DC
First bar: 15% DC
Bottom of arc: 20% DC
Middle of arc: 50% DC
Top of arc: 75% DC
Top bar: 80% DC

To set this up, you would build a table similar to what's attached below.
You can set the scaling however you like. If you'd like the full sweep of the arc to only be 20*F or 200*F, it is possible.

Stock Oil Pressure (S4 values)
The stock OP gauge uses a Yellow with Red stripe wire that can be found at the back of the gauge cluster or under the hood if that wiring still exists.
I used Generic PWM 2 for oil pressure. Same process as setting up the CLT.
I wired a spare injector wire directly to the OP cluster wire.
For Oil Pressure, I found that the window of duty cycle was much tighter. I played around with adding resistors and found that it added a lot of lag to the reaction times.
With no resistor, below are my values:

Freq: 11.1hz
0psi: 0% DC
30psi: 4% DC
60psi: 12% DC
110psi: 18% DC

The oil pressure gauge was still rather slow to react. I'm not exactly sure how it compares to the stock gauge as it never really worked on my car. Either way, the gauge would at least work as a reference gauge to let you know all is well.

Stock Boost Gauge (S4)
For Boost, the stock sensor uses 2 wires on the cluster (Blue/Wht & Black/Wht). One is a ground and one is power, I'm honestly not sure which is which. I was able to PWM one to ground with an injector output and connect the other to voltage and get a usable range at different %DCs. The problem I found with the boost gauge is that it is very noisy. The higher the freq, the better, but it still made a lot of noise, I plan on swapping out my gauges for S5 NA for higher RPM tach and a volt meter, so I didn't do any more testing.

I hope this helps everyone and let me know if you have questions.

Good information! I'm thinking of a similar use for one of the spare outputs on my Cosmo to drive the factory ammeter as a boost/vacuum gauge.

But I'm a bit confused. On the FC, all of the gauges but the boost gauge are independent of the ECU. The tach runs off the trailing coil, the oil and coolant have their own sensors with connections on the engine harness. Was it just a matter that you removed all the existing wiring for your install?

As for the noisy boost gauge, try adding a capacitor between the signal and ground of the boost gauge. Go for 3.3uF and see if that dampens the jitters. Might even take a much lower value if the impedance of the gauge is very high. A 1uF capacitor with a 1K resistor in parallel with it.

That's true, but I'm switching to 4 spark outputs and D585 coils for ignition and I have an S5 engine with S4 gauges, so the coolant temp sender is not longer compatible. I believe the late '88 cars came with a 1/8" BSPT sender that is compatible with S5 rear iron and S4 gauges. All other S4s use an M10 sender and S5 use 1/8" BSPT. Mostly, I just don't have the stock wiring anymore. Whatever was in the engine harness is long gone, and my drivers side harness was pretty cracked and many connectors were missing.

I am planning on testing the stock oil sender to see if the reaction time is much better than the PWM output. The latency of the CLT I can live with but I'd like to have a reactive OP gauge. Either way, I have aftermarket gauges in the center stack that are much more accurate. I mostly don't want the stock gauges to be dead lol.


Thanks, that's good info. Adding a resistor did help dampen it, but I'm not sure how much hassle the boost gauge is worth. I did test it as a meter for water-methanol duty cycle and it seemed to work well, but I think I'd rather just have a volt meter from an NA car.

Glad to see after all these years, you're still in the game, Shaine.

Hey, man! How's it going? It's been a while (ten years?). I still remember giving you a ride in my beat 86 Sport with blown shocks and a bad diff mount lol

The stock oil pressure gauge is fairly heavily damped, like the coolant an fuel gauge. I believe the boost/vac and voltage are the only servo style gauges. All others are thermal strips.

The thermal gauges respond to PWM very well. The servo style gauges, as you have discovered, giggle because their reaction time is much faster.

I've been well man! Haha yea I remember that, from the sound of it, it has come a long ways.