Hello all, I'm working on adapting the aftermarket engine management in my '84 S3 to power the stock gauges. I need to know the following:

What type of signal drives the stock tach? Is it a typical 12V square wave tach signal? Is the tach looking for a 6, 8, or 12 cylinder signal?

What type of signal drives the stock speedo? Sine or square wave? What PPM? Which wire is the speed signal on the back of the speedo?

Finally, is it possible to upgrade the bulbs in the stock instruments to LEDs? The original bulbs are very dim at night and hard to read. If so, what type LED?

 

I am using a mid-90's LT1/4L60e engine/trans and went with a Dakota Digital SGI-100BT: Universal Speedometer and Tachometer Interface to drive the speedo and tach. The instructions for setup and adjustment are very good, I believe the output of my trans is 4000ppr and the Jag speedo is looking for 8000 pulses. This interface has connections for this and other combinations. The tach is even easier as I remember, my PCM tach output went to a connection on the interface and the output to the Jag tach was setup and adjusted onboard the interface. Have not had any problems in two years. I don't have any affiliation with Dakota Digital.
P.S. This interface is completely adjustable for tire size, gear change, etc. for the speedo.

Jag Specialties may have something that helps also.

PP.S. I also changed the bulbs to LED's in all the dash gauges, Classic Cars LED in the UK has excellent selection and shipping and delivery time was cheaper than some of the vendors I checked in US. Again, no affiliation.

Dave

 

That tach's are different between the 6 and 12 cylinder cars, so I assume that the signal they are expecting to see is different. There is power and ground going into the tach and a wire connected directly to the negative side of coil for the speed signal.

So whatever signal is present at the coil is input to the tach, I'm sure you could put an oscilloscope on it and see what the waveform is.

 

I should have thought to message you Dave, as you have a similar setup to mine. I have a 100BT on the way now, should be here to experiment with on the weekend. Previously I did actually hook up the tach to the output from the Holley, but scaling was wrong, so I'm hoping the Dakota can take care of it.

Unfortunately the coil and the original 6 cylinder engine are long gone - replaced with a 6.0 V8 LS. I don't have access to a scope, either...

 

I had to use an adapter for Speedo, similar to Dave’s, but mine didn’t do tach too, on Jaguar specialty website there are tips and one is how to convert the tach, I want to say I had to solder ina a 330 ohm resistor, can’t remember off hand, also used their trick for my check engine light! I’m running lt1 as well!

 

Thank you, I already soldered in the pull-up resistor per the Jag Specialties website, but the calibration seems to be incorrect. Hopefully the DD converter fixes that!

 

Ok, still haven't got it nailed down yet. I got the SGI100BT installed and am having trouble getting the tach working. To verify that the actual tachometer itself works, I hooked the tach signal from the Holley directly up to the tachometer (no SGI100BT). It works and reads rpm (although low). Keep in mind I have the 330ohm resistor soldered in per the following link Jaguar Specialties: Tech Tips 3

With the SGI-100BT installed inline, the tach doesn't work. It has a good 12V switched power source and ground and turns on with the ignition. When I hold the SET button with the engine running it shows a number bouncing between 048 and 052, which rises with RPMS. I assume that means it has a good input signal from the Holley. I confirmed this by verifying that I have a 14V AC signal on the input wire from the Holley. However, the tach still does not work. I tried both the Normal and High Voltage outputs per the diagnostic guide but didn't have any luck.

So in summary, I know I have a good input signal and a good tachometer.

The only thing I can think of is a setup error that I'm missing, or that the tach has a 330ohm pull-up resistor soldered in.
Dave, do you recall which output settings you used on your unit?

 

Electronics are the bane of my existence, but I'll try to remember and pass on what I have in my records.

Tach: In my case ('94 LT1/4L60e) RED 13 (wht) is the tach output from the PCM. I connected that to the INPUT of the 100BT and connected the WSU (white/slate/blue) Jag tach wire to the NORMAL (output) on the 100BT. I honestly don't remember doing any adjusting for the tach, I had the PCM tuned for 750rpm in neutral and 650rpm in gear and that's what the tach read with the above connections. The speedo was more complicated, but you say that turned out OK. I am including a copy of speedo instructions for setup and adjustment without the car moving, might be of some help. You may have already done this, but if you look at the tutorial on the DD website it shows the steps to adjust the tach.

I'm sorry, but that's all I have on this issue. One thought that comes up and you mention, maybe the 330ohm resistor (that you added?) needs to be removed when using the 100BT. I didn't follow the Jag Spec. suggestion. Disregard the 100BT link, doesn't work.
SGI-100BT TESTING
On 6/30/2020 9:55 AM, Tim Jorgenson wrote
Dave,
You can always do the "rEF" test signal out on OUT1 to verify your speedometer responds.

To test the outputs and speedometer reading while sitting still:

1. Begin with the key off.

2. Press and hold the SET switch while turning the key on. The display will show “ - ”.

3. Release the SET switch. The display will show “SPD”.

4. Tap the SET switch and then tap the INC switch until “ref” is shown.

5. Tap the SET switch. The display will show “33” and be supplying a 6.3VAC, 33Hz signal on OUT1 and OUT2 (O.C.) level depends on P-U volts.

6. Tap the INC switch to select “33”, “67”, “133”, “267”, or “533”. The output signal will change immediately.

7. Tap the SET switch or turn the key off to turn the output signal off.

8. Note what frequency & indicated speed on gauge that works on OUT1 or OUT2 then plug the values into this formula to find what PPM the ECU or gauge wants for input PPM: (Hz x 3600) / MPH indicated = PPM (ECU/GAUGE)



Thanks Kindly,

Tim Jorgenson (TJ)

Tech Support

Dakota Digital, Inc

Email: TimJ@dakotadigital.com

O.C. = OPEN COLLECTOR (OUTPUT TRANSISTOR IS TURNED OFF UNTIL IT GETS DC VOLTS FROM DESTINATION DEVICE. THE OUTPUT IS CONTROLLED BY THE DC VOLTS THE DESTINATION DEVICE FEEDS IT. IF YOU GIVE IT 5VDC THE OUTPUT WILL BE APPROXIMATELY 5V PULSES, 12VDC AND PULSES WILL BE 12V PULSES BUT RMS ON THE METER SHOWS AVERAGE SO AC VOLTS ON A METER WOULD SHOW ABOUT 7 VOLTS AC WITH 12VDC COMING IN.

P-U = PULL UP RESISTOR. RESISTOR TIES BETWEEN INPUT AND + VOLTS POWER (USED TO ACTIVATE AN O.C. OUTPUT OR TO LIFT A WEAK SIGNAL UP INTO VIEW SO IT CAN BE "SEEN").

AC = ALTERNATING CURRENT (TYPICAL FOR HOUSE POWER OUTLETS AND LOW VOLTAGE SIGNALS SUCH AS SPEED AND TACHOMETER SIGNALS)

PPM = PULSES PER MILE (MEASUREMENT OF SPEED SIGNAL)


Thanks Kindly,

Tim Jorgenson (TJ)


Dave

 

After putting the kids to bed, I spent Saturday evening tinkering with the tach. I snipped out the resistor and resoldered those leads, returning the tach to OEM spec. Just for testing purposes, I left the tach loosely installed with the signal wire hanging loose.



I fired up the engine and was met with a working tach! After a few minutes comparing the RPM readout on the Holley handheld to the analog tach, I used the Dakota Digital app to apply a correction factor and bring them in line. Success!

 

I'm glad you found the solution, I'm usually still mystified when something electronic works for me.

Dave