Getting Tach to work with LS RPM Feed

[PDC]

I’ve got an LS9 in my 1973 Trans Am. Shop that did the install wired up the original gauges to work with the senders and bulkhead outs for the Crate LS9. This is the info I found for making the stock tach work with the LS signal:

“A tachometer signal is included in the bulkhead connector (see below). This is a 2 pulse/rev output which may correspond to a 4-cylinder setup in some tachometers or transmission controllers. Note the signal is a low voltage square wave, some tachometers or transmission controllers may need a pull-up resistor in order to read the signal, similar to a 5000 ohm, 1⁄4 watt resistor– this detail is left to the user. The following circuit has worked for numerous devices – the resister value may need to be changed if your device does not read this output properly“

This was done correctly, as the tach does read the correct RPM. But here is my problem: If I roll on the throttle gradually to medium, the tach ready and climbs appropriately. But if I roll on the throttle quickly, the tach will bottom out to 0. It will come back to life when I let off the throttle and then it will read appropriately as long as I am applying gradual throttle. But it will bottom out to 0 again the next time I roll on the throttle too quickly.

Any ideas on where I might start to get the tach to follow what’s going on during more ‘enthusiastic’ throttle application?


Problem 2 - my fuel gauge works, but the tank is genuinely Empty when the gauge reads 1/2 (ask me how I know). At legit 1/2 tank, the gauge reads 3/4. So as long as you know this and fill up when the gauge gets between 3/4 and 1/2 you are fine. I’d like it to read correctly. Any ideas on how to get the fuel gauge to work through the full sweep?

Thank you.

[dhutton]

Tough to diagnose without a scope but try 1k and then 10k for the pull-up resistor.

On the fuel gauge it sounds like your sender is the wrong resistance. I think your Trans Am would use a 90 ohm sender.

Don

[PDC]

Tough to diagnose without a scope but try 1k and then 10k for the pull-up resistor.

On the fuel gauge it sounds like your sender is the wrong resistance. I think your Trans Am would use a 90 ohm sender.

Don

Thank you very much. After reading your reply, I went on a deep dive reading as much as I could about resistors. Electronics and wiring are by far my weakest subject. I can compound and correct paint with the best of them, and given time, I can hog-ring upholstery, hang door panels, and install headliners. But electrical …

I did find this online and wanted to make sure I’m reading it correctly:


“A rule of thumb is to use a resistor that is at least 10 times smaller than the value of the input pin impedance. In bipolar logic families which operate at 5 V, the typical pull-up resistor value is 1-5 kΩ. For switch and resistive sensor applications, the typical pull-up resistor value is 1-10 kΩ. If in doubt, a good starting point when using a switch is 4.7 kΩ. The disadvantage when using a larger resistance value is that the input pin responds slowly to voltage changes. In high-speed circuits, a large pull-up resistor can sometimes limit the speed at which the pin can reliably change state.”

This sounds a lot like what I am seeing on the tach - as long as I am changing rpms gradually, the tach can keep up. When I change rpms too abruptly, the tach cannot keep up and drops to 0. Does this symptom suggest that I may actually need a lower value resistor as opposed to a higher value?

Or should I go up in value because the ‘switch’ in my scenario is my tach which was designed to operate at 12 volts - so 10k ohm resistor???

Thank you again. I’m not sure how to fix this yet, but I am sure that I have a better functional idea of the issue than I had yesterday.

[blitzer454]

Go lower than what is presently provided. If a 5k is currently being used then I would try a 1k. If there is no resistor then use a 5k.

[dhutton]

Thank you very much. After reading your reply, I went on a deep dive reading as much as I could about resistors. Electronics and wiring are by far my weakest subject. I can compound and correct paint with the best of them, and given time, I can hog-ring upholstery, hang door panels, and install headliners. But electrical …

I did find this online and wanted to make sure I’m reading it correctly:


“A rule of thumb is to use a resistor that is at least 10 times smaller than the value of the input pin impedance. In bipolar logic families which operate at 5 V, the typical pull-up resistor value is 1-5 kΩ. For switch and resistive sensor applications, the typical pull-up resistor value is 1-10 kΩ. If in doubt, a good starting point when using a switch is 4.7 kΩ. The disadvantage when using a larger resistance value is that the input pin responds slowly to voltage changes. In high-speed circuits, a large pull-up resistor can sometimes limit the speed at which the pin can reliably change state.”

This sounds a lot like what I am seeing on the tach - as long as I am changing rpms gradually, the tach can keep up. When I change rpms too abruptly, the tach cannot keep up and drops to 0. Does this symptom suggest that I may actually need a lower value resistor as opposed to a higher value?

Or should I go up in value because the ‘switch’ in my scenario is my tach which was designed to operate at 12 volts - so 10k ohm resistor???

Thank you again. I’m not sure how to fix this yet, but I am sure that I have a better functional idea of the issue than I had yesterday.
That is why I suggested 1k first. 10k was just a Hail Mary shot if 1k doesn’t work….

Try not to over think this…. You’ve got two electronic engineers helping you here… :)

Don

[79 Camaro]

Back in the day when I was line tech at Chev dealership. When tested gas gauges we had a black box that we hooked to the wire that goes from the tank to the gauge. The black box had a dial on it and went from empty to full. You turned the dial and the fuel gauge should move to match. If it did your tank sender is toast.
What are you using for a tank and sender? On my Tank's Inc tank the fuel level sender is adjustable.

[Hotwire]

Are you using stock gauges or aftermarket? Do you have the fuel level sending unit (potentiometer) going through the ECM before gauges or going straight to the gauges? (PCB and IC layout engineer :bananna2:)

[PDC]

Autometer to the rescue:

https://www.autometer.com/ls-plug-play-harness-with-tach-adapter.html?gclid=EAIaIQobChMIx8WTp8CR-QIV3mxvBB2hwgSnEAQYAiABEgLaGvD_BwE&gclsrc=aw.ds

I’m going to start here and then see if a simple ground (or possibly a sending unit) will have my fuel gauge working correctly.

As far as the fuel gauge - it is the original GM gauge - along with the rest of the dash cluster. Temp, oil pressure, volts and speedometer all work. Tach is jumpy - but Im betting that will be a straight-forward fix. Fuel sweeps form 1/2 to full instead of empty to full. The LS conversion in this car was finished before I purchased it, so I am learning along he way what was (and was not) done during the install. I am betting the ‘easiest’ thing to get the factory fuel gauge working was to bypass the ECM - which might actually be easier to trace and trouble-shoot?

[dhutton]

No interface on the ECM for a fuel gauge as far as I know. It is always connected directly to factory gauge. I’m going to guess you have the wrong sender as I suggested above. A bad ground doesn’t explain the consistent half sweep imho. It’s possible but not likely to be so consistent.

That autometer tach setup is like using a sledge hammer to kill a gnat….

Don

[PDC]

No interface on the ECM for a fuel gauge as far as I know. It is always connected directly to factory gauge. I’m going to guess you have the wrong sender as I suggested above. A bad ground doesn’t explain the consistent half sweep imho. It’s possible but not likely to be so consistent.

That autometer tach setup is like using a sledge hammer to kill a gnat….

Don

I have amassed an impressive collection of sledgehammers over the years. All sizes and weights.

[PDC]

Quick follow up. Floatless Sender from Tanks Inc has the fuel gauge working properly from empty to full. Looks like the arm and float on the sender was contacting one of the baffles in the tank preventing it from full range of motion.

Dug under the dash to trace the tach wiring and discovered I already had an Autometer tach box under there. Ground was sketchy. Cleaned and snugged the ground and all is good. Tach is rock steady from idle to redline.