71 firebird ground issues

[71LS5Formula]

Here's a fun one. Lights off. I ohm from the engine to the battery and its less than .5 ohms. Turn tube lights on and it literally jumps to 7.2 ohms. Anyone else run into this before? This is all on the ground side.

[dontlifttoshift]

You can't check resistance when there is current flowing through a circuit.

[71LS5Formula]

I'm going from the intake/engine to the ground side of the battery. They resistance is jumps up when the lights are turned on.

[dontlifttoshift]

Right, what I said still stands. When you measure resistance with a digital volt/ohm meter, it sends a small current through the circuit, if you add current to that circuit your reading is meaningless.

You have proven that something in the lighting circuit is chasing a ground through the engine block back to the battery. It could be anything, dash lights, idiot lights, headlights, etc. That is caused by a weak ground somewhere in the system.......it could be inconsequential.

What problem are you trying to solve?

[71LS5Formula]

I'm having problems with my fitech system. For some reason it's picking up rpm noise. I check the tach signal wiring for the system and its setup liked it's needed. Away from other wiring. The other way it can pick up rpm noise is it's having a grounding issue.

[rohrt]

How do you know its picking up rpm noise? What is the issue.
In some cases bad regulation of the alternator can cause some A/C ripple but you would need a scope to see that.
If you have a radio in the car do you hear noise through the speakers?
Also most electronics like radios will have their own voltage regulation circuit.

If you suspect bad grounding you can always add more grounds to test the theory of a bad ground. Use some jumper cables from the battery to the intake and see if the issue is still there or not.
You should have a few factory ground straps from the motor the firewall and to the frame.

[71LS5Formula]

I'm having problems with my fitech system. For some reason it's picking up rpm noise. I check the tach signal wiring for the system and its setup liked it's needed. Away from other wiring. The other way it can pick up rpm noise is it's having a grounding issue.

[rohrt]

Im not a fitech expert but I think it would help to know what your seeing that make you say it picking up rpm noise.

If it is related to the Alternator you could disconnect the belt and try running off the battery.
And like I said you could run some extra ground to the carb base the (-) of the battery.

[71LS5Formula]

Sitting in the car while idling you can watch the tach start to bounce out of the norm. Watch the handheld that came with the fitech and you can see the rpms jump to around. It hits the rev limiter at 5000, starts to do what it's suppressed to and cut the engine off. Even gives three code for rpm noise.

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Even gave codes for bad grounds

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But everything seems to be checking out

[dhutton]

Do you need a pull-up resistor? Where are you getting the tach signal from? Post a link to the manual if you have one.

Are you supplying a 12 volt signal to an input looking for a 5 volt signal?

Don

[71LS5Formula]

Tach signal is from the tach slot on an hei distributor. There are 2 12 volt feeds to run a fitech. One is from the battery direct. The other is from the fuse box for key on. What happened is i took the car out for shake down run for about 50 miles. Before that i would be making short trips around town or to and from work. Worked perfectly. While on the shaker down trip the car the car performed perfectly. I made a stop at a store so they car sat for about 10 or 20 minutes in the parking lot. Come back out and start the car. Fires up like normal. Start to drive off and starts having it's problems.

[dhutton]

Tough to troubleshoot without a scope on the tach signal but I would be tempted to try one of these:

http://www.speedwaymotors.com/Classi...lter,5613.html

Don

[rohrt]

Isolate the issue the best you can. I would be tempted to throw the carb back on with all the FiTech connection disconnected and see if the tach is still bouncing.
Again very easy to run more grounds to see if that helps.

[71LS5Formula]

Can't run a carb. I have an intank pump setup. I'm waiting oin fitech too wire me back to see what they think.

[dhutton]

Try the filter I listed....

Don

[rohrt]

get creative. slap the carb back on. disconnect the fuel pump and all the FiTech stuff. Plug the carb inlet and fill the carb from the vent hole with gas. It will run for a few seconds. Should tell you if the tach is still bouncing.

I would assume the FiTech is not the issue but who knows. That is why you troubleshoot things. Could be some loading issue. Could be crossed wires, Could be a bad HEI, etc...

[71LS5Formula]

Plan is to disconnect my tach wire tonight to see if that fixes it.

[71LS5Formula]

Disconnected the tach signal wire to the dash and it makes no difference. Waiting on fitech again.

[71LS5Formula]

Well. I sat and watched the handheld yesterday. The problem starts when the engine hits 140 degrees.

[dhutton]

Is this the system where all the electronics are in the throttle body?

If it is it is possible that the logic thresholds are moving as it heats up and you are getting reduced noise margin on the tach signal. Still think you should spend $30 on that filter I posted. Install it between the distributor and ECM....

Don