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.

- - - Updated - - -

Even gave codes for bad grounds

- - - Updated - - -

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/Classic-Instruments-SN20-Tach-Filter,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

[71LS5Formula]

Installed the filter and made things worse.

[dhutton]

Installed the filter and made things worse.

Can you tell us which wire was connected to what.

Should have been red wire to tach signal output on distributor, yellow to Fitech tach signal input and your tachometer, black to engine ground. Would be worth trying with and without your tachometer connected.

Do you have an ohmmeter? Could you measure the resistance between the red and yellow wires and the red and black wires with the filter not connected to anything.

Could you post a pic of your distributor and the wires to the Fitech unit.

Thanks,
Don

[71LS5Formula]

Red wire from distributor, yellow out to efi and tach and black ground.

[dhutton]

Red wire from distributor, yellow out to efi and tach and black ground.

Could you try it without the tach connected.

Thanks,
Don

[71LS5Formula]

I have before i installed the filter and the same results.

[71LS5Formula]

Could bad plugs cause this issue?

[dhutton]

Since the problem gets worse at hot I would not guess the plugs but I guess you never know. Next thing I would try would be to add an insulating spacer between the intake and throttle body to reduce the temperature of the electronics in the throttle body.

I don't know if you have room for something like this:

http://www.ebay.com/itm/66-72-Chevrolet-Cars-With-Holley-Carburetor-Heat-Shield-Between-Intake-Carb/351157865184?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Or maybe something like this:

http://www.ebay.com/itm/Carb-Carburetor-Spacer-1-2-Phenolic-Insulator-Open-Center-Holley-Carter-AFB-/391433904117?hash=item5b23475ff5:g:5BwAAOSwUUdXD84 g&vxp=mtr

A quick test to see if it would help would be to hit the electronics part of the throttle body with freeze spray to see if the problem disappears.

Would be good if you could post some pics of your setup.

Don

[71LS5Formula]

Can't use any spacers. Not enough room.

[dhutton]

Can't use any spacers. Not enough room.

What about the first one, it doesn't add much height at all.

Don

[71LS5Formula]

running a Pontiac 455 with the factory intake and ram air breather pan.

[dhutton]

If the Fitech throttle body fits I would have thought the first one could be made to fit. Sorry other than the other things I suggested above I am out of suggestions.

Is the heat passage under the throttle body plugged in the intake manifold gasket?

Don

[71LS5Formula]

No need to be sorry. Thank you for the advice that you have given. I'm gonna try plugs tomorrow. I did put a new control module in and coil.

[dhutton]

I think you need to lower the temperature of that throttle body somehow.

Don

[71LS5Formula]

right now as I'm trying to figure things out the hood is open and breather pan is off.

[dhutton]

What is the thermostat temperature? 180? If that thermal passage is open under the throttle body it will be heating it up. What about shooting the throttle body with compressed air to cool it to see if things improve?

[71LS5Formula]

160 degree thermostat. Aluminum intake. I think i need to try another distributor to eliminate it, but i don't have one.

[dhutton]

One more question. Mechanical or electric fans?

Don

[71LS5Formula]

Mechanical clutch

[71LS5Formula]

Riddle me this. Riddle me that. Why would disconnecting the vacuum advance correct my problems?

[dhutton]

No idea. Does it go away as soon as you disconnect the vacuum advance? Or does it take a while?

Don

[71LS5Formula]

Everything clears up. Turned the car off and hooked it back up. Problem started immediately. I'm gonna try a different port from the factory spot to the back off the throttle body and see what happens.

[dhutton]

Only thing I can guess is that maybe you are running too much total advance but I don't know enough about the Fitech system.

Don

[71LS5Formula]

I don't think that's the issue since its happening at idle.

[dhutton]

With the advance disconnected advance the timing and see if it happens.

Don

[71LS5Formula]

I think its the location of the of my vacuum port for it. The factory has it in one of the intakes runners, so it gets vacuum from just one cylinder and its causing to much fluctuation. I'm gonna try one of the ports on the throttle body today.

[dhutton]

Maybe it is a ported versus manifold vacuum source issue. Ported would give less advance at idle since the source is before the throttle blades.

Don

[71LS5Formula]

Went to the port on the throttle body and almost the same results. So either the vacuum advance is bad or the weights and springs are not right.