Tach Problem - Team Camaro Tech
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post #1 of 10 (permalink) Old Jan 3rd, 05, 07:43 PM Thread Starter
 
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I feel like I don't know anything as I have to ask about everything but here we go again.
I just installed my engine harness which I bought altered for an HEI distributor. I plugged all wires as they they should go. However, when I tried to turn the car on, it wouldn't happend. Once I removed the brown wire that goes connected to the tach plug on the distributor she turned on. Once I plugged the brown wire back in, it immediately turned off. What's wrong? I want my tach to work. What can I check? I know someone out there has the answer. I would greatly appreciate the help as always.
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post #2 of 10 (permalink) Old Jan 4th, 05, 09:18 AM
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Jim
 
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The only thing I can think of is the brown wire is grounded out somewhere and this is keeping the HEI from working. Years ago you could add a wire off of the coil's negative terminal (which also sends a pulsed signal for a tach) and run it through a toggle switch and then to a ground and if the switch was on and the negative coil terminal was then grounded out the engine would crank but never start.
I wonder too if maybe there are two brown wires in the haness that may have been inserted into the wrong holes in the bulkhead connector ?.
You may have to trace the wire back into the car. Possibly the tach is bad ?
Jim

1974 Spirit Of America Nova (being restored), 1973 Nova Custom, 1968 Chevy II (Garage Find 2012)

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post #3 of 10 (permalink) Old Jan 5th, 05, 07:45 PM Thread Starter
 
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Thanks for the reply SOA but the Tach is new. Purchased it from Classic Industries. My car didn't come originally with gauges so I converted and bought all the correct wiring. I tested the brown wire and there is no current on the negative or positive side. The only thing I can think of is that maybe the plastic circuit behind the dash wasn't correct for a tach but I checked and it should work for both a tach and speedo. I am stumpt hopefully someone can come up with an answer or suggestion.
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post #4 of 10 (permalink) Old Jan 6th, 05, 05:19 AM
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Jim
 
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Even though something is new it still may not be right. I have installed car audio and other electronics for 20+ years and while rare when dealing with major manufacturers there is still a low percentage of out of the box defectives.
Ok. Lets say the tach is fine and the HEI is fine when the tach wire is not plugged into it. To see if the wire from the HEI to the tach is either in the wrong slot on the bulkhead connector or is possibly shorted somewhere, unplug the tach input signal at the back of the tach itself (the wire that goes up to the HEI). Now if the tach wire is plugged into the HEI and the car runs then it is either something in the tach or the wiring after the point where it is disconnected by the tach. If the engine does not run with the wire unplugged at the back of the tach and plugged into the HEI then there is a problem between the disconnected wire at the back of the tach and the plug on the HEI.
Jim

1974 Spirit Of America Nova (being restored), 1973 Nova Custom, 1968 Chevy II (Garage Find 2012)

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post #5 of 10 (permalink) Old Jan 6th, 05, 05:30 AM
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Jim
 
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Something else to add, you stated that maybe the plastic circuit behind the dash is not correct. I don't know what year you are working with but when I converted the dash on my 74 Nova the flexible circuit board was the same between a tach dash and a fuel gauge dash however on mine the trigger wire (tach signal wire) was in the same location as the fuel sending unit wire. I cut the wire at the plug and then routed the wire off of the plug to the coil's negative terminal and then extended the wire off of the loom that goes to the sending unit to the console fuel gauge.
I kinda wonder if the harness you bought has a problem or if a pair of wires got mixed up and inserted into a wrong terminal hole on a plug. If somewhere the tach wire off of the HEI is somewhere connected to the fuel gauge wire. If the tach signal wire off of the HEI is connected to the fuel sending unit wire (this wire can be a zero-90 ohms of resistance) then that could be keeping the motor from running.
Jim

1974 Spirit Of America Nova (being restored), 1973 Nova Custom, 1968 Chevy II (Garage Find 2012)

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post #6 of 10 (permalink) Old Jan 6th, 05, 07:22 AM Thread Starter
 
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Thanks again. I'll look into it this weekend and let you know.
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post #7 of 10 (permalink) Old Jan 21st, 05, 06:22 PM
 
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funny you post i just finished with this same problem.it was the printed circuit board had burned in a spot and didnt complete the circuit.
installed a new board.it sound like your not getting a complete circuit.how about dash ground?
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post #8 of 10 (permalink) Old Feb 6th, 05, 08:22 AM Thread Starter
 
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O.K. I spend over six hours trying to find out the problem and it seems as if the tach is no good. I checked the circuit board and even spent time putting on a second layer of the plastic covering on the back end of the circuit board (the end that touches the metal part of the dash). I also tested it with a tester and everything was fine. So I then tested the brown wire. If not connect, I don't get any ground. However, as soon as I connected the plug to the rear of the gauges, I got ground. I even narrowed it down to the positive connection of the tach. so I get ground as soon as I connect the positive side of the tach (brown wire to positive). I then checked the tach and connected the negative terminal and the positive terminal and got current going through. I don't get any current when I connect the negative to the s terminal or when I connect the s terminal to the positive. Therefore, I must assume that SOA-Nova was correct when he stated that the tach is probably defective. Am correct in saying, that I should get no reading when connecting the negative and positive terminals of the tach?
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post #9 of 10 (permalink) Old Feb 6th, 05, 01:12 PM
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Jim
 
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I think you have a bad tach from what you are telling us.
You do have the dash grounded when you are testing this ?.
If the tach is mounted in the dash use your meter (on the ohms scale) and measure the resistance between the ground terminal (or the case/it might be the same) of the tach and a metal spot under the dash. You should be reading close to zero ohms resistance (you may read 0.?? but this could be due to the connection points and the meter). If that is OK then verify you have ignition 12V on the back of the tach. If these two readings are fine then disconnect the wire or terminal that is on the back of the tach that goes to your HEI's tach terminal. The car should run now.
Now to verify the tach is bad leave it running and connect the wire from the HEI's tach output to the back of the tach. If the engine dies then the tach is bad.
If you want to verify it is the tach and maybe not something screwy behind the dash, you should know what terminals on the tach go to where. Wire the tach under the hood to the battery for power and ground and then run a jumper to the tach output on the HEI. If it kills the motor under there then the tach is defective as you have bypassed everthing else.
If you do send it back and they repair it but don't replace it with a new one ask them what might have caused the problem from their service tech and when you get the new one wire it up under the hood to verify it's operation.
I'm lucky at work as I can test things on a test bench to verify they work or not. Electronics are not perfect and I have had defective equipment right out of the box and it's brand new.
Jim

1974 Spirit Of America Nova (being restored), 1973 Nova Custom, 1968 Chevy II (Garage Find 2012)

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post #10 of 10 (permalink) Old Feb 6th, 05, 04:05 PM Thread Starter
 
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Jim,

I beat you to your idea. Today, I removed the tach from the dash and ran a wire directly from the HEI output of the distributor to the tach input to the gauge and then ran a ground wire to the tach. The car immediately turned off. It must be the tach. I contacting vender tomorrow.

thanks
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