1968 Tic-Toc-Tach & Gauge testing question

To anyone out there in the group,
I am restoring a 1968 Tic-Toc-Tach dash cluster to install into a non-tach car. I was wondering if anyone knows how to bench test the
tachometer. I have basic electronic tools and knowledge, except I don't know what kind of signal should come from the tach. Does it come straight off the tach, or is there some other box it goes to before it goes to the tach? The housing is stamped with "COIL" and my wiring diagram doesn't show anything except for a 18 GA brown wire coming off the coil going to the firewall bulkhead connector, and then another 18 GA brown wire going to the dash cluster connector.
If it's DC, I 'm thinking I should be able to simulate it on the bench and vary the voltage as necessary to see if the tach works, but I don't really want to just start running different voltages through it without knowing how the tach should be reacting. If it's AC, then does anyone know what the voltage and cycles range should be?
Also, there's a small round circuit board attached to the gauge housing with a couple of capacitors, a diode, a transistor, and possibly a resistor. Does anyone have a circuit diagram for this (including the values of the components?) Does this unit rectify an AC coil signal to a DC voltage for the tach?
Also, the clock does not work when hooked up to 12 V DC. I have heard thats not unusual. Does anyone have a method to convert the clock to a quartz movement (or a way to repair the original?) I could probably do it myself if there wasn't too much modification work necessary. If it's really difficult, is there someone who rebuilds these units and does good work for a reasonable fee?
I will also be adding a set of original console gauges to the car at the same time. Is there a way to bench test the low fuel sending unit? I'm guessing it senses the voltage that goes to the gauge and trips when it gets to a certain value. Anyone know the value it trips at (or if that's not right, how to bench test it?)
You can e-mail me at ricanjan(nospam)@gte.net just remove the (nospam).
Thanks, Rick in Seattle
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To answer one of your questions, the tach responds to an AC signal. I have a 67 Camaro that I converted over to an HEI system. This distributor outputs a 1.7V peak square wave. That seems like an odd voltage but that's what I measured. The bottom of the square wave is at 0 vollts and the top is at 1.7V. Increasing the engine speed increases the frequency of the signal. I measured 8 pulses per engine revolution. The signal that I measured has a LARGE spike at the low to high transition. It is about 30V in amplitude and short in duration (a few tens of microseconds). I don't know if this is normal but it seems very high. This may be damaging to my tach but I have no other HEI igntion to compare it to. I don't know the signal output of a points style distrubutor (if that's what you're running) but since my tach responds to an HEI signal it must be similar. On my car the tach receives it's signal directly from the HEI tach terminal on the distrubutor. When I had a points-style distrubutor I believe the same wire ran off of the coil. (Negative side or positive side I don't remember). You should be able to run from the coil to the tach trace on the printed circuit on the back of your instrument panel.
To answer your clock question, if it's the same mechanism as a 1967, the clock can usually be repaired. These clocks were completely mechanical. Remove the clock from it's housing. and you'll discover that a 12V solenoid winds the clock spring. If you take the clock out of it's housing to reveal the mechanical workings you can see this solenoid. If you connect 12V to the clock terminal and ground to the clock case, the spring should wind if the solenoid is good. Usually it is. You can tell if the spring is wound by looking for two electrical contacts. If these contacts are touching and you apply 12V but nothing happens, the solenoid may be electrically open. Then you're out of luck unless you want to remove the solenoid and rewind it. (I've never attempted that). If they're not touching, this means that the solenoid has wound the spring but the lubrication the factory used on the moving parts of the clock has dried out. I fixed mine by soaking it with a light oil (WD-40 for instance).
The winding spring drives a oscillating spring that causes a flywheel (for lack of the proper term) to rotate back and forth. Find this wheel and carefully twist it 1/16 to 1/8 turn with your finger. Be very careful as this is the most fragile part of the clock. If you're careless, and twist it too much, the shows over. If you do it correctly, you should see it oscillate once or twice then stop. By patient twisting of this wheel with your finger and lubricating the entire clock mechanism (all gears and moving parts) it should start operating. The clocks I've repaired in the past can get so wet from the oil that they begin to drip. That's okay. I saw a guy actually immerse the clock in WD-40 completely. He left it there for a day. I prefer to not waste that much oil. With patience you can do it by just spraying the mechanism. Keep lubricating and twisting the wheel. It might take as long as an hour but, if the gears are not damaged, the clock should come back to life. When operating properly the clock spring should drive this wheel between a 1/4 to 1/2 turn.
Set it up on your bench and let it run. You can put the hands back on it to make it easier to monitor. It might stop. If it does lubricate it again and give the wheel another twist. When it's running properly, the clock spring will slowly unwind and about every 1 to 2 1/2 minutes the contacts for the solenoid should touch. You'll hear a little "click" as the solenoid winds the spring. Then it's good for another few minutes.
When I fixed my clock I let it run for about 2 days on the bench. I wanted to make sure that it was running smoothly before reinstalling it.
Once it's running smoothly, you then reinstall it and calibrate it to keep correct time. Do this by monitoring how many minutes it gains or loses during a 24 hour cycle. If it gains or loses 1 minute a day, pull on the knob that you use to set the time and advance or retard the clock 12 hours. This means alot of twisting but it will speed up/slow down the clock 1 minute per day. (I can't remember now which way you're supposed to turn it if it's running fast or slow but it does have the ability to be calibrated.) If it's running 10 minutes slow per day this means rotating the clock ten 12-hour clock cycles. You can get it to keep amazingly accurate time, but this is predicated upon how well you lubricated it before.
just remove the

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Gary,
First off, thanks so much. That has given me lots of great info. You must have a scope to see the wave shape. Wish I had one! You mentioned the output was coming from the distributor. (is it because your HEI has an integral coil?) Something below 13.8 V would seem right since the coil is constantly charging and discharging and it would draw it down. I intended to run a points type breaker ignition, or possibly install a Pertronix magnetic trigger unit in the stock distributor unit. AC makes sense because the coil is charging 8 times per engine revolution.
Does anyone have a circuit diagram for the printed circuit assemblly that goes with the tach?
On the clock, once the WD-40 loosens up the mechanism, did you go back and use another oil on it? WD-40 seems to dry out or evaporate fairly quickly since there is so much solvent in it. Great to know about the winding solenoid. I will watch for that.
Does anyone have any info on the voltage the Low fuel sensing unit should trip at?
Also, does anyone know if the resistance through the temp sending unit varies between 1968 and 1969? I hooked up a temp sender on the bench and used a lighter and some ice to cool and heat the sender. I got a response on my gauge that indicated it was reading correctly for hot, but the gauge needle does not bottom out on cold. I don't really want to mess with the gauge. If I knew the correct resistance range associated with a certain temperature I could check the sender. Does anyone have that info?
Again, thank you David!!!!!!!!!!!!
PS. There sure seems to be a lot of stuff that's not associated with Camaros that makes it onto this newsgroup. Too bad something can't be done to filter that junk out.
Regards, Rick
Note: These are all stock GM parts, not repro, and are correct for a 1968 Camaro.
On Sun, 08 Feb 2004 07:43:22 GMT, "David R. McCoy"

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Hi Gary,
Yes, I had access to a scope and yes, the HEI has an integrated coil in the distributor. I was having trouble with my tach but it seems to have calmed down now. The only thing I did differently was to regap the spark plugs from 0.035" to 0.045". I was accustomed to gapping them for a points style distributor (0.035"). This HEI can handle larger gaps. I haven't looked at the waveform but I would expect a stronger signal now because now the coil has to produce a larger voltage to jump across the larger spark plug gap.
I don't have a schematic of the tach.
With regard to the clock, I didn't use any other oil. My clock latest clock has been ticking away for about a month now and it seems ok. It might not be a bad idea to use another oil but it must be very thin because the flywheel and oscillating spring are tiny and cannot overcome damping that would occur with a thicker lubricating oil. Try it and see. I would be interested in hearing about your results.
I have the low fuel sensing unit on my car but haven't connected it to the light yet so I can't answer your question, however see below on testing the fuel gauge.
GM temp sending units worked the same from at least 1967 up through 1985-1990 (later ones may be the same too, I'm not sure). The temp sending unit is a very high resistance when cool (meaning engine is cold). On these GM temperature gauges, if I remember correctly, a 100 ohm path to ground should deflect the gauge halfway across the scale. That's approximately equivalent to 180F. Shorting the sending unit wire to ground should cause it to deflect all the way to the right. Note: the temp sending unit is not a linear device, that is, infinite resistance is ~100F (left side of the gauge), 100 ohms is about 180F (midway) and 0 ohms(wire grounded) is 250F (right side).
Fuel gauges and oil pressure gauges (the later ones that use an electric oil pressure sending unit) are both tested the same way. These devices are linear. Grounding the wire should correspond to empty on a fuel gauge or 0 psi on a electric oil gauge. A resistance of 40-45 ohms should be half scale and 80-90 ohms should give you a full deflection.
You can test temp, fuel and the later electric oil pressure gauges with a few resistors. When I was turning wrenches for a living I had a potentiometer that went from 0 to 200 ohms or so. I put it in a box and put an indicator knob on the pot. shaft. There were a couple of leads coming out of the box with alligator clips on them and I marked the surface of the box to indicate 45 ohm, 90 ohm and 100 ohm positions. Then I disconnected the wire from the sending unit, connected one of the alligator clips to this wire and grounded the other alligator clip. By varying the pot. value I could check the gauges very quickly.
If you want to test the low fuel sensing unit, disconnect the fuel sending unit wire near the fuel tank and vary the resistance between it and ground until you get the light to come on. Grounding the fuel wire is equivalent to an empty tank. This should turn on the sensing unit if it's working correctly. I think you're wanting to test the sensing unit to see if it works. This will accomplish your goal.
It's been 14 or 15 years since I've done any of this so I may not be remembering the resistance values correctly. If you wanted to be sure about the temp gauge do this:
Get a pot of water on the stove and put a themometer in it. BE CAREFUL NOT TO BURN YOURSELF! Immerse the temp sending unit in the water (you can suspend it with string if you want) and raise the temperature of the water to 180F or a bit higher making sure that the thermometer isn't touching the bottom of the pot. You want to measure the temperature of the water, not the bottom of the pot. When you get to this temp, take the pot off of the heat and allow the sending unit and water temperature to stabilize for a couple of minutes. After a few minutes, use an ohm meter to measure the resistance of the sending unit between the terminal and the body. This will give you the resistance value that will deflect your gauge 1/2 way. If you do this, let me know what the resistance is. I think it should be around 100 ohms but it's been along time since I've worked on these things.
just remove the

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Hi Gary,
One more thing. If the temperature gauge isn't bottoming out on cold, make sure that the needle isn't rubbing against anything. It may be trying to go to cold but can't because of an obstruction.
just remove the

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Gary,
Do you know the steps to remove the clock assembly out of the tachometer?
Thanks, Rick
On Mon, 16 Feb 2004 14:44:36 GMT, "David R. McCoy"

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Wow! That was a long-ass, comprehensive answer to a seemingly mundane point of restoration. Kudos to your knowledge and accumen. If I ever need any early Camaro info, I know who to turn to. I almost finished my whole beer while reading it.
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Thanks,
I hope you had another beer. I'm sure you didn't plan on finishing an entire beer reading my response!
I used to work at a Chevy dealership as a mechanic (1988-1990). I learned that clock repair trick from one of the more senior mechanics there. I wish I learned more. He has forgotten more than I ever knew. I quit to go back to school and got an undergraduate in Electrical Engineering so I have a bit of knowledge in electrical diagnosis as well. (Just enough to be dangerous!)
I went for a drive in my 67 RS/SS today. Wonderful! I bought it in 1990 and it sat in a garage most of that time. I dreamed of driving it again while working my way through school.
What year/type of F body are you driving?

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