Well.... That HEI would be your problem. :Smarty: Call Year 1 and see if they have any recommendations when using the tach with HEI. If they don't know, try a tach shop. I haven't looked into it, and don't want to steer you in the wrong direction. It would also be possible that you damaged the tach from the increased currents due to the HEI (high energy ignition). :bglasses:
Dan, I know alot of people using the stock tach with an HEI, with no problem whatsoever. Is the Year One Tach different from the stocker?
I understand why you think that. One other point is that I used to run a MSD 6AL with a Tach Adapter (no less) and the tach still had the same problem!
tach problems I believe your problem is caused by the HEI. I have a factory tach in my SS 454 Monte, with an HEI, and it does the same thing. They must need some kind of filter. I do not think it is the tach. If anyone knows about this filter, let me know. Rich
I think the problem is too much voltage out of the HEI for the stock tach to handle. Years ago while trying to fix my broken stock in-dash tach, I brought an o-scope home from work. Looking at the scope traces, the stock points igniton was delivering about 25 volt peaks on the tach lead (looked just like a proper high-voltage spark profile, except not at 25,000 volts). With the HEI ignition in, I was measuring about 40 volts peak on the tach lead (ever tried to grab the HEI tach lead with the engine running? You WILL feel the 40 volts!). The turns ratios are about the same for the HEI and points coils, so the spark voltage is "reflected" (divided by the turns ratio) back to the tach lead. I made a schematic diagram (can't find it now) of the tach, repaired the tach, but had no solution for the extra voltage. It worked fine for a few years but is now dead again. I feel that the extra HEI voltage needs to be somehow reduced or translated to a lower (near stock) level.
OK Maybe we're getting somewhere! I feel more hopeful that I can find some level of resistance, a resistor (Ohm value) that will help. I'll keep adding resistance until either 1) the tach stops working or 2) the problem is solved! I wonder if a small transformer or in line capacitor would do the trick.....?
Probably the best thing might be about a 15 volt (or thereabouts) Zener diode, reverse (I think) biased. They can be used as an inexpensive voltage regulator. They will drop voltage across them equal to their voltage rating. A resistor might work, the current draw by the tach should be low, so the current draw times the resistance will tell you how much voltage is dropped. If you had access to a scope, you could figger out something that would work.
Interesting. Maybe that's why the resistor seems to be ineffective. I am sure that the tach draws VERY little current... I'll call Radio Shack about the Zener diode. Are we fairly sure about the volt rating? Ideally, should it be about 12v? Thanks!
I'm guessing that since I saw 40 vs 25 volts on the scope trace, you should drop the voltage by about 15 volts to equal what the stock ignition put out. Reverse biased will be with the "band" of the diode towards the tach lead.
OK, I'll talk to the people at Radio Shack. Just today, I tried using a scavanged regulator out of a power supply. The tach didn't mess up, but at higher RPM it showed at least 1000 RPM low. Obviously unacceptable. Man, I am experimenting with everything! Maybe I'll be a "specialist" after everything's said and done??
Well... I was thinking an I.C chip (op amp) to clamp the voltage. It has been a LONG time since I had to fool around with electronics... ou: To make things worse, 10 yrs ago I pitched all the electronic crapola I had sitting in the basement. :error:
I know what you mean. I took electronics in High School (graduated in '86) but still remember some of it. I do recall something about Op Amps. I'm going to try the Zener Diode thing soon....
Talked to my buddy He said an op amp would work, but a power transistor (probably a PNP type) would be simple to install. The connections to the trans go as 12v power (from the neg side of the coil, or tach output connection at the MSD box), ground, then the output of the tran to the tach. He told what trans he'd pick, but I don't remember the number...something like a 309? o No: Ask at radio shack.... :bglasses:
Hmmm. Well, I did call Radio Shack yesterday and they do have a 12v Zener Diode. I think I'll research that on the web today first. Thanks for all of your help!
That's the funny thing. I checked the voltage comming out of the HEI with the car running today. On the DC scale and a digital meter, it reads ~ 12.58 volts. OK, that looks normal. BUT when I switched to AC scale (thinking that pulsed DC is similar to AC) it read over 30 volts! Is that the problem? I don't know. I will hook up a 12v Zener like you have mentioned, BUT according to sources I have found, a normal Zener configuration has a resistor in series - AND the voltage drop occurs Across the doide and I think I'd have to ground the other side. I'll post a pic of what I found soon, but it does not look like it will work if I just run it in series on the tach lead. Then again, maybe it will. I honestly don't know.
The tach signal is a complex one; has a 30-40 volt peak, 10-15 volt flat section, coil ringing around 12 volts, a zero volt flat section,etc. There are pics of this on the web - look for "coil primary voltage trace". Actually, you can diagnose your ignition system from it! A DC meter will read the DC component of the signal - kind of like the average. An AC meter will try to read the AC RMS component that is around 60Hz (unless it is a true AC measuring meter that has no frequency limitiations - those are thousands of dollars). Regular handheld meters will rectify (try to convert it to DC) the signal, then display it using the same hardware that measures DC. So that number really won't mean much. I got a piece of hardware that converts my laptop into a o-scope. It was free at the time, but has some limitations built into it. It simply converts voltages into digital and then you can plot it on the laptop. You can also do it with your sound card - plug it into the microphone input (with the proper voltage scaling) and "record" it. Then plot it. Try it with the resistor and the zener both.
Here is the schematic. Obviously, I only have a tach output from the dist. and an input to the tachometer. I am not clear on how I would hook up the other side of Zener Doide....
Close Andrew, But the resistor and the Zenor should be in series connection between either the neg side of the coil, or the MSD tach connection, and the pos tach lead. In other words, the pos tach lead connects to the bottom of the Z diode as drawn (or output), and the resistor connects to the Z diode as drawn, and for HEI, the other side of the resistor connects to the neg side of the coil (the input), and for MSD the connections, it goes to the tach connection on the MSD box. Correct me if I'm wrong on the connections guys. The Zenor should work just fine, it will block the a/c current being induced from the high engergy output of the spark plug wires, and it will clamp an output voltage. I assume you would want to purchase the diode with 12-15 DCv range. :bglasses:
