Update on my S4 project

[Tom H]

Before opening up the tach and getting out your soldering iron, try adding a loop or two to the pick up clip. And while you are at it, be sure that the metal U clip makes good metal to metal contact with the inner part at both tips. Might even clean the tips a bit. If that metal clip does not make good contact at both ends, you do not have a good transformer core and the signal from the ignition current does not get well coupled into the tach.

I myself have replaced the main capacitor on my tachs, based on write ups. but I have serious doubts that the old caps go bad. These caps are simple film caps and are not prone to age failure.

You say the needle just sits at 3000 or so. Does it sit there even with power off? Or does it go back to zero when OFF? If it does not go to zero when off then either you have a broken return spring or it needs some very light lube.

Tom

 

[puff4]

Tom, let me first say I'm definitely *NOT* an electrician or electronics expert - I really stink at it, frankly.

My symptoms - the tachometer works smoothly across the range, and in cool weather it works pretty accurately, keeping pace with the Sunbeam's original design - i.e. with all the meter needles - oil, water, ammeter, tach, speedo and fuel gage pointing straight up at 60 mph with 1/2 tank of gas (don't you just love it when an aircraft designer designs automotive instrumentation!?). However, when the weather gets hot the tach starts reading high, at varying amounts between 100 and 1000 RPM higher.

Is that because the capacitors are failing? Or it it because the heat causes additional resistance in the induction wire going to the tach? Or does the heat increase the resistance across the clip's contacts instead?

It would be really great to get to the bottom of this problem, since I understand that 'reading high' is a common problem, and I don't relish ripping apart the tach, and I'm sure that helpless little tach won't appreciate having my inexperienced and fumbling fingers inside it, either.

 

[Tom H]

Kevin, I'm an electronics engineer and I understand pretty well how our tachs work, but I have never fully analyzed the design to understand all that goes into it. I do know there is a termistor - temperature dependent resistor - somewhere in the circuit, and that is obviously intended to compensate for some variation due to temperature. I'll look into that in more detail tonight. I don't know what temp effect is being compensated for. I know it is not the transistors. But I am 99% sure this does not indicate a bad capacitor. Hmmm. I have not heard any previous complaints about temperature variations in our tach. I've heard erratic (jumpy, sticky) operation, Non working, inaccurate in general, but not temperature error.


Tom

 

[sammaw@bellsout]

Thanks for the help Tom. What I have seen it do is start at zero, sometimes it will hang there a short while and then jump to 3000. Other times it will go there as soon as it starts. It does move while operating at normal speeds, but tends to hover in the 3000 range +\- 500.

Is the transformer core and loop external to the case?

Sam

 

[Tom H]

Sam, The transformer is half inside and half outside the case. There is a half core inside, that looks just like the metal U clip that is on the outside. Inside there are many turns of small wire wound around that half core. (maybe a couple hundred turns) . Then the metal U clip forms the other half of the core. And it needs to make solid contact with the tips of the inner half core. Then you have one loop or so of the white ignition (wire to the coil) around the outside half core. The combination is the transformer - a couple turns outside the case and many turns inside the case. White wire has high current (but little voltage - like 1-2 millivolt pulses- across that little loop). The transformer action then converts that little voltage into a higher voltage (about a volt or so) with much less current. It takes about a volt or so to turn the transistor inside on.

I'd say try adding an extra turn on the white wire and see if that cures your problem. And be sure the U clip makes good contact. Adding that extra trurn is not trivial because the wire is a continuous wire from ignition switch to coil (or to the ballast resistor). It's OK to cut the white wire for this purpose. Just be sure you make a good re-connection.

Once you have it operating consistantly then you can calibrate it by adjusting the variable resistor. Most of us take the assembly out, and then put a hole in the case right by the adjustable res, so you can adjust it after it is installed back in its case.

You coming to the Invasion? If I make it (50/50 now) I will bring cal equipment and spare parts for Tachs.

Tom

 

[sammaw@bellsout]

Thanks tom, great explaination and instructions. This car has not been run in probably 7 years, could it be possible that it needs lubrication too?