It's magic!

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canyncarvr

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I've been running a 'blue' ignition box for a few years. Got it from the bay. I don't know what it is or where it came from. It works, the car ran fine.

I have a VDO white-face tach that I am fond of. The thing started getting flaky a couple years ago. The needle would jump around at lower RPM mostly and at higher RPM a good bit. When the car died, TU no start/run just out of the driveway, I found that the car DID start/run when the tach was disconnected. OK. My used-to-be-flaky tach has a real problem.

My blue box died recently. I put in a Standard LX-101 box I got from the bay. The module came in a Standard box, has a Standard label, probably is a 'real' Standard module.

Car starts/runs fine.

BUT...my MAC Tools ignition tester said the spark is a bit hotter. I've never trusted the MAC tool to do anything but tell me 'something' is in the wires. Still, the higher readings piqued my curiosity a bit. I hooked the VDO back up: It's perfect. Solid as a rock.

I tried for months to find someone to 'fix' the VDO. A repair outfit VDO referred me to didn't bother replying to any query. A second repair outfit said it would cost more to fix it than to buy a new tach. That tach isn't available any longer.

I've run with no tach for years...and it was the ignition module?

That's my story. At least for this afternoon.
 
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I recall long ago I had a similar problem, tach started acting erratically, engine started to miss, then after I had shut it down, it would not restart. Seems that the tach shorted out internally, fried the coil and ignition module, tossed the tach and never had another problem.
 
Perfectly reasonable why a tach attached to coil- would/could screw up anything in the ignition system.

My VDO is maybe just behaving for the short-term. It's been flaky for years, every time I've tried it (hoping something fixed itself) it's been flaky...with the blue box.

I'll think positively until then. It works! It's magic! I'm happy!
 
I've been running a 'blue' ignition box for a few years. Got it from the bay. I don't know what it is or where it came from. It works, the car ran fine.

I have a VDO white-face tach that I am fond of. The thing started getting flaky a couple years ago. The needle would jump around at lower RPM mostly and at higher RPM a good bit. When the car died, TU no start/run just out of the driveway, I found that the car DID start/run when the tach was disconnected. OK. My used-to-be-flaky tach has a real problem.

My blue box died recently. I put in a Standard LX-101 box I got from the bay. The module came in a Standard box, has a Standard label, probably is a 'real' Standard module.

Car starts/runs fine.

BUT...my MAC Tools ignition tester said the spark is a bit hotter. I've never trusted the MAC tool to do anything but tell me 'something' is in the wires. Still, the higher readings piqued my curiosity a bit. I hooked the VDO back up: It's perfect. Solid as a rock.

I tried for months to find someone to 'fix' the VDO. A repair outfit VDO referred me to didn't bother replying to any query. A second repair outfit said it would cost more to fix it than to buy a new tach. That tach isn't available any longer.

I've run with no tach for years...and it was the ignition module?

That's my story. At least for this afternoon.

Good thing you didn't say black face tach that could be a problem:eek:
 
If the tach works with a higher 'spark' value ignition box, your R1 resistor (first off input) may have gone south, giving it a higher resistance than designed. This schematic (guy found that R1 resistance decrease = lower coil voltage to trigger) is based on an older Volvo Tach made by VDO and used the kettering points system trigger and its 150V-300V induced coil voltage to trigger the tach. The 100:1 windings of the coil increase this induced voltage to >15,000V @ high tension line to distrubutor. He says the R1 resistor is not needed for ignition box derived tach triggers (MSD, 'tach out' on CD boxes):
"...low voltage tachometer signals such as those coming from electronic ignition units or EFI systems... If you want to adapt one of these VDO tachometers to work with those systems, you can simply solder a wire across R1, shorting it out. Never do that for a high voltage points/condenser system, but it will work fine with low (5-12V) tachometer signal systems..."

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If you have a multimeter and a soldering iron, I bet you can trouble shoot about any through hole mount component on that tach's small PCB.
 
If the tach works with a higher 'spark' value ignition box, your R1 resistor (first off input) may have gone south, giving it a higher resistance than designed.

That is a great point!

I've had the tach apart to resolder everything I could get to in an attempt to 'fix' it. No reason I'd recall a resistor on the signal input line...wasn't looking for such a thing.

What I do recall being inside the tach: A LOT of flatpack ICs! I could not have guessed at such a complicated setup for a tach. It is a shift indicator tach with an electronic set-point. That adds some circuitry. Still, the thing looks more like a computer processor than a tachometer.

I'll make a note of your resistor input notes on my tach paperwork. Next time I have it apart, I'll check on it.

Thanks!
 
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IC driven could make all ^^ this ^^ analog stuff out the window. Never associated VDO with ICs...
 
IC driven could make all ^^ this ^^ analog stuff out the window. Never associated VDO with ICs...

Retro or flatpacks...there is still the need that the input to a tach be dialed down. I've not measured what's on the coil- of the Mopar system, but it's hot enough (high voltage) to make you take notice! No way that kind of voltage is going to be running around in a tach.

Pretty good chance there is a similar clamping resistor in any tach.

Now I just have to look inside to see what I can see!
 
Retro or flatpacks...there is still the need that the input to a tach be dialed down. ...No way that kind of voltage is going to be running around in a tach.
with this voltage unbuffered through a bad resistor, you'd get the magic smoke....Here is the wonky VDO analog tach reverse engineered theory for nerds:
"...
The input circuit consists of a 150K Ohm resistor R1 connected to AC coupling cap C1 connected to a 10K Ohm base current limiting resistor R2 connected to the base of Q1. D1 protects the base of Q1 from going negative, which would hurt Q1. When the input voltage pulses high, C1 initially looks like a short circuit until it starts to charge, so the current going into the base is determined by the input voltage across R1 & R2, a total of 160K Ohms. Using Ohm’s Law, we see that, in order to get the 1mA base current we need to turn Q1 on, we need a (trigger) voltage of 160K * 0.001 = 160V........All this led me to wonder how the tachometer input could get such high input voltages, I thought about inductive loads (coils) causing voltage spikes when they are switched off, and that is what actually happens. A quick internet search led me to https://www.picoauto.com/library/automotive-guided-tests/primary-voltage/. Figure 5 and its corresponding paragraph give a good explanation of the phenomenon:
“The vertical line at the centre of the trace, called the 'induced voltage', is above 200 volts. The induced voltage is produced by a process called magnetic induction. At the point of ignition, the coil's earth circuit is removed and the magnetic field or flux collapses across the coil's windings. This in turn induces an average voltage between 150 to 350 volts (Figure 5). The coil's High Tension (HT) output is proportional to the induced voltage. The height of the induced voltage is sometimes referred to as the primary peak volts.”..."
 
Yep. Which is why getting your body parts on either the coil- or the ignition module TO3 case when the car is running is going to hurt.

'Magic smoke': I don't want to see that! I used to have a can of smoke to use if bad things happened and it got out. I used it all, don't have no more.
 
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