Tachometer Problem, not RPM sensor

My tachometer in my 1982 300 TDT is not working. I have replaced the RPM sensor, but this does not seem to be the problem. I have two of them now and it works pretty much the same with the guaranteed good
junkyard one and also the brand new VDO one.
Mostly it doesn't work. Sometimes if I pull and yank around the wire harness from the sensor near the crankshaft balance pulley on the front of the engine, the tach will work properly for a short time.
But mostly it just sits dead. But when I tap the horn button on the steering wheel, the tach jumps to life for a split second. Repeated very quick taps get it to hover around 1000 rpms. The horn and the tach are on the same fuse.
Any ideas?
Thanks.
Paul Fretheim
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By RPM sensor do you mean the pickup at the flywheel, or the screw cap gizmo (tach amplifier) on the fender which converts the signal and drives the tach?
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The RPM Sensor is the barrel shaped gizmo on the fender. The part near the crankshaft balancer pulley (on the front of the engine) is called the TDC Position Sensor.
Paul

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No, the barrel shaped gizmo is called the tachometer amplifier. Check the service manual.
The part

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The tach on my 1981 300SD dopesn't work either. But sometimes it does. I have been told that putting a cigarette butt into the amplifier to increas pressure on mating parts can help. Sometimes it does. But I have also noted that when engine temperature climbs above 100 C when the air conditioner is running on a hot day the tach will start working properly. Turning various things on and off will also affect it. Must be some very strange electrical connections in there.
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Regarding putting something into the amplifier to increase pressure, I think either there must be an older and newer version of the tach amplifier, or this makes no sense to me. The ones I have seen have no place to put anything to increase mating pressure. The electronics portion is contained in the cap, doesn't come out and there is no way to put anything behind it to increase down pressure on the pin connections. They are assembled with the floating electronics portion having retaining tangs on it. Once it's pushed into the cap during assembly, they pop back out, holding the floating portion inside, with no way to ever take it back out without sawing the top of the cap off. Which I did on one. And even then, the electronic portion, which isn't much, is embedded in silicone caulking, so you still have no hope of repairing a bad solder connection, etc, which is probably what the problem really is.
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I don't think the problem with my tach is the amplifier/RPM sensor ribbed barrel shaped thing on the fender because I have two new ones, one I bought on eBay used and a brand new VDO one and the tach works the same with either one installed. I haven't had time yet to check the solder on the circuit board of the tach itself, but I am doubtful that is my problem because sometimes the tach "pins" at the high end, and I can't see how that would be caused by a broken connection in the board. I am going to try that next though.
I am also wondering if the TDC sensor is working intermittently.
wrote:

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snipped-for-privacy@inyopro.com says...

dunno. Sometimes it works. Anyway, I have given up on the damned thing, though it pains me to have non-working gadgets no matter how little utility thay really have. The tach isn't really necessary.
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