Re: Auto electrics ? experts please


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Posted by D Sherman [72.47.9.228] on Monday, June 20, 2011 at 16:06:58 :

In Reply to: Auto electrics ? experts please posted by Kevin in Ohio [50.41.180.150] on Monday, June 20, 2011 at 13:26:10 :

It's hard to guess, but my guess is the design of the gauges is not very good. Most modern vehicles charge at 14.5 volts. 13.8 is the bare minimum needed. In any automotive system you're going to have spikes as high as 40 volts from time to time, and competently-designed electronics must handle that without damage. I've seen some amazingly bad electronic designs recently that leave out all the protection components that I would normally put in. Perhaps the engineer was overruled by the marketeers and bean-counters who think factory cost is all that matters, or perhaps the engineer is some kid right out of school who thinks that if he can make the circuit work one time on his bench with a lab power supply, it's good to go.

Regardless of the details and regardless of them being "American-made", at this point I would want nothing more to do with that brand of gauges. Yes, you could get in there and add your own protection network to the gauge cluster power supply, with a linear voltage regulator to drop the voltage to a nice steady 12.6 volts, a ~20 volt 5 watt zener in front of it to protect the regulator from load dump spikes and reverse connections, a suitable resistor upstream to protect the zener during serious spikes, and few capacitors of different kinds (.1 uF multi-layer ceramic in parallel with 10 uF tantalum is a good combination) to slow down the narrowest and wickedest spikes so the zener has time to catch them. Oh, and there should be a fuse upstream of the whole thing to keep thing from catching fire if all else fails. This is all stuff that should be inside any automotive electronic device, but these days often is not.

My feeling is you're going to keep having problems with this brand of gauge because they're badly designed. You might swap them out enough times and find one that holds up. The factory guy is never going to say, "We left out 10 cents worth of parts in order to keep the price low." He probably doesn't even know. All he knows is that he can usually get most complaining customers to go away by telling them some technical-sounding mumbo-jumbo.

The company guy you talked to sounds like he was blowing smoke and has no idea what's going on. Sure you could put a scope on the tach input and you should see some sort of pulses going into it that vary in frequency with engine speed. Aside from that there's no way your brother the EE could be expected to know what they ought to look like, since that all depends on the design of the system. In any case, a bad coil should not make the tach go up in smoke.





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