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RPM sensor
I have a 1970 911t. I have recently upgraded to a jacobs ignition system , which in turn, produces a bad signal at the tach.(very irratic) I know I need a ballist resistor, but electrical is not my specialty. If anyone knows the proper resistor for this system, I would greatly appreciate your input.
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05-21-2003, 12:06 AM
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Join Date: Apr 2002
Location: Siskiyou County, CA
Posts: 309
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Is the bad signal that's erratic, or the tach reading?
If the signal/tach reading is only intermittently bad, then common sense would suggest that the wiring and/or connections are at fault. If it's the tach reading being erratic (which I'm assuming it is), then does the car run properly? If so, which from the sound of things is the case, then - well, I've got the wiring diagram here, but I have *no* clue how to read it, it's not like anything I've ever seen before. I'll be happy to scan it, though, if you want. Cheers 
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'90 C2 Coupe - 104k miles |
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05-21-2003, 12:53 AM
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The car runs great, always has. The tach works A1 when the stock coil is hooked up. Once i installed the jacobs coil and ignition, the tach is giving false reading. It likes to jump up to around 6000 and just as fast as it jumps, it comes back down. The engine does not increase the RPM, just the tach. I am curious as to if anyone else has the jacobs system, and if so, have you encountered this issue as well? I have contacted jacobs about the issue and I just as well should of consulted the pine tree in my backyard...No help whatsoever...just told me to get a ballist resistor, which I already knew I needed
 but couldn't specify which resistor I needed.
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05-21-2003, 02:35 AM
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Join Date: Apr 2002
Location: Siskiyou County, CA
Posts: 309
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OK, I've found some stuff that may be helpful... not quite sure how I missed this earlier.
Regarding the RPM sensor: "The resistance between terminal A and B must be 170-210 ohms, and between A and C 220-300 ohms". That'd be the first thing to verify. Then (this is in theory, not in practice), tracing the signal (voltage and current) going to the tach with the original system and the new system should tell you where the problem lies (and therefore where the ballast resistor needs to be installed) - comparison of voltage and current measurements from the old and the new systems should tell you what size the resistor needs to be... that's something to think about, at least.
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'90 C2 Coupe - 104k miles |
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05-21-2003, 03:19 AM
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