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Location: Boulder Colorado
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72 MFI tach question

Just got the engine back in the car and got it running. Tach is not responsive though, so checked the back of the tach and nothing was unusual. Checked power to tach and I have 12 v so that isn't the problem. Important wires were connected correctly, 12 v red, black purple (dist signal) and brown (ground). Figured I'd swap a different tach in to see if I by chance had a dead tach...nope, did that twice and I am pretty confident that I don't have three dead tachs.
Checked connection to the dizzie and the spade is tight. Checked the wiring schematic and the purple black first goes to the speed switch (11) and then into the harness up to the tach. So what and where is the speed switch? What does it do?
Maybe I have a continuity problem somewhere in the purple black wire? I suppose at worst I can run a long jumper from the spade on the side of the dizzie up to the spade on the back of the tach...and if that gives me an active tach, then I have a wire continuity issue somewhere. But if that doesn't give me a tach, I will be stumped. Suggestions?



Last edited by speedo; 10-06-2019 at 03:19 PM..
Old 10-06-2019, 03:13 PM
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No tach

I now understand the speed switch and am guessing I have an RPM transducer issue.
If someone can weigh in on the effects of a bad transducer that would narrow my focus.
Or point me down a different path...
Old 10-06-2019, 04:41 PM
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Lash
 
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speedo

found this thread on early 911 S registry

Non Operating Tachometer

hope this helps

Lash
Old 10-06-2019, 05:03 PM
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From another thread,

Around August of the '71 production year, Porsche changed the tach to work directly from the points. Before that, the tach was triggered from a 'filtered' signal derived directly from the high voltage discharge A-lead of the CDI/coil. The 'filter' was the round-can "Intermediate Unit".

I thought my tach kept working even with transducer disconnected but I am not around my 911 ATM.
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74 911, #3
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Old 10-06-2019, 05:05 PM
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Hmmm...

Quote:
Originally Posted by manbridge 74 View Post
From another thread,

Around August of the '71 production year, Porsche changed the tach to work directly from the points. Before that, the tach was triggered from a 'filtered' signal derived directly from the high voltage discharge A-lead of the CDI/coil. The 'filter' was the round-can "Intermediate Unit".

I thought my tach kept working even with transducer disconnected but I am not around my 911 ATM.
I am owed the original instruments for this car. Previous "caretaker had the car stored and he worried the instruments would be stolen so he removed them. I pulled extra instruments off my shelves to install for now. It is reasonable to expect that the three tachs I installed...were pre-71. So I need to look through my supply and try to find one 71 and later and try it out before I get crazy and start fixing things that might not need fixing.
Old 10-07-2019, 06:31 AM
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The earlier (up to Aug 1971) tachs require a high-voltage pulse to trigger. For '72, the tach is triggered from the points (low voltage). So, you need an adapter to make an earlier tach work. I would pitch my little TACH-ADAPT product, but since you may be able to obtain the original tach for the car, that may be unnecessary.

The MFI speed switch (transducer) should not affect the operation of the tachometer. It 'hangs' on the same signal as the tach, but presents a very light load (12K Ohms), so unless something is completely shorted inside, it has no impact. Also, the tach will still work if it is removed. The purpose of the transducer, in conjunction with the microswitch on the throttle linkage and a fuel shutoff solenoid on the MFI, is to coordinate the shutoff of fuel during lifted throttle at higher rpms. It shuts off fuel to the idle circuits to improve emissions and stopping any popping in the exhaust. The idea is that the fuel does not need to be on if you are decelerating under closed throttle conditions. Whenever the throttle is re-opened, or when the rpms drop below around 1700 fuel is turned back on so the engine can start or idle. (Yes, I fix those too. They are notorious for having some of the parts literally fall off the little pc-board inside, simply due to corrosion/oxidation. Certain components had bad plating on the leads and they just 'rot' off! If you shake one and you hear stuff rattling around inside, that's a giveaway.)

Last edited by Bob Ashlock; 10-07-2019 at 08:48 AM..
Old 10-07-2019, 07:22 AM
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tach wasn't really an issue

I found a tach dated 72 on the shelf, plugged it in and everything works as it should.
Glad I didn't start ripping things out and replacing. Thanks for the wisdom guys!

Speedo

Old 10-12-2019, 12:24 PM
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