xonix_digital Posted June 8, 2010 Share Posted June 8, 2010 Hey guys, I know there have been a couple of threads about this but I didn't find any that were helpful. My Tach is acting sleepy. Basically I mean that When I rev it up to about 2k it does nothing at first and then will eventually rise up really slowly. Then after I let the car idle, the needle stays there around 2k. this is not RPM specific as the same thing will happen at 1.5k or 3k if I'm driving around. So I downloaded the FSM from XenonS30 (Thank you), and I have been looking through the wiring diagrams and I cant for the life of me find where the Tach gets its signal from! I know for a fact that the Tach itself is good. Please bail me out of my ignorance! Quote Link to comment Share on other sites More sharing options...
Zmanco Posted June 9, 2010 Share Posted June 9, 2010 You need to tell us what year car before we can help you. Quote Link to comment Share on other sites More sharing options...
xonix_digital Posted June 9, 2010 Author Share Posted June 9, 2010 You need to tell us what year car before we can help you. Woops! My fault. I park a 1973 Datto-san 240z. I forgot that the EFI cars are probably going to be different. I just re tipped the Fuel Tank Sending unit connections also. Those seem to be working much better. And my front right headlight is very dim and I have yet to figure out why. I'm going to hit it with a multimeter tonight or tomorrow to find the cause of that. Not sure if any of those things could be related but i thought I would toss them out. I really just need to know where the hell this tachometer gets its signal from. After looking through the wiring diagram front to back I can't find the sensor anywhere. Quote Link to comment Share on other sites More sharing options...
galloguy05 Posted June 9, 2010 Share Posted June 9, 2010 (edited) I had a problem similar on my 75 280z when I re-did the interior. One of the harness plugs that plug into the firewall was loose. It made my right side lights dim and the a/c not work. My tach wasnt hooked up at the time so I cannot comment on whether or not that was affected. I really cant remember if the car started and ran when I had this happen either though. Might be something to check though? I have no idea if the 240z's have the same type of harness there on the passenger side firewall. edit: Just did a quick google search and someone said that the 240z and early 260z get the signal from the + side of the coil and the late 260z and 280z's get it from the negative side of the coil. Im not sure where or how this works if it is the right info but maybe it will steer you in the right direction. Edited June 9, 2010 by galloguy05 Quote Link to comment Share on other sites More sharing options...
Zmanco Posted June 9, 2010 Share Posted June 9, 2010 The 73 has an inductive (or current triggered) tach and is wired in series (in line) with the supply to the coil. Basically every time the coil fires it is in effect creating a pulse and the more pulses per second, the more the tach needle moves. You should get the wiring diagram for the car - it would have answered your question about how it's connected. These tachs are famous for failing as they are almost 40 years old now. If you can find a known-good one to test with, I'd expect it to solve the problem. I doubt what you're describing has anything to do with wiring. One simple thing to check - pull the tach out and make sure that the needle hasn't come loose and isn't rubbing on the face. It could be as simple as that. Quote Link to comment Share on other sites More sharing options...
xonix_digital Posted June 9, 2010 Author Share Posted June 9, 2010 Zmanco, which wiring diagram have you seen that shows that? I was looking at the scanned FSM on XenonS30.com and did not see that anywhere. I really appreciate your help though. Galloguy, that makes perfect sense. the Right Headlight is dim and it looks like it is wired into the same harness as the tach. I will check that harness for a loose connection. If that turns out to be the fix I'll mail you a beer. Quote Link to comment Share on other sites More sharing options...
Zmanco Posted June 9, 2010 Share Posted June 9, 2010 Here's a copy of the wiring diagram for a 73 240Z attached. I don't recall where I found it. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted June 9, 2010 Share Posted June 9, 2010 One-Dim-light and a side of noodles please... Seriously, the one dim light means that the right light is not getting 12v directly. Â It is getting feedback voltage through the other headlight. Â The cause is probably a bad fuse, fuse box, or connectors for the fuse box. Â Check all of those things. Â Take out the fuse box, clean it up along with the connectors, even if they appear clean. Quote Link to comment Share on other sites More sharing options...
xonix_digital Posted June 9, 2010 Author Share Posted June 9, 2010 ZMan This diagram is lightyears better than the one in the Scanned FSM I have. Thank you very much for posting! cygnus, I had no idea that that was possible. I will run through every fuse there is. Thanks guys! Quote Link to comment Share on other sites More sharing options...
xonix_digital Posted June 10, 2010 Author Share Posted June 10, 2010 (edited) One-Dim-light and a side of noodles please... Seriously, the one dim light means that the right light is not getting 12v directly. Â It is getting feedback voltage through the other headlight. Â The cause is probably a bad fuse, fuse box, or connectors for the fuse box. Â Check all of those things. Â Take out the fuse box, clean it up along with the connectors, even if they appear clean. cygnusx1 I owe you one. Email me your address and favorite brand and I will mail you a beer. http://dirtys30.blogspot.com/2010/06/thank-you-cygnusx1.html Edited June 10, 2010 by xonix_digital Quote Link to comment Share on other sites More sharing options...
xonix_digital Posted June 14, 2010 Author Share Posted June 14, 2010 ZManco, You were correct. My Tach needle was just resting on the face of my gauge. Do I really have to pull the whole dash out to replace the gauge? Big thanks Zman! I thought sure it was a wiring issue. Quote Link to comment Share on other sites More sharing options...
Zmanco Posted June 14, 2010 Share Posted June 14, 2010 No need to pull the dash. The tach is held by 2 wing nuts up under the dash on it's back. It's a bear to see them but if you lie on your back and feel around you should be able to find them. One of mine was very tight so I had to get a small pliers up there to break it loose. And don't forget to remove the dash lights (they just pull out). Once loose, you can just pull the tach out towards you while sitting in the driver's seat. Quote Link to comment Share on other sites More sharing options...
xonix_digital Posted June 14, 2010 Author Share Posted June 14, 2010 (edited) That's music to my ears! I started to pull the dash yesterday to try and pull the clock and replace with a Wideband but it proved to be a lot of work just before the move (buying a new place). Great to hear that I can get the gauge out without yanking the whole mess out. You are the man Zman. PS. I did try stuffing my fat self under there to try and see how to get the thing out but failed miserably. One of those things you really hope nobody saw. heh heh Edited June 14, 2010 by xonix_digital Quote Link to comment Share on other sites More sharing options...
xonix_digital Posted June 15, 2010 Author Share Posted June 15, 2010 Just pulled the tach last night and tore it apart. If the Gauge face was flexed to the right it wont spin, if to the left a certain degree it spins free. Just stuck a washer under the gauge face on one side and put the screw through it. Worked like a charm! thanks again everyone for all the help. Quote Link to comment Share on other sites More sharing options...
p-factor Posted June 16, 2010 Share Posted June 16, 2010 (edited) Hate to hijack this thread, but I didn't want to make a new topic with such a similar problem. I have a 74 260 but I have a 327 V8. When the car is off, the tach is at zero, but from startup to shutdown it is usually pinned at the top of the red arc, but when I use my turn signal the needle drops 300 rpm or so in relation to the blinking of the turn signal light. Every once in a while it drops down into the normal range and seems to indicate pretty close to normal rpm and functions normally for a few minutes. Does this sound like a tach problem or a wiring/sending problem from the distributor? Edited June 16, 2010 by p-factor Quote Link to comment Share on other sites More sharing options...
Zmanco Posted June 16, 2010 Share Posted June 16, 2010 Does this sound like a tach problem or a wiring/sending problem from the distributor? If it never read properly I would lean towards a wiring issue, especially since you have a V8 swap. If the input lead to the tach sees a constant +12V I would imagine it would peg the tach at 8k (although I haven't tested this). This could happen if it was connected to the wrong side of the coil. But if it really does occasionally read the correct RPM for a short while, then it sounds more like the tach is the culprit. Of course, I am assuming that all the wiring is solid and there are no intermittent connections. I would start by looking over all the wiring, especially what was changed for the engine swap. If you can find a known-good tach to swap, you'll narrow things down quickly. Quote Link to comment Share on other sites More sharing options...
Kevlars30z Posted June 16, 2010 Share Posted June 16, 2010 i have a 240z it had a sleepy speedo and one day i was going 50mph and out of nowhere it just dropped to zero ever since then it doesnt work. :/ ill fix that later. Quote Link to comment Share on other sites More sharing options...
beermanpete Posted June 20, 2010 Share Posted June 20, 2010 The tach in 73 240 occasionally goes crazy when the interior of the car is very hot, like when the car has been parked in direct sunlight for a while. The needle floats mid-scale regardless of engine speed or pins at 8K. when the car cools down it works fine and compares well to my service tach/dwell meter. I suppose the electronics have become temperature sensitive with age. Any of you guys have a schematic for the internal circuitry of the tach? Perhaps I can fix it. Quote Link to comment Share on other sites More sharing options...
Zmanco Posted June 20, 2010 Share Posted June 20, 2010 I'm attaching a zip file with a bunch of info from when I thought I'd try to repair my 73 tach. There's schematic included. In the end I couldn't find a convenient source for the germanium transistors so decided it would be easier to swap to a later 280z voltage triggered tach, and indeed it was. I don't work in the semiconductor industry any more, but perhaps someone who does could find a source, or an alternate part number. But unless you're obsessed with keeping everything "original" (in which case I doubt you'd be spending much time on this site!) it's far easier to swap in the later 280z tach. Just use the mechanism and keep your old face and needle and it will still look stock. Tach repair.zip Quote Link to comment Share on other sites More sharing options...
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