240zip Posted March 15, 2013 Share Posted March 15, 2013 I have a MSD 6AL, Pertronix set-up with an aftermarket Mallory mechnical advance distributor. The tach on the car used to work just fine, now any electrical load causes the tach to bounce all over the place. Even without a load it bounces. You have to guess the RPM as someone in the middle of the 'bounce range'. When a load is applied (brakes or turn signal) the bounce is even more pronounced. The tach bounced in harmony with the turn signal. As the load is removed, it bounces up. Otherwise the car drives fine. While likely not related, the battery is dead. I figured this was due to inactivity over the Winter, but it now won't even take a charge. I'm going to run out and get a new battery today. It was a few years old (I think around 4 or 5). I know that Big Phil had a similar issue with is turbo car that seemed to happen when his car was cold. This still lingers when up to regular operating temp. Quote Link to comment Share on other sites More sharing options...
BluDestiny Posted March 15, 2013 Share Posted March 15, 2013 Possibly corrosion behind gauge or your tach could be going out. My 260z did that every now and then. I'd start the car and it would go crazy and never stay still even though the car was idling. Then after about 20 min driving it would be back to normal. Then hit a bump and it would go crazy again. I didn't care enough to fix it since I was going to do a full gauge swap, but then I sold the car. Quote Link to comment Share on other sites More sharing options...
240zip Posted March 15, 2013 Author Share Posted March 15, 2013 Did your tach bounce when you used the turn signal or brake? I guess an easy (relatively speaking) test is to swap out the tach. Not exactly what I wanted to do this weekend. Quote Link to comment Share on other sites More sharing options...
BluDestiny Posted March 15, 2013 Share Posted March 15, 2013 No, no other electrical affected it, and the amp meter in the car (if working/accurate) was rock steady. Have you measured your voltage under load/idle? Quote Link to comment Share on other sites More sharing options...
Xnke Posted March 15, 2013 Share Posted March 15, 2013 This is an easy one guys... The tach didn't bounce before...but the battery is dead now. Any electrical load causes the tach to flip out... So... Get a new battery and run the system at full voltage and I bet it goes away. Quote Link to comment Share on other sites More sharing options...
240zip Posted March 15, 2013 Author Share Posted March 15, 2013 I'll swap the battery before I do anything else. Quote Link to comment Share on other sites More sharing options...
240zip Posted March 15, 2013 Author Share Posted March 15, 2013 I did some digging around and another factor might be my dual electric fans and the aftermarket fuel pump in the rear of the car. I'm going to guess all that extra electrical gear is putting stress on the electrical system. I saw this in a Lotus forum: my tach which jumps around whenever the aftermarket electric fuel pump (which is powered off the coil) clicks on? I have a Twin Cam (with standard points, not the Luminition), and the problem is that the tach seems to "droop" for a split second. The frequency of the droop is proportional to the engine speed and throttle position. Therefore, at idle the tach will droop for a split second about every second or so (which corresponds to whenever the fuel pump clicks on). But at heavy throttle at around 4k and above, the tach droops so frequently that it basically never goes above about 3000 to 4000 rpm. So after reading this post about Luminition, my hypothesis is that the tach may also be confused by the current which gets diverted to the electric fuel pump. Any one else have a problem like this? I guess the obvious fix is to wire the fuel pump into some place else, so where does everyone suggest?Yep, not much doubt since the tach is current activated, not voltage activated. The easy fix for you is to drive the fuel pump (which draws a lot of pulsed current) with a relay (which will draw a negligible and constant current) actuated by the the line that presently drives the fuel pump, and drive the fuel pump through the relay to fresh solid wires from the battery (with an inline fuse, of course). A standard 30 Amp relay and socket can be had at a NAPA or Carquest place. The same is true for anything else that is attached to the ignition line. BTW, since this is a current path problem your problem should also be cured by attaching the fuel pump to the ignition wire on the 'upstream' side of the tach since the pump current would no longer be pulled through the tach sense loop. I'd recommend the relay as it will relieve the rest of the wiring (possibly the key switch) from the fuel pump load and increase reliability due to shorter power line runs for the pump. Seems to the same problem I have. Anyway ... we'll see if the new battery solves the issue first. Then I'll think about some things I likely need to do like get a more powerful alternator. Quote Link to comment Share on other sites More sharing options...
Heroez Posted March 23, 2013 Share Posted March 23, 2013 Did your tach bounce when you used the turn signal or brake? I guess an easy (relatively speaking) test is to swap out the tach. Not exactly what I wanted to do this weekend.My tach in the 260z (280z Type Tach) will jump to the turn signal at times. Most of the time it just lays there. Ive been trying to think of why it does that. Care to offer a clue? Stock electronic distributor. Quote Link to comment Share on other sites More sharing options...
30 ounce Posted March 23, 2013 Share Posted March 23, 2013 Mine did that too until I replaced the voltage regulator. Mine was bad but I suspect your dead battery is the cause. Quote Link to comment Share on other sites More sharing options...
Heroez Posted March 23, 2013 Share Posted March 23, 2013 Would have been a good time to put in a 60 amp or better internal regulater alternator. No more browny frowny. My battery is fine. Just got a ton going on. Quote Link to comment Share on other sites More sharing options...
240zip Posted March 25, 2013 Author Share Posted March 25, 2013 I have a 70amp from a 280ZX turbo and the adapter plug from MSA. I agree that the alternator needed to go. The battery was old and needed replacement. Swapping the alternator and battery was so much easier than going into the dash. Quote Link to comment Share on other sites More sharing options...
BluDestiny Posted March 25, 2013 Share Posted March 25, 2013 So that fixed the problem? Quote Link to comment Share on other sites More sharing options...
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