2.7 Floating Tach

Porsche_monkey · 11-17-2004, 09:24 AM

My tach has started to work sporadically, generally sitting at zero, but it appears to begin working, or alt least showing something, as I get up to 3,500 or 4,000 rpm.

(I have the original ignition system, everything ran fine up to yesterday when I ran my battery dead. Recharge via trickle charger overnight.)

Anybody else had this?

mede8er · mede8er's Garage

Mine did the same just before my battery died...new battery, no more float......

Porsche_monkey · 11-17-2004, 09:34 AM

I thought I'd read that before, but I couldn't find it by searching. I don't doubt what you say, but it seems curious to me, because I don't see the connection. the tach does run from the ignition right? Not from the generator (like a diesel)?

RoninLB · 11-17-2004, 09:38 AM

Quote:

Originally posted by PBH

I don't see the connection. the tach does run from the ignition right?

it's powered from the CDI. How did you kill the battery ?

Porsche_monkey · 11-17-2004, 09:41 AM

Just rebuilt the engine, completed in June. Car has been driven only occassionally since. Then I let it sit for 6-8 weeks. Tried to start: 'dead'. Trickle charged battery. Fired up and ran right away, but no tach.

mede8er · mede8er's Garage

In my case I just picked my car up from the seller....car had been sitting for over a year...on the way home I stalled her out and it wouldn't restart (dead battery) got a jump and took her home with tach floating wildly.....replaced battery the next day and it's been fine for weeks now....go figure......

RoninLB · 11-17-2004, 10:10 AM

Quote:

Originally posted by mede8er

....go figure......

I never found out very much about tachs.. the CDI has a pulse shaping circuit for the tach. The weak battery issue is interesting.

The usual suspects are points, connections including the 2 into 1 on the engine comp harness on the left, installing an MSD, and a gauge glass installed in reverse.

Porsche_monkey · 11-17-2004, 10:28 AM

I will double check my electrical connection, I know I left it undone once. But since it is floating, rather than dead, I assume it is still plugged in properly.

randywebb · 11-17-2004, 11:54 AM

clean or replace points

if that's not it, start saving for a new cdi box and carry a cell phone while driving.....

Porsche_monkey · 11-17-2004, 11:55 AM

Sorry should have mentioned, points are brand new. Cdi box is possible though, being 30 years old...

kach22i · kach22i's Garage

Quote:

Originally posted by PBH
Sorry should have mentioned, points are brand new. Cdi box is possible though, being 30 years old...

Still it could be the points.

If you do a points search you will find some people claiming that they need to be readjusted soon after installation, that there is a break-in peroid.

This spring my points acted similar to what you have described, they apparently were bad right out of the box. Almost never happens I'm told, but sure enough it happened to me.

mede8er · mede8er's Garage

I'm thinking the bad battery...sucking the life out of the electrical system....is compromising the tach reading.....

RoninLB · 11-17-2004, 01:58 PM

Quote:

Originally posted by mede8er

I'm thinking the bad battery...sucking the life out of the electrical system....is compromising the tach reading.....

hmmm.. maybe the fact that the alternator had to charge up a weak battery caused a diode to weaken and cause an AC ripple. The AC noise hits the CDI and continues to the tach electricals.

if this happened the tach would stay screwed up with a new battery..

kach22i · kach22i's Garage

RoninLB, you are the points expert I was thinking of.

Looks like you are on the job again, still learning from you, it's a way of life.

unclebilly · unclebilly's Garage

MY tach has behaved strangely since replacing the points. When the car is warm, the tach floats up to 3000 rpm at an idle, under throttle it is usually well behaved. I just haven't gotten around to readjusting the points.

RoninLB · 11-17-2004, 02:38 PM

Quote:

Originally posted by kach22i

Looks like you are on the job again, still learning from you, it's a way of life.

have a good time.. I've made every mistake possible at one time.
I now keep rolling by not making the same mistake twice. LOL

sometimes the needle spring will slowly weaken with age causing the floating sensation. I haven't had it happen yet in 160k mi, although some have said that 90% of the time a tach prob can be fixed with just changing the points.

Howard M · 11-17-2004, 03:46 PM

The signal from the points goes to the tach and the CDI, so I don't think it's a CDI problem (the points are the trigger for the CDI). The tach is ground sensitive. Double check the connections in the rear (purple striped wire for the points signal, I think, and brown for the ground). While you're back there, make sure the ground is a good one.

Howard

randywebb · 11-17-2004, 03:53 PM

break-in period -- that little block might have a sharp edge on it when new - would cause rapid wear and hence gap change as it wore down to a wider amt. of material. I thought Bosch rounded theirs off, tho?

Ron! I would never admit to having made every mistake possible at one time.... that would include a wide variety of sexual mistakes... Har!

RoninLB · 11-17-2004, 04:32 PM

Quote:

Originally posted by randywebb
that would include a wide variety of ... Har!

try timing 4 of them together + me getting satisfied in one night and you learn what proper timing is all about. Drink tons of water and cycle every 45min and it's a banging good time.

So anyway, don't let your meatloaf.

RoninLB · 11-17-2004, 05:11 PM

Quote:

Originally posted by Howard M
The signal from the points goes to the tach and the CDI, so I don't think it's a CDI problem (the points are the trigger for the CDI). The tach is ground sensitive.

yep.. I twisted the 6 pin with the 3 pin CDI.
The 3pin CDI tach was designed to use grounding from the distributor points. The 6-pin signal from the CDI-unit has the pulse shaping circuit.