Tacho unreading- effect ECU in anyway?

[fnukle]

Hi :),

Ive got a 1988 3.5l EFI classic.
tacho under reads ie. nothing at idle and starts reading around 1500 but obvously 1000-2000 rpm lower than it should.

i was told by the previous owner that its just the retifier on the back of the alternator but my research is telling me otherwise and im not very clear on the matter.

However
my main question guys is will this effect the way the engine runs, ie does the ECU use the same info the tacho gets and fuel mix accordingly?

thanks Pete

[fnukle]

I just hooked up a multimeter to my battery to give me an idea of what the alternator is doing.

key off-engine off= 12.80-12.60V~ (batterys not in great condition by the looks)
key on-engine off= a 0.10V drop (expected)
key on-engine on= 12.40-12.50v (without any use of throttle as yet, battery is being drained)
key on- engine on=13.60~+v once throttle is touched and returns to idle.
seems alt isnt charging at all.
its not untill i blip the throttle that it starts charging, even when it returns to idle it continues charging.
is this expected? is this the regulator or retictfier?


perhaps putting a smaller pully wheel on would fix the problem(of underreading tacho) but would of course increase volts(taken care of by regulator) and more load on the engine therefore less of the limited power and even more costly on fuel hehe.

i dont see how the rectifier could be the cause as all it does is change ac into dc. so must be either alt with too big of a pulley or where ever the alt sends the tacho info from is broke.

is that a seperate item i can replace or is it time i go for a decent alt?

[peter r]

G`day Pete ,

Pressuming it`s the std flapper system fitted to your 88

The Ecu uses the pulse from the coil/dissy doesn`t use tach pulse .

The correct size pulley is either 62 or 68mm , can`t remember .
Has to be the right size for the tach to read right .

It gets its pulse from one of the phases in the alt and other than getting the pulse has nothing to do with how the alt works . The wire can have a break which sometimes will give an intermitant reading but is more likley to be the brushes or the diodes .

There is also a multifunction unit , it is also know by another name , in the instrument housing that can play up , the tach signal goes to it before it goes to the gauge .

A bad earth somewhere can also be a cause .

Being EFI if your battery aint great and your alt is suss there is a good possibility that if these are corrected your miss in your other post will be gone as well .

When you start it does it seem to fire when you release key , as in wind , wind , wind let the key return and it fires , if so it`s telling you it`s having a loss of voltage under cranking .

Low voltage upsets everything on EFI engines .

All the best , peter

[fnukle]

wow awesum reply thanks for all the info Peter :)
yea its the flapper(afm has the piece of metal that opens as the revs increase)

good news about the ecu using pulse from coil.
i've got a brand new sandard duty battery that i'll put in parrallel to increase amps which should make the difference if that is indeed the problem.

i'l report back tomorow with the results
thanks
Pete