Carrera51

Hi Guys:
My tach in my 84 Carrera started acting up recently. When I turn the ignition on, the needle swings up to 7K and sits there. If I blip the throttle, it tries to swing backwards. I removed it and unplugged it, then plugged it back in and it acts normal for a few minutes, then does it again. Is the tach going bad or could this be a ground issue. This seemed to start after changing the fuel filter and then connecting a fuel pressure gauge to the fuel rail when diagnosing a fuel problem that turns out to be my fuel pump pressure dropping at high RPM.

Any suggestions on helping me sort out this problem will be appreciated.

Thanks,

whalebird

I think I would take a volt meter and check the alternator output at the battery first. Eliminate an overvolt situation first. Maybe try another DME relay just for kicks as well. I would start with some of the basics.

Carrera51

Thanks. I did do a DME relay swap when I was sorting out the fuel issue at the track. DME relay is OK. Will check alternator output.

Thanks for your help.

Houpty GT

Maybe the new relay is defective. Swap it back and see what happens.

I have an LM-1 air fuel ratio meter hooked up to my car and if the LM-1 is not turned on then the tach will not turn on or act funny. Do you have anything like that on your track car?

Carrera51

Houpty:
No air-fuel ration meter. Stock motronic 3.2. Put the old DME relay back in when I discovered it wasn't the problem. Going to try borrowing another stock tach and see what happens. Funny thing is that I turned the ignition on today to roll the windows up and the needle did not move so I am definitely stumped at this point.

Thanks,

whalebird

I know it's hard to tell at this point, but possibly an ignition switch is going south - they can do some strange things

ron mcatee

Maybe a resistor has failed.

dshepp806

Could easily be a ground issue,..or the instrumentation is failing (due to a component, if anything). The tach swap out will tell you lots, for sure.

As Bird suggested, start simple. Get that voltage measurement for sure, as this malimpact other things, as overvoltage is not a good thing,..especially in the DME world.

As Bird stated, that frikin ignition switch can wreak havoc in ways that are utterly confusing.


Looks like you've some good starting points.

Best,

Doyle

whalebird

Shepp. I was thinking of you when I posted above. I wondered if you could look at the schematic and tell if the relay itself has a provision for overvold protection. The diagram may be on the side of the relay itself.

dshepp806

You speak of the DME relay? I don't think so,..but will look. One would look for protection on both the coil side, as well as contact(s) voltages, I would presume,..in a perfect world.,...I doubt I'd find any such conditions.

I've often wondered (moreso) about the DME protection from overvoltage, as I understand it to NOT be the case,....never delved into the prints but wanted to study a protection approach (zenered, at the least), to see if there was a viable protection approach (MOD) to the DME when these conditions occur. I read one thread (a while back) where a fellow lost his DME because of this condition being allowed many days with no monitoring of voltage to tell the poor chap what was going down,..ultimately, he started seeing the RPM gauge go beserk,...well,..you know the drill. Let it go a few days more,..then the DME failed on him...not good. I could never get a clear answer from the gent as to what specifically failed within the DME due to these conditions.......maybe Loren will chime on his bench findings as to OV conditions causing DME failure. AGAIN,..I never really studied the prints any further.....maybe when the Fall comes,..this year...,..I'll indulge my anality.....

Mark: be sure to monitor that voltage at various RPMs. No spiking!

Best,

Doyle

Ed Hughes

I'd vote for the tach, these things cannot last forever.

Lorenfb

"as to OV conditions causing DME failure."

That's correct. The over-voltage condition does occur with the 3.2 as the
Valeo alternator regulator fails in that mode causing a catastrophic failure
to the DME ECM. It can also cause the tach to function sporadically, as
the tach signal is a direct function of the alternator voltage. This occurs
often on the 911SC with the Valeo alternators too.

Bottom line: An internal regulator alternator was a poor choice for the
air cooled 911/964/993. But then again the accountants saved Porsche
the cost of one 36'' wire.

whalebird

Shepp/Loren. I mentioned in an earlier thread the mercedes part known as an overvoltage protection relay. It is a part found on most benzes thru the mid 90s. I have wondered if one of those could be grafted into the circuit structure. Here is the first link I found just for a referance.
http://www.thebenzbin.com/mercedes-hard-start.html

theiceman

+1 for us external regulator guys .....

rusnak

Wait a second. We can add a 10 amp fuse/relay to protect our very costly DMEs from overvoltage?

Where do I get in line for that?