Troubleshooting Fiero tach w/ 3800SC (Page 1/2)
sdgdf JUN 02, 05:27 PM
This weekend I'm going to try to work on a few wiring projects on my 88 coupe, first of which is my tach. It hasn't worked since I got the car. My idea now is to cut the tach signal wire coming from the ignition control module and run a wire directly to the tach. At this point I want to take the tach out of the car and try to get it working manually on my workbench.


I can hook the tach signal wire from the 3800SC's icm directly to that bolt that says coil? Then just 12v hot and ground and it should work? If that doesn't work, does that mean my icm is bad?

From what I know, there's another wire to runs to the pcm that gives it a tach signal, and this other wire is an old-school tach signal that is supposed to go directly to a tachometer. In the Grand Prix its not actually used but it still has signal.

I was also thinking of running a wire up to the tach wire that runs through the center console, but on this site http://www.nathanbittinger....roaddiction/c203c500 I don't see a tach wire there at the C203 connector? How is it routed from the C500 into the interior and to the dash???
olejoedad JUN 02, 08:42 PM
Tach signal is from C500-C3.

Front harness/engine harness plug pin E will get you the tach signal. White wire.

Hook it to C500-C3, no filter needed.
sdgdf JUN 03, 04:53 PM

quote
Originally posted by olejoedad:

Tach signal is from C500-C3.

Front harness/engine harness plug pin E will get you the tach signal. White wire.

Hook it to C500-C3, no filter needed.



I appear to have signal at C500, but no signal up in the dash. So my next step was this plan and running a wire manually to see the tach work at all. I've bought a couple tach boards off ebay just in case the 1 in here is bad. I was also wondering how to connect the wire up to C203, and in what way the tach signal gets from C500 up into the interior?
olejoedad JUN 03, 05:23 PM
It goes to the dash from the C500.

What plug and contact are you checking at the instrument cluster plugs?

The C203 connector has nothing to do with the TACH circuit.

The PCM has nothing to do with the TACH and the tach output from the PCM with not drive the Fiero tach.
sdgdf JUN 03, 09:06 PM

quote
Originally posted by olejoedad:

It goes to the dash from the C500.

What plug and contact are you checking at the instrument cluster plugs?

The C203 connector has nothing to do with the TACH circuit.

The PCM has nothing to do with the TACH and the tach output from the PCM with not drive the Fiero tach.



C3 connector, the 1 on the passenger side of the gauge pod. The white wire on the top passenger side? I’m not getting anything there.
olejoedad JUN 03, 11:02 PM
Are you checking continuity from C3-18 to C500-C3?

You should have continuity.
sdgdf JUN 04, 10:58 PM

quote
Originally posted by olejoedad:

Are you checking continuity from C3-18 to C500-C3?

You should have continuity.



Ok well I connected a wire directly from E wire on the 3800sc icm to c500-c3 and the tach works. The needle moves as I rev it but it’s reading 0rpm at idle and as I rev it only goes up to 1k. I need to wire up and obdII connector so I can use my scantool to read actual rpm. This is progress. The car is an 88 L4 and it has a V6 tach in the dash. What do I need to adjust with the tach to correct the signal?
jelly2m8 JUN 04, 11:20 PM

quote
Originally posted by sdgdf:


Ok well I connected a wire directly from E wire on the 3800sc icm to c500-c3 and the tach works. The needle moves as I rev it but it’s reading 0rpm at idle and as I rev it only goes up to 1k. I need to wire up and obdII connector so I can use my scantool to read actual rpm. This is progress. The car is an 88 L4 and it has a V6 tach in the dash. What do I need to adjust with the tach to correct the signal?




The Stock V6 tach works without mods if it is connected properly as olejoedad replied right below your first post. Possible you have a faulty tach

[This message has been edited by jelly2m8 (edited 06-04-2023).]

sdgdf JUN 05, 12:58 PM

quote
Originally posted by jelly2m8:
The Stock V6 tach works without mods if it is connected properly as olejoedad replied right below your first post. Possible you have a faulty tach




I just changed the board on it a couple weeks ago, could it also be possible my icm is bad? Or I have a bad connection in the gauge pod or something? The light bulb for the tach flickers as I'm driving around. Its a FL car and there's light corrosion everywhere including the interior. I can swap boards again to see about the tach itself.
olejoedad JUN 05, 02:23 PM
FYI, forum member JGunsett can troubleshoot and repair your existing tach at a very reasonable price.