Now that I know I'm getting 17-18psi in 3rd gear and who knows what in 4th gear...I've moved up the air/fuel gauge install up on the task list.

OK, signal I go to the DME.
Where can I get a good ground? The A-pillar maybe? Where can I get a hot wire (positive) from?

thanks,
Adrial

 

I took the 12-volt switched positive off my stereo, and ran a ground all the way from the gauge to the negative side of the battery. There's a trade-off there: the wire run is very long, but you're sure that you're getting a good ground.

On second thought, I probably should have just scraped a bit of paint off the a-pillar and grounded to the chassis there.

 

Chassis grounds for sensitive instruments like the A/F meter are not a good idea. If you don't want to run all the way to the battery, try the negative side of something in the fuse/relay board (you can attach from inside the car... look up when under the dash).

Also, if you don't have, or don't plan to install, a 4-wire O2 sensor, then you might try externally grounding the O2 sensor... I did this with an automotive clamp on the O2 sensor body wired direct to the neg batt terminal. Makes a bit of a difference... every little bit helps.

Good Luck!

 

Wow! Skip expresses approval of an engineering decision I made! I think I'm gonna faint...

Then I'm gonna rip off his idea about grounding the sensor body

 

You pretty much have to do BOTH of the steps that Skip mentioned. Because if you have a faulty chassis ground that requires you to run a negative line directly to the battery, then your DME is not going to be seeing the same O2-sensor signal that your air-fuel gauge is getting. Then you also have to ground the O2-sensor's body.

I also recommend using a double-shielded co-axial cable to run the O2-sensor signal to your air-fuel ratio gauge. Why would Porsche use such a heavy-duty cable if they weren't worried about interference? The O2-sensor output is a very weak signal. So if you don't use a double-shielded coax cable, again, how can you be sure that your air-fuel ratio gauge is getting the same signal as the DME?

 

My wire run goes from the back of the gauge, down the a-pillar, all the way across the inside of the dash to behind the glove box, out the grommet there, to the inboard side of the battery and then straight down the firewall to the O2 sensor. All that is stranded 16 gauge.

Seems like the only part that would be susceptible to engine noise is the short (2ft) run down the firewall. It *is* right next to the block, but... I notice that my O2 sensor wire is stranded 12 gauge, and runs almost a foot straight up before it plugs into the firewall connector.

Do you really think it'll be worth it to run double-shielded coax for less than 16" of wire?

(BTW, I have an old one-wire sensor... is it a good idea to upgrade? I mean, does the heating feature really do anything other than give you accurate readings faster?)

 

Double shielded coax cable is just TV wire isn't it?

 

Yes, it's the same type of cable. But due to the variations in diameter and thickness of the dielectric insulator, the impedance of the cable and the resultant signal that gets across can be different depending upon the type of cable used. I would try to get another cable that's identical to the one used in the Porsche harness.

[quote]Do you really think it'll be worth it to run double-shielded coax for less than 16" of wire?<hr></blockquote>Well, the length of the coax cable run from the back of the engine, through the firewall down to the DME. That's about 3-feet of cable. And it can pick up interference along the ENTIRE way. Especially going through the firewall because it's going at 90-degrees. And then there's all the other wires it runs parallel to the ENTIRE way as well.

The point I'm trying to make here, is that you want the air-fuel ratio gauge to see the identical O2-sensor signal as the DME. If you vary any of the wiring of your gauge differently than the wiring of the DME (power, ground, O2-sensor), then your display will indicate something other than what the DME sees.

 

Please elaborate more on the details for an A/F meter's hook up. I plan on installing one also but after reading this I want some detailed info on the correct or recommended procedure. Danno, please give me some specifics on the coax cable that you recommend. What numeration does it carry and where do I get it. Is hooking up to the DME's input suggested, if so , how is it done.

 

RacerX's website has a detailed procedure on how to hook up to the DME on the later cars. If you do it that way you don't need a coax cable.