Tach Reading off

[burrsg101]

Ok I've searched for quite some time and all I keep coming up with are people having tach problems after a swap (e to de, or an sr or rb swap) which isn't at all my issue. I just helped my buddy swap a 97 ka into his 91 after his 91 got rod knock (we reused the 91 dizzy) and I ended up getting the car from him. The tach now is reading way off (says im at 3500 rpm when I'm at more like 23-2400). He said he was having problems with it doing that before the swap as well. I'm assuming the tach is just garbage, but want to be sure it's not something else. Is there anything else I should check first?

[compactfean]

What harness are you using?

[burrsg101]

The original 91 harness. Only harness that got used from the 97 was the sub harness for the intake manifold/throttle body. It's apparently intermittent though as it started working perfectly last night for a bit. When it's not working though it seems like it reads almost exactly double what I'm actually spinning.

[compactfean]

I believe in the s13 harness there is a resister between coil negitive and ecu but I can't remember if that's sohc or dohc. Maybe check that out and see if its going bad.

[burrsg101]

Do you know where that's located?

[compactfean]

I dont, I did a write up on debugging problems with the s13 gauges and realized in the wiring diagram, there was a difference from sohc to dohc on the way tach signal is wired and also the resister but I haven't seen the resister and where its at.

[burrsg101]

Well I've narrowed it down to being something under the hood. It only goes haywire when it rains(there's a stupid aftermarket hood on here that has a mini cowl basically that's just sitting open over the dizzy). So I can assume that's my issue. Oh well I planned to throw either a stock hood or a silvia conversion on anyway.