Need help with making Tach work again!!!

[Stainbox12]

I recently did an engine wire harness swap, i had a 90 harness, and i swapped it with a 91, i bought the coil, power transistor, 91 gauge cluster, the cas, the only remaining thing is the ecu that is 90. Now, the problem is that the logger show the rpm's, but it doesn't display on my cluster, neither the water temp. I need help on this one. Thanks.

 

[Talesin]

Make sure that they're plugged in under the hood. The tacho has one wire directly behind the intake manifold, on the driver's side, with a connector in the middle. When I had my new engine put in, they forgot to plug it in. They thought they blew the cluster, went through all kinds of hoops, just to find the unplugged wire, plug it in, and it started working again instantly. Said something about a 'line filter', but it sounded more like embarassment. Same thing with the gauge cluster temperature sensor, it's the one on the thermostat neck with only ONE wire. Mine was broken off, so I crimped on a spade connector and it works just fine. The two-wire sensors are for the A/C and ECU. To test if it's the sensor that's dead or a wiring issue/dead gauge, just touch a grounded wire to the connector while it's plugged in. If the wire path to the gauge is OK, and the sender is just bad, the temp gauge will spike all the way to hot.

 

[luv2rallye]

The 90 had a signal from the coil pin 3 go to a noise filter (aka tacho interface) and it's output went to the instrument tach. The 91-94 got it's instrument tach signal from the power transistor pin 4. All of these wires (on both) were white.

 

[Stainbox12]

The 90 had a signal from the coil pin 3 go to a noise filter (aka tacho interface) and it's output went to the instrument tach. The 91-94 got it's instrument tach signal from the power transistor pin 4. All of these wires (on both) were white.

That means then, that it should work right? because i changed, the transistor, coilpack, and gauge cluster, along with the whole new 91-94 harness, the filter has nothing to do here. As i said, i can see the rpm's through the datalogger, so the problem must be the gauge itself correct?

 

[Talesin]

I'm pretty sure that the ECU gets its tach signal from the CAS, and figures it out from there, given that it's what tells the coilpack to fire in the first place. The tacho interface is just for the gauge cluster.
I'm uncertain if the gauge cluster tacho line is even part of the main engine harness, honestly. Could be the proverbial 'wild hare' just poked through the firewall to where it needed to go, since it was so close.

 

[luv2rallye]

That means then, that it should work right? because i changed, the transistor, coilpack, and gauge cluster, along with the whole new 91-94 harness, the filter has nothing to do here. As i said, i can see the rpm's through the datalogger, so the problem must be the gauge itself correct?

Well you'd think so. I don't know where the datalogger is getting it's signal but I'd suspect it's from the CAS as talesin says. The signal from the '90 noise filter or the '91 PT both go to ECU pin 109 but that's labeled "fuel pump control" which the ECU uses to run the FP only while getting spark. So to know for sure if your gauge is bad you'd have to connect an aftermarket tach onto the signal that now goes to the stock tach gauge (PT pin 4). Or you'd think if your FP is turning on (assuming it wasn't rewired) that signal would be there. Keep in mind you still have a 90 ECU which might give you other problems. Example pins 6 and 14 are swapped: http://www.dsmtuners.com/forums/showthread.php?t=51539#post1865524.