I'm building up an 87' LT with a 350 TPI. I wanted to know what harnesses would work. 85-89 ?? The car has NO harness in it now except for what is attached to the dash.Car had NO tach and 85mph speedo. I have switched out the gauge cluster for an 89 IROC's. Any tips?

 

Your '87 5.7 is a mass air flow (MAF) type setup and uses a cold start injector. Any '85 through '89 harness & ECM should work, with minor modifications ('89 does not use a cold start injector). You could also use a '90 - '92 harness, if you bought a manifold absolute pressure (MAP) sensor, installed it on the plenum, and plugged the cold start injector port from the fuel rail - the MAF sensor would no longer be used. In any case, make sure the PROM is correct (for a 5.7), and make sure the correct knock sensor is used.

Tim

 

What about my guage replacement? Am I going to need to due the dash harness as well? Car had NO tach before, it was a big a*s fuel guage and had digital pressure guages. Now it has a different guage pod eniterly without digital and a 7K tach. Thanks again for the advice.

 

The instrument cluster harness should not have to be changed out. The clusters up through '89, except the Berlinetta & T/A digital dash types have a speed buffer built into the cluster that provides the ECM with a 4000 pulse per mile signal. The GM digital dash setups had a separate (yellow box) speed buffer on the drivers side, under the dash. In the '90 & up ECMs, the speed buffer is in the ECM & a vehicle speed sensor (VSS) is used to feed it - no cable drive. In this case, the ECM feeds the speedo with the 4000 pulse per mile signal.

Tim

 

Wouldnt he have to use an 87 harness, because in 88 they went to serpentine systems and the plugs would be on the opposite side.

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Yes, but I'd consider swapping sides for the alternator & A/C wiring to be among the minor modifications.

Tim

 

Just as a quick addendum, the '85 is not compatable with the other MAF years, if you run an '85 harness, then you havee to make sure you get an '85 MAF.