Swapped a 95 LT1 into 86TA. The thirdgen tachometers work with LT1.

Tach reads zero. Only code thrown was Code 11: Malfunction Light failure or SES light failure. I'm seeing 12V at tach output wire, until i rev the engine then it drops a couple volts. Also seeing 12V with key on but engine off. I hardwired two tachs (one at a time) just in case the tach was bad or the connnector was bad, still nothing. Took a resistance measurement from A13 to tach, it was a good connection. Freescan shows RPM and car drives just fine. Also, reprogrammed the pcm with a madz28 tune (b/c i haven't gotten to it yet til now) and still nothing. My PCM is socketed. I also have no speedo output but i haven't looked into the speedo problem yet.

What should i try next?

 

Is the VSS signal getting to the PCM? In a 4th gen. it's on the yellow and purple wires going to the VSS in the tranny but don't know a thing about the 3rd gens.

 

i haven't checked out the speedo yet. But the vss wires are the same color for LT1 and thirdgen electronic vss. Yellow and purple. Its not throwing any codes. Actually most of the wires that connect to the same stuff for Lt1 and thirdgen are the same colors. And i might not have the right speedo adjustments in the pcm for the 700r4. I have to get a laptop next and see if the pcm is putting out the correct speedo signal on the next test drive.

A guy at thirdgen.org's message boards told me he had the same problem with the firebird tach. He used a 8000 rpm tach from a 350 tpi firebird, problem solved. So, i'm going to give that a try next.

 

Have you checked the fuses for the gauge panel?

 

The PCM basicly just reads the tach signal and passes it on, so if you read the same on the input to the PCM as on the Output from the PCM then the problem is from there on out.

I put an 94 LT1 into 92 S-10 pick-up and used an after market tach so I really have no experience with the swap in a third gen. But the speedo in the LT1s was modulated differently than previous vehicles, You can use the VSS signal to the speedo from the PCM but it must be modulated (or converted , can't remember "proper" term). The S-10 swap sites can tell you where to get the box that does it all.

 

According to the 95 PCM pinout data on Shoebox's website, the signal at pin A13 for the tach wire should be less than 0.5V with the key on, engine off.

 

Put the 8k tach in, still not working. Which exact wire can i wire in the tach to other than the A13 wire?

I've check fuses, wire connections, taken the tach out of the cluster and hard wired it. I've also tried 3 tachs, 2 gauge clusters.

If i cant get the right values in programmed in the pcm to get the 700r4 speedo working, then i will buy a dakota digital sgi-5 box. But first i need to hook up a laptop while driving to see if the pcm is getting zero mph like its showing on the gauge.

The other thing i have done is completely taken out vats and programmed it out. Does the vats box have to be connected/wired up? Any chance it could affect the gauges.

its also possible that the wire from the a13 connection to my tach could be cut and touching another cut wire that has 12V on it, but i highly doubt it. I might cut the wire after the A13 connection and see if it still has 12V with key on, engine off.

 

If your problem is getting 12 volts, quit playing with fuses and go direct. At least until you prove out the function of the tach.

 

The problem appears to be that he is getting 12V on the wire from A13, when he's supposed to be seeing less than 0.5V KOEO.

 

The other issue is that he is trying to read a waveform with a multi-meter.... the tach is a On/Off signal at different frequencies from 33Hz to 800Hz, thus your multi-meter won't tell you much excpt for what Injuneer is stating about KOEO setting where there should just be the 0.5V or less.

The PCM maynot be outputting the correct signal to the tach wire or there is some other issue with that wire.

The PCM does not feed-through the signal either. It's filtered in the PCM to remove noise, then fed to the Microcontroller and on through the wire to the dash.

If you want, hook the tach right to the coil pack instead of the PCM, that's where it was always done old-school.

 

i went through the coil, white with black stripe wire and my tach now works. My multimeter measures frequency also, and the frequency i measured at A13 never really changed that much when reving the engine. Someday i'll swap pcms with someone and see if its the pcm or something weird with the wiring.

For the speedo i'm going to swap the vss in the 700r4 with one from a 4l60e and then press on a reluctor from a 4l60e. Now i just hope the pcm will output a speedo signal, find out soon.