Tach troubleshhooting
 

I finished the swap of the 4.8 into the firebird and my Tach isn't working. Engine is from a 2001 Pickup.

Outside of being connected into the wrong location in the ECM or the C100 in the 92s harness, how can I tell whether I am actually getting output from that wire? I had a 1000 ohm 1/2 watt resistor on the back of the cluster from the +12 pin to the tach input and I got nothing and the Tach output resolution - high and low, in HPT, were both set to 3 and I still got nothing. I pulled the resistor and tried changing that value to a 5, but still nothing. Next I will try putting the resistor back on the circuit board with the 5 set and see if I get something, and I will double check my pinouts

Can I use a logic probe to test the output on that pin from the ECM to see if I am getting output?

Read some over at HPT forum. I didn't realize what the tach output resolution was, it is to show what the engine RPM is to the gauge since the same ecm is used across platforms and since I have a V6 tach in there I need to set them to a 4, I believe to get the proper reading. I thought it was to change the output power of the tach, I read a long time ago there is a way to boost the output through the ecm without using a pull up resistor, but I don't remember where or how the guy was able to do it.

I thought the v6 tach wouldn't work with a v8? Do you have a extra cluster to swap and try? I have a few if needed.

It should work, that is what I am reading on the HPT site about setting the output for different tachs, originally V6 or I4's

I think I have like 3 more clusters, at least two of them are from a V8 car but the V6 has the higher redline than the V8 TPI cars

The tpi cars had the 8k tach as well, unless you mean the red mark is actually in a different place on the v6, I never noticed.

The Orange line on the V8 starts at 5K and on the V6 it starts at 5500, they both have designations to 8k, it's not a big difference but there's no point in changing it.


The problem existed between the keyboard and chair. When I did my spreadsheet there were two places I with the tach info, on different pages. When I made the tach wire originally I made it a little too short and it was in R10, then when I finished the harness I remade the tach wire I must have looked on the other page where I had put the wrong pin. The book says engine speed instead of tach, so I must have read the "speed" in vehicle speed and put that pin down instead. I moved the wire and fixed my spreadsheet now. I tried without the pull up resistor and it still didn't work, but once I added that it worked right away. I also changed the output resolution to a 4 in both fields and the tach is really close, enough so that I don't care about the difference, it's like within 50 Rpm.

Still wondering if a logic probe will detect output on the wire, I'll have to try it before I button the dash up.