Tach troubleshhooting

[PolarBear]

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?

[PolarBear]

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.

[//<86TA>\\]

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.

[PolarBear]

Originally Posted by //<86TA>\\

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

[//<86TA>\\]

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.

[PolarBear]

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.