Tach swap issues.

[Andy.B]

I just wrapped up a 2.7i build, and am having some issues with the 7k tachometer. The car is a 325 that was manufactured 10/86, and the tach is from a 91 318. It is a motometer tach, and I kept the 325e coding plug. The problem is that the tach reads progressively higher the faster it revs (idle is accurate, but actual 2.5k reads 4kish). Just to see what would happen, I swapped in the 318 plug, and have the same problem. The strictlyeta writeup(as well as other threads) state to use the original plug (the 325e coding plug), but that clearly isn't working out in my case.

Do I need a 325i coding plug, or is my 7k tach broken?

[Gregs///M]

You need a 325i coding plug...

A 325i coding plug will interpret tach signal from a 6 cylinder car with 7k red line.
Your 325e interprets a 6 cylinder car with 5k red line.
Your 318 interprets a 4 cylinder car with 7k red line.

Thus you need the 325i plug.

[jlevie]

The coding plug for the 325e should work. Both 325i and 325e are six cylinder engines and all the coding plug does is to tell the tachometer whether the engine has 4 or 6 cylinders.

It sounds like you have a bad tachometer.

[nando]

wrong coding plug will make the econometer read incorrectly. and since the tach has a different swept range, it also affects the tachometer output - so you can't use a 325e plug with a 325i or vice versa.

[Andy.B]

I borrowed an I coding plug, and the tach read correctly. It now makes me wonder how many people out there with 2.7i swapped ETA's are driving around with incorrect tach readings, since most guides I read say to retain the original coding plug.

I hooked up a timing light with a tach when I broke in the cam, and used a zip tie to hold the rpm. Then, when I drove it the next day, the rpm's seemed a lot lower than they did the day before. I hooked the timing light back up and checked the tach and noticed the discrepancy.