How-to Use HVAC Display as Tachometer

[ghlkal]

Did you know you can check engine RPM on your HVAC (C68) display? Why? Well, my tach is way off and I wanted to see how far. I ordered a Tech 1 which didn’t work, so I’m wondering how could I see how far off the tach was.

As I was troubleshooting an AC problem, I discovered that in addition to displaying HVAC fault codes, the HVAC display could show other parameters too.

In my 1990 FSM, this is on page 1C1-5

To display fault codes from the HVAC system, press and hold both [fan up] and [fan down] buttons simultaneously for 5 seconds. Pressing the [fan auto] button after that will display any HVAC fault codes (eg, I had code 09 – low Freon). Now, if you press [fan up] you can move through a number of additional parameters.

Parameter 6 is engine RPM (RPM / 25). After pressing [fan up] to display “6,” press [fan auto] to see the value.

At idle my tach showed just a tad above 1000 RPM. Parameter 6 on the HVAC showed 26. So 26 * 25 = 650 RPM.

(As a noob, I keep looking for some information that I can share to help pay back all the great info I get here – I searched and looked in solutions, and I didn’t see this … but, it’s probably posted and many of you may know this)