How accurate should the ACT/ECT sensors be?

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How accurate should the ACT/ECT sensors be?

December 29, 2005, 05:08:29 PM

For whatever reason, my mpg dropped ~20% on my last series of trips between Birmingham/Huntsville (~100 miles). I normally get (indicated) 30-31 mpg. My last tank netted 24.8 mpg.

KOEO netted 11s
KOER netted a 34 (vacuum to the EGR is bypassed) and 42/92 (EGOs rich)

I went to check the ACT/ECT first and got the following results.

ECT - 63.9 kOhms - Actual temp: 49.3 deg. F.
ACT - 68.2 kOhms - Actual temp: 49.5 deg. F.

Vref - 4.575 volts

I took the temperatures with a K-series thermocouple after the car had sat overnight for 11 hours.

63.9 kOhms corresponds to 34.8 deg.
68.2 kOhms corresponds to 28.4 deg.

What is the normal error range for these sensors?

How accurate should the ACT/ECT sensors be?

Reply #1 – December 29, 2005, 06:39:16 PM

From '84 Shop Manual:

ECT typical resistance.

58.75 Kohms at 50 degrees F
40.5 Kohms at 65 degrees F
3.6 Kohms at 180 degrees F
1.84 Kohms at 220 degrees F

The manual doesn't give similar numbers for the ACT.
In the "pin point tests" it says:

Check ACT sensor--engine off
With engine warmed up.
Key off, wait 10 seconds.
Harness disconnected from ACT sensor.
Resistance between 1100 and 58,000 ohms is good.
Resistance lower than 1100 or greater than 58,000 is bad.

Check ACT sensor--engine running
Harness disconnected from ACT sensor
Run engine for two minutes
Resistance between 2400 and 29,000 ohms is good.
Resistance lower than 2400 or higher than 29,000 ohms is bad.

How accurate should the ACT/ECT sensors be?

Reply #2 – December 29, 2005, 06:43:47 PM

The ECT and ACT use the same values.

A variance of 15% is considered "normal"

The values you list are lower than what my charts show, so I can't see which is closer to "correct".

How accurate should the ACT/ECT sensors be?

Reply #3 – December 29, 2005, 08:27:17 PM

I got the numbers from fordfuelinjection.com

I created some trend-lines in Excel based on their data. The only reference I have only goes to 50 degrees. After looking over their data again, it seems they just made up numbers below 50 deg. :rolleyes:

Their data from -40 to 32 deg is linear. Riiiight...

So, the supposition from my first post is all wrong due to basing it on faulty trend-lines.

I'll just wait until the weekend (no daylight when I get off work) or a warmer morning to get >50 deg. data.