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Discussion Starter · #1 ·
Yesterday my check engine light came on and flashed 3 times indicating the inlet air temp sensor.

When I measured the voltage across the sensor it was 1.58V, this was at a temperature of 10 deg C. I also measured the rersistance of the sensor and it was 3.0K Ohm so the question is whether these values seem correct?

The values in the Haynes book are 1.5V @ 40 deg and 2.4V @ 20 deg so the voltage does seem on the low side but then it also says they are 'approx' so I'm not sure if the sensor is OK.

I was going to just replace it but I've just been to my local dealer and they quoted me £52 for the sensor (Saab part no 9174442)!
 

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At that price, would it not be worth getting the dealer to plug it into the diagnostics to confirm the problem? The check engine light has been know to show up occasional "faults" that never reappear. E.g. my lambda sonde a few years ago, still functioning correctly.
 

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Discussion Starter · #3 ·
Hi Mark,
The thing is that I removed the battery to reset the ECU and the light stayed off until I'd been driving for about 15 mins then came on again. I checked/cleaned the contacts and wires but all look fine.
 

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You can get more data here.

I put this into a spreadsheet here.

From that, at 10C you should read 3.5V or about 4K ohms.

Since your resistance sounds about right, but your voltage is way off, I'd guess at a bad connection or bad wire?
 

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Discussion Starter · #6 ·
thanks for the replies, esp. John for the sensor voltage and resistance figures. Going along with the suggestion that it's a wire/connection problem, with the sensor disconnected I made some measurements and found the following:
The White wire is grounded (0.0 Ohm to -ve battery).

With Black wire is at +4.93V with respect to ground.

This seems strange as in the info John provided it says that the Black wire is ground

Anyone else found this?
 

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Can this be of any help!
You stated voltage problem at a sensor, the voltage amount you indicate may not be the issue, more so the supplied voltage from your ECM. The processor (ECM) is the voltage control distribution center for the feed back sensors to deliver signals for the process to get optimum fuel efficiency, basically.
The hot and ground leads from sensor connect to the ECM, the voltage from the ECM should be ~constant~, from my experience worked on Japanese and US cars they use 5 volts as the signal.
This is my first Saab, purchased a 98’ 9000cse on eBay last August and have not explored this brand of ECM so far! My guess is a power failure within the ECM.
Check other sensors to get a voltage value that would show a constant volt value.
Wise to check impedance of sensors.
Continuity on all wiring.
Jim
 

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I think the NTC is probably in a circuit like this



or




although there are many ways these can be configured.

The reason I'm curious is that I'd quite like to figure out how to tap off a true engine temp reading from one of the existing temperature sensors, and they are probably set up in a similar way.

James, if your meter supports it, try measuring the resistance between the white wire and battery -ve. You could also try putting a 2k4 resistor in place of the sensor and measuring the voltage across it.
 

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Discussion Starter · #10 ·
Thanks. I measured the resistance between the white wire and battery -ve before and it was 0.0 Ohms, strange as the info suggests that this wire should be the signal.

When a get my engine back together (head being skimed today), I'll figure out the problem with the inlet temp. sensor. I did however find that the earth tag that should be bolted to the inlet mainfold was not connected so I'm hoping this was the problem.

Will confirm next week (assuming I get the car back together at the weekend..!)
 

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Discussion Starter · #11 ·
Well I've used the car to and from work this week (about 140 miles total) and the check engine light has not re-appeared. I am assuming therefore that the problem was the earth tag on the inlet manifold not being connected.

Thanks JohnCC for your help, worth remembering that the documentation you have shows the incorrrect polarity though.

Regarding your actual engine temp. gauge, are you intending to buy an engine temp gauge/sensor or make something yourself? Not sure how electronics/software minded you are but there are small microcontrollers available (from people such as Microchip) that have analogue inputs and could be used to convert an analolgue voltage into a signal suitable to drive a display. This would however need some simple s/w code. Let me know if more info would help.
 

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I've just had to fix this on my newest Carly, it dropped to base boost over the weekend but no check engine light.

Bit of head scratching, all apc circuit looked ok, di and plugs good too, fuel ok.

I'd also had the TCS recalibrated the previous day and the garage had passed on all the error's code logged, one of which was inlet temp sensor OS error.

Cleaned up the 2 very corroded pins, reconnected the plug and full mid red boost again


Andrew
 
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