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IAT sensor damage, work-around advice?
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markvanh Offline
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Post: #1
IAT sensor damage, work-around advice?
I started sensor mods on my 07 Subaru Forester 2.5X with the Intake Air Temperature (no H gen yet).

Studied the wiring diagrams from my Svc manual CD and under the hood, 5 wires go to my combo MAF/IAT sensor: a fatter 12V supply wire, plus IAT signal & ground, MAF signal & ground wires. With IGN key on & engine off, the IAT signal voltage (signal-ground) was:
2.92V at 89 degF, and
3.26V at 71 degF.

I installed a 10K Ohm pot between the supply V and the IAT sensor wires. In attaching & adjusting the pot and measuring V & R with my Fluke meter, I think I "broke" the thermistor. I didn't think it possible, but when I took off the pot & returned to orig, the IAT value on my Scanguage went to 40 degF below actual and I got a CheckEng code for IAT sensor range.

Panic. I searched hoping to buy a new Subie IAT thermistor and not the whole MAF/IAT unit ($360 from the dealer). No luck.

After the incident the thermistor was sending too high V, so to workaround I connected the same pot between IAT signal & ground wires to bleed off some V, and I've now routed wires to my dashboard storage box so I can adjust it from inside. With this I manually adjust the IAT value on the Scangauge to 10 degF above the OAT sensor value readout in my dash display. Now the IAT value to ECU is fixed unless I turn the pot. Bit of a hassle but my plan was to have a pot inside anyway.

Theories? My IAT thermistor seems wounded. How'dee do dat?
I thought a pot in parallel with the sensor meant connect between the supplyV & signal wires. Or should it be between signal & ground wires?
Think it's fixable? I'd much prefer to set an offset degF that moves with temp. Any advice is greatly appreciated.
Mark

07 Subaru Forester 2.5X non-Turbo Auto Trans
(This post was last modified: 07-01-2008 09:05 AM by markvanh.)
06-30-2008 03:26 PM
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colchiro Offline
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Post: #2
RE: IAT sensor damage, worked-around. Advice?
I'd connect a pot to the IAT sensor wires for now. Adjust it to read about 10 deg warmer than outside temp on your ScanGauge. That will give you a little time to find one in a junk yard. You don't want to permanently eliminate the sensor since it will affect your mileage and cold start driveability.

Rick

Links: Documents / Tuning for Mileage | Toyota Sensors | Autoshop Sensor Tutorials
06-30-2008 05:25 PM
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markvanh Offline
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Post: #3
RE: IAT sensor damage, work-around advice?
Thanks Rick,
Do you mean leave the pot as is, between the IAT signal & ground? If i eventually replace the sensor, which 2 wires should the pot be connected between? In the attached pic, the wire on the right is V supply, and the 2 wires on the left are the IAT ground & signal.
Mark


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07 Subaru Forester 2.5X non-Turbo Auto Trans
(This post was last modified: 07-01-2008 09:08 AM by markvanh.)
06-30-2008 06:22 PM
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colchiro Offline
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Post: #4
RE: IAT sensor damage, work-around advice?
markvanh Wrote:With this I manually adjust the IAT value on the Scangauge to 10 degF above the OAT sensor value readout in my dash display. Now the IAT value to ECU is fixed unless I turn the pot. Bit of a hassle but my plan was to have a pot inside anyway.

Mark, are you now able to set the IAT temp to 10 deg above outside temp? If so, you're done for now.

Otherwise it probably should be connected the same as the Tuning for Mileage page indicates, just use the pot instead of a 10k ohm resistor since it'll need a lot less than 10k, (maybe closer to 1-2k) to get the right temp. I think when I disconnected my IAT sensor it said about -40 F. Just make sure you leave the pot at midrange before adjusting it. I don't think zero ohms would be good for the ECU. Hmm

Rick

Links: Documents / Tuning for Mileage | Toyota Sensors | Autoshop Sensor Tutorials
(This post was last modified: 07-01-2008 03:15 PM by colchiro.)
07-01-2008 03:10 PM
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markvanh Offline
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Post: #5
RE: IAT sensor damage, work-around advice?
I can set it 10 degF above the OAT, but I have to move the pot anytime the OAT goes up or down. Thanks for posting those autoshop articles, lots of good info in there. Thermistor descriptions have 2 wires, the Vref and signal, which is confusing.

There are 5 wires to my MAF/IAT sensor. The #1 (YL) one is sl. larger gauge wire and measures 12V constant. Voltage on the #4 wire (Br) varies with temp, so I'm sure it's the IAT signal. Sensor mod first attempt I connected the pot between the #1 & #4 wires. The workaround is now connected between the #4 and #5 (YR) wires; #5 measures 0V, I think it's IAT ground.

Looking again at the wiring & I/O diagrams I'm not sure-- the I/O table shows the MAF with 3 wires: signal, shield & ground. On other ref'd pages, the #1 (YL) wire comes from the main relay, and the #5 (YR) shows common connected to other sensors, so looks like either a ground or shield.

When I go to mod my ECT thermistor, equally confusing. It's a 3 wire, one looks like signal, one appears to be routed to the dash temp gauge, and the third looks to be common connected to other sensors, so a ground or shield? I had thought a thermistor had a Vin and a signal out, possibly also a ground?

Can anyone help me figure out which wires to connect the potentiometer to?
               

07 Subaru Forester 2.5X non-Turbo Auto Trans
(This post was last modified: 07-14-2008 11:29 AM by markvanh.)
07-02-2008 08:59 AM
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texasdanml430 Offline
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Post: #6
RE: IAT sensor damage, work-around advice?
can you pull your maf out of the intake plenum and look at it? i THINK most thermistors just look like small diodes. 2 wires. on my Bosch maf, you can see the thing, hanging out there in the airflow to measure temp. then, you could trace the leads back to your connector with an ohmeter.

it sounds to me like your iat thermistor is shorted, or maybe you just need to reset your ecm.

personally, i 'cheated' by carefully tack soldering a swamping resistor across the iat thermistor, inside the disassembled maf.

hope that helps.

i notice your wiring diagram is as cryptic as most. (what the ^&*^& is 'K' anway??)
07-14-2008 01:28 PM
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markvanh Offline
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Post: #7
RE: IAT sensor damage, work-around advice?
Good idea Tex. I'll try it. I've seen pics on a Subaru owner's forum of the MAF/IAT opened up, IAT thermistor just a little stub sticking down. Diagrams cryptic as they must be. : ) The "K" references 3 Pgs earlier where it shows going to a relay from the ignition, I suspect a common supply V.
Thanks! I'll report what I find. -Mark

07 Subaru Forester 2.5X non-Turbo Auto Trans
07-14-2008 11:38 PM
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ncmountainmadness Offline
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Post: #8
RE: IAT sensor damage, work-around advice?
markvanh,

I buy quite abit of my electronic components from

http://www.JAMECO.COM

Now I personallly have not dmamged one, but the thought of maybe doing so has come to mind. Therefore I have looked for said componenent.
Thermistors is an item that they sell, and yes they look exactly like the one it appears you may have broken. However, the thermistor has a resitance value, and this range you would need to be aware of prior to ordering. YOU didnt by chance take an ohm reading prior to all your tinkering did you?
However, if you plan is to offset it with a resitance to increase your IAT, you can get by with that for now which sounds like what you are doing anyway.

First I would ask, Have you reset your code?

2nd ,rarely, but on occasion, some digital multi meters have been known to send trace amounts of voltage out of the test leads and in some cases Fry whatever is being tested. But, ive never heard of such on resitors, varistors, or thermistors. Most commonly seen on transistors or IC's. I doubt you fried your thermistor, BUT you might have tripped a code in testing it with a digital VOM.

Most all IAT's are nothing more than a variable resistors going to ground. Changes value as the temp rises. CTS's are the same. To fool you merely parallel another resistor to ground to alter the outcome. Most common parallel value is in the 5k range, but a 10k should work as well, you just wont have quite as fine a tune.


As for where to place your pot, I cannot tell much by the schematics you have provided. Basically you want to place your POT across the IAT signal and its associated ground. One end of pot on signal, center tap on ground, or vice versa either will work. Most IAT/MAF combined sensors have the IAT leads as both on one end, the other 3 would be the MAF. Now I do see a shielded wire, pin # 3/white wire. I have seen some MAF's that have had shielded wires, but cannot swear this to be fact on your car. If so, I would guess pins 1 &2 to be the IAT. See if you can find a diganostic section that gives the details of each sensor individually. Individually will show the sensor itself in full electroninc detail. From there you should get wire colors and ECU pinout designations. However, dont trust all you read, colors are usually 100% accurate, but pinouts may not be. Using a continuity checker confirm from point a to b. My recent work on my Toyota showed that not all prints are correct. When doing Continutiy checks disconnect ground or power of the battery. No need to risk making mistakes.Once youve confirmed your pin locatinos from there you can do your connnections.

wish I could be of more help, and hope what Ive offered atleast points you in a postive direction.
07-15-2008 01:01 PM
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markvanh Offline
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Post: #9
RE: IAT sensor damage, work-around advice?
Thank you NC,

I searched the Jameco site for thermistors, and maybe that could be a solution. Been interested in a good online store for electronics like multi-turn pots of the right size & range, switches and ammeters.

Before I connected the pot the first time, I measured R at 2 temps only... my notes say (YL -> Br):
1.185 KOhm @ 71 degF
1.115 KOhm @ 82 degF
The YL wire is labeled #1 pin on the first diag, and the Br wire #4. The YL measured 12V consistent, and the Br V varied with temp.. I figured constant supply V in, variable Ohm & V out with temp-- those are the 2 sensor wires to parallel. I don't know if those 2 R measurements are enough to match a thermistor for range & value.

I reset the code with the scangauge right after I looked it up. Later it occurred to me to reset the ECU so I discon'd the batt and held the brake pedal down for 30 sec, then measured the V of the Br signal wire and it was still high.

When I can, I'll see if opening up the sensor I can verify the 2 sensor wires and go from there. Thanks for the tip on batt-off during continuity checks--I don't remember if I did tried that before. Had thought it's all low V and a resistor would be tough to damage.

Your experience & advice help a lot-- thanks again. -Mark

07 Subaru Forester 2.5X non-Turbo Auto Trans
07-16-2008 10:53 AM
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texasdanml430 Offline
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Post: #10
RE: IAT sensor damage, work-around advice?
FWIW, that about what my thermistor measures too, in terms of ohms at certain temperatures. with an ohmeter clipped across it, i can pinch it tightly between my hot little fingers and watch the resistance go down, or hold it in free air and blow gently on it and watch the resistance go up.......mine looks a lot like a 1N914 diode, but i know its NOT..
07-16-2008 12:37 PM
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