85 MAT sensor question
Thread Starter
Intermediate
Joined: Aug 2010
Posts: 34
Likes: 7
From: Lancaster Califonia
85 MAT sensor question
Looking to find out if an AutoXray scanner correctly reads the MAT temp on a stock 1985. Mine reads 203* all the time. Followed the diagnostics in FSM up to the point of replacing ECM. The connector has 5.03 volts across the pins. Temp reads 203* with it disconnected. I jumpered the pins with 10K resistor and still reads 203*, should have read about 40*F. All other functions the AutoXray reads appear to be correct (RPM, Engine Temp, etc). The MAT reads the same 203* when doing a scan KOEO or in monitor mode with engine running.
Thanks for your help!
Thanks for your help!
Thread Starter
Intermediate
Joined: Aug 2010
Posts: 34
Likes: 7
From: Lancaster Califonia
Thanks for the input MrWillys. I think the ECU was upgraded between 85 and 90. I'm hoping someone with an 85 will chime in on this. The other thing I noticed today was it always shows open loop when in the monitor mode on the scanner, never goes to closed loop. It seems to run fine, any way to tell if it is in open or closed loop?
Thanks!
Thanks!
Drifting
Joined: May 2013
Posts: 1,736
Likes: 31
From: Reno Nevada
Thanks for the input MrWillys. I think the ECU was upgraded between 85 and 90. I'm hoping someone with an 85 will chime in on this. The other thing I noticed today was it always shows open loop when in the monitor mode on the scanner, never goes to closed loop. It seems to run fine, any way to tell if it is in open or closed loop?
Thanks!
Thanks!
Thread Starter
Intermediate
Joined: Aug 2010
Posts: 34
Likes: 7
From: Lancaster Califonia
The AutoXray scanner is the EZ Scan 6000, not 600 C that I miss typed in an earlier post. I just read the manual again and see nothing that talks about a switch (10K). It does state:
"NOTE: [GM] On carbureted vehicles and a few fuel injected
vehicles, the on-board computer is placed into Service
Mode, a mode which fixes the timing and the idle speed.
If the idle speed on your fuel injected vehicle is affected
during scanning, AutoXray recommends that you do not
drive your vehicle while it is scanned."
When I connect the scanner to my 85, it does change the idle to 1000 rpm so it must be going into service mode.
"NOTE: [GM] On carbureted vehicles and a few fuel injected
vehicles, the on-board computer is placed into Service
Mode, a mode which fixes the timing and the idle speed.
If the idle speed on your fuel injected vehicle is affected
during scanning, AutoXray recommends that you do not
drive your vehicle while it is scanned."
When I connect the scanner to my 85, it does change the idle to 1000 rpm so it must be going into service mode.
Drifting
Joined: Jun 2013
Posts: 1,590
Likes: 60
From: ft myers FL/Munising MI
when I bought my 85, I looked around for scanners and was led to believe they didn't work well with the 85....just checked out that ez6000...kind of pricey but worth it if you get valualble info......does it read all those things on your 85...tps/ a/f/ timing etc? hate to ask after pricing that but is that the best unit out there for the 85?...does it matter that the engine has been modded?....roller rockers/ no smog egr... thanks Paul 
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Thread Starter
Intermediate
Joined: Aug 2010
Posts: 34
Likes: 7
From: Lancaster Califonia
I actually bought the AutoXray when I had to work on my 99 Tahoe. The only way to correctly set the Cam Retard is with using a scanner like this. It works on the 85 Vette but is very limited as it only checks in service mode which sets the idle to 1000 rpm so you can't check IAC counts, etc. I'm sure there are better scanners out there for OBD I systems.
Drifting
Joined: Jun 2013
Posts: 1,590
Likes: 60
From: ft myers FL/Munising MI
I actually bought the AutoXray when I had to work on my 99 Tahoe. The only way to correctly set the Cam Retard is with using a scanner like this. It works on the 85 Vette but is very limited as it only checks in service mode which sets the idle to 1000 rpm so you can't check IAC counts, etc. I'm sure there are better scanners out there for OBD I systems.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
It also does something to the spark advance, but I don't remember the details. There are other things that are affected. You'd need to look in the FSM to find out what they are. A Google search might turn up the answers.