'90 Vette is overheating
Thread Starter
Racer
Joined: Aug 2008
Posts: 275
Likes: 0
From: Florida
'90 Vette is overheating
My '90 vette is overheating. This is what I have done so far.
1. I've jumped A & B posts in the ALDL connection and both fans come on.
2. I turn on the AC and both fans come on.
3. I hooked up TunerPro and watched as the temperature rises and watch to see if the "Fan 1 Requested" or "Fan 2 Requested" changes, and they don't. They do not come on. Temperature got to around 236 degrees F.
4. While TunerPro was still attached, I turned on the AC, both fans came on and both "Fan 1 Requested" and "Fan 2 Requested" changed from Not Requested, to Requested.
So, it appears there is a problem somewhere with the fans turning on. Anyone have an idea what to do next? I'm at a loss.
1. I've jumped A & B posts in the ALDL connection and both fans come on.
2. I turn on the AC and both fans come on.
3. I hooked up TunerPro and watched as the temperature rises and watch to see if the "Fan 1 Requested" or "Fan 2 Requested" changes, and they don't. They do not come on. Temperature got to around 236 degrees F.
4. While TunerPro was still attached, I turned on the AC, both fans came on and both "Fan 1 Requested" and "Fan 2 Requested" changed from Not Requested, to Requested.
So, it appears there is a problem somewhere with the fans turning on. Anyone have an idea what to do next? I'm at a loss.
Pro
Joined: Mar 2007
Posts: 526
Likes: 1
From: San Angelo Texas
I had this problem and all it was was the temperature switch on the block. I think there are two, one for the gauge and one for the fan relays. I want to say it was the one on the passenger's side cylinder head with a little wire on it above the exhaust manifold. I believe there is another sensor on or close to the thermostat housing. Sorry for all the "maybe's" but I did this a while ago and I am not with my car. Hope this helps.
BTW, way to go on all the other diagnostics. You already verified the other stuff is working so I am confident the temp sensor is your issue.
BTW, way to go on all the other diagnostics. You already verified the other stuff is working so I am confident the temp sensor is your issue.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
If I remember correctly, your '90 grounds the Main Relay with a Coolant Temp Signal of 226; secondary 228; or a/c pressure signal north of 200 psi for the primary, and something above that for the secondary. Your scan confirms the a/c side is working, that the ECM can ground the Relays and therefore, the Relays are good. The only thing left is the ECM - you need a new one if the scan indicates a CTS at 236 and neither fan is cranking with the a/c off. The CTS is below the throttle body and you can verify it's accuracy by checking the temp signal with the engine cold - should be close to ambient - rise to the thermostat once started and when that opens, fall back a few degrees before rising to the threshold for fan operation (and when the ECM is working right), temp should fall when the main fan comes on. Secondary threshold on these L98's, even with the '90 redesign, generally isn't reached with the a/c off, so you may only see the 1 fan when the system is the way it left the Factory.
Thread Starter
Racer
Joined: Aug 2008
Posts: 275
Likes: 0
From: Florida
Well, I figured out the cause of the fans not coming on...
I hooked up my TunerPro software to monitor the temperature and to see the "Fan 1 Requested" or "Fan 2 Requested" flags.
There is a sensor on the passenger side of the block between cylinders 5 & 7 with a single wire on it. I let the car get to temperature, about 230 degrees F. I popped that wire off and a couple of seconds later Fan 1 kicked in! The temperature dropped to 219 degrees F and fan 1 shut off. I watched as the temp climbed back to 228 degrees F and the fan kicked in again. I watched it cycle back and forth for about 15 minutes, and all is well!
That sensor apparently controls the dash guage also, because the dash guage shut down.
Thanks goes to Mechguy79 for giving me that idea to pop that wire off and see what gives. I'd still be scratching my head if it weren't for him!
I hooked up my TunerPro software to monitor the temperature and to see the "Fan 1 Requested" or "Fan 2 Requested" flags.
There is a sensor on the passenger side of the block between cylinders 5 & 7 with a single wire on it. I let the car get to temperature, about 230 degrees F. I popped that wire off and a couple of seconds later Fan 1 kicked in! The temperature dropped to 219 degrees F and fan 1 shut off. I watched as the temp climbed back to 228 degrees F and the fan kicked in again. I watched it cycle back and forth for about 15 minutes, and all is well!
That sensor apparently controls the dash guage also, because the dash guage shut down.
Thanks goes to Mechguy79 for giving me that idea to pop that wire off and see what gives. I'd still be scratching my head if it weren't for him!
Melting Slicks
Joined: Feb 2008
Posts: 3,286
Likes: 6
From: Palm Beach
Something is screwy then because the CTS in the front of the intake measures the coolant temp and provides the input to the ECM. The sender you speak of only supplies the analog gauge.
Did you note the CTS reading? Did it agree with the analog?
Did you note the CTS reading? Did it agree with the analog?
Thread Starter
Racer
Joined: Aug 2008
Posts: 275
Likes: 0
From: Florida
I looked up what that is on the side and from I could tell (I have the parts manual and diagrams on CD), it said it was a Knock Sensor. That really doesn't make sense to me. Why would the fan suddenly work when disconnecting the knock sensor???
Thread Starter
Racer
Joined: Aug 2008
Posts: 275
Likes: 0
From: Florida
Ok, now I'm more confused than ever.
I decided to leave that wire on the sensor on the passenger side of the engine and run the car. I monitored the temperatures and fan 1 kicked on at 228.65 degrees F and shut down at 220.55 degrees F. It continued to cycle through like this for 20 minutes.
I decided to leave that wire on the sensor on the passenger side of the engine and run the car. I monitored the temperatures and fan 1 kicked on at 228.65 degrees F and shut down at 220.55 degrees F. It continued to cycle through like this for 20 minutes.
Last edited by stevenhmiller; Sep 30, 2008 at 09:27 PM.
