Hey guys, I'm having some problems with my car overheating. It is a 90 corvette. I have searched the forum and have found some great information which I have done, but I am having some problems that weren't addressed before. One problem is that the fans aren't turning on. They both work if I turn on the a/c, but for some reason the computer doesn't turn them on period. I thought that it might be a problem with the ECM so I bought a rebuilt one and that didn't fix it. The other problem that I am having is that I can't get a scan tool to communicate with the car, however I am able to run a test mode and turn the fans on that way. I'm not sure what is causing me not to be able to connect to the car. There isn't a wire in the E slot on the OBD connector, but there is one in the M position. I don't have the service manual for the car so I don't have any wiring diagrams. We also have an 89 and we have a manual for that but doesn't do me any good seeing as how the computer is different. Do you guys have any ideas as to why the fans aren't turning on from the computer? I know that I can throw on a fan switch, but I don't want to just do a temporary fix. I already cleaned out the whole coolant system including checking for debris and all that stuff. This is what I have finally found, cooling fans not turning on. I do have a jet performance chip plugged into the ECM, but I don't think that would change anything. I am wondering if my stock chip might have a problem not allowing the computer to turn on the fans at the stock settings. I am hoping that I am remembering everything here, but I'm stuck and don't want to throw money away like I did on the ECM. Any info is greatly appreciated.

Thanks guys,
Brian

 

The fans get turned on when the temperature sender indicates to the ECM that the temp is high enough to operate the fan. You might have a defective temp sender or wire connection to the sender. A scan tool allows you to see what temp the sender is reporting. I think you have a major problem if you cannot get a scan tool reading. Also, order the service manual and electrical diagnosis manual for your car if you are going to work on it, they are indespensible.

 

I found a tip on here that lets you use the auto climate control to tell you the temp reading in Celcius and then you just convert it to get the reading. In my stepdad's manual he showed me last night where it said that if you can't connect with the computer that either the signal wire is grounded somewhere or the ECM or the PROM is bad. I already replaced the ECM and taking the drivers side lower dash off we checked the wiring to the OBD port and it all looked fine. The sensors are both new like I said earlier and using that method with the auto climate control it is showing me that my gauge is reading correctly (close enough that the fans should be turning on). I just didn't want to order a PROM and have that not be the problem. Thanks for your reply.

 

ECM drives the fans for a/c and non a/c and the Relay driver(s) or ground is from the same ECM pins. Specs for a/c are something around 228 psi from the switch on the high line and if it's charged correctly, it will make that within seconds of compressor engagement; ie, the fans should crank on shortly after the compressor. And since yours seems to do that, the ECM is doing it's job. If you want to verify it, hook up a Manifold Gage Set.

Non a/c drives the primary with 226 degrees from the Coolant Temp Sensor and the secondary at 228 degrees. Verify those temps if you can get your Scanner working, but since the Drivers work for a/c, suspect something with the CTS.

 

Thanks for the response. I don't know what could be wrong with the CTS though. I replaced the one in the intake manifold and also the one in the passenger side head. It didn't change anything. Do they ever end up bad brand new? That might be a possible problem but I would think that they would find that out during their quality control process. I guess I could try one that would turn the fans on sooner like one of the hypertech sensors or something. Is that what you would recommend? It looks like there was a fan switch on the car at one point. I'm not sure where the switch was located, but I see the same wire going to each relay has be stripped and taped up. I could throw on a fan switch to do it manually, but I wanted to fix it properly in case someone was driving my car and forgot about the switch or something that the computer would turn the fans on. Tonight I tested the M pin on my OBD plug and it wasn't grounded out or anything so I'm wondering if my PROM is what is making it so I can't communicate with the computer with the scan tool. I would also like to get that fixed as well. Normally just driving the car doesn't get really hot unless I'm in heavy stop and go traffic (not that often), but while sitting waiting in the staging lanes for autocrossing it gets real hot. I was told that temporarily I could just short A and B terminals or whatever and have the fans just run constantly. Anything bad with doing that so I could race this weekend?

 

The ECM only uses the Coolant Temp Sensor in the manifold. The Sensor in the right head - which works on the same principal - is for the Display, but not connected to the ECM. It's important to see what the ECM is seeing as the CTS is used for fuel delivery too - the cooler the Coolant Temp, the more fuel supplied to the injectors. If the Sensor is new and the wiring is good, but the scan shows the wrong temp, then the ECM is bad. You should start by scanning it with the engine cold (though I know you will have to get that working first). The signal should be close to if not the same as the outside air temp. After you start it, the signal should rise to the thermostat and when that opens, drop a degree or two. It should then rise to the threshold for Main Fan Operation which is 226 degrees on a '90. The fan should then come on and drop a few degrees and keep it below that threshold as long as it's running. You can also troubleshoot the ECM reference by checking for 5 volts on the Yellow wire at the Sensor with the harness disconnected.

Shorting the Diagnostic Link pins will turn on the fans but also put it on backup. That isn't going to give you the best performance and if it isn't getting the right fuel you're not getting max performance either.