How does the ECM know when to kick on the fans? Is it from thge coolant temp sensor? Does the temp guage get its reading from this same sensor? I changed to a 180* Tstat and now the fans won't come on unless the AC is on. Both fans worked right before the Tstat change. I have not done anything to make the fans come on sooner. I am wondering if I may have damaged the sensor when the throttle body was taken off. The temp gauge still works but the fans don't come on unless the AC is turned on.

 

One temp gauge feeds the dash gauge and another the ECM. Most likely the temp does not now go above the temp required to turn on the fans.

You can:
1) have the EPROM changed to bring on the fans earlier
2) install a switch to bring on the fans manually
3) install an aux fan switch in the block between #1 & #3 plugs and wire into both fan relays.

 

After installing the new Tstat I let it run and the temp guage climbed and it started to boil out the overflow tank. The fans never came on. It cam on when I turned on the AC.

 

The temp sensor to the ECM is under the TB. Disconnect and check it's resistance cold and warm. The ECM should throw a code if this temp sensor is too high or too low, though I don't know what those limits are.

If you can lay hands on a scan tool, find out what temp the ECM is seeing.

Did both fans come on with the A/C or just the secondary fan? If the latter, check the main fan relay.

 

The main fan comes on at 228 F and the auxiliary fan comes on at 238 F.
The a/c will close the main fan relay turning on the fan, so your problem must reside in the ECM or the temperature sender under the TB. A scan tool will tell you right away if the temperature sender is reporting normal temperatures to the ECM. Or, you can measure the voltage on the temperature sensor under the throttle body. When cold (ambient temperature), the sensor has high voltage (manual doesn't say, but I would guess over 5v). At the operating temperature (180 F), the sensor output should be 1.5 to 2.0 volts.
If the sensor is shorted or the wire to it is shorted, a code 14 will be set. If the sensor is open circuit, or the wire to it is broken, a code 15 will be set. Check for any codes set first thing.


[Modified by jfb, 11:25 PM 10/12/2002]

 

There was some discussion as to this area being prone to "air pockets" which keep the sensor from reading correctly until the air is purged. Supposedly, later models have a fitting of some sort in this area, but you could try just unscrewing the fitting a bit to let the air/coolant out...I suppose... :rolleyes:

 

I pulled the codes--there were none. I replaced the ECM about a year ago. I checked the fan operation right before I changed the Tstat. It was working properly. Now that the Tstat has been changed the fan never came on unless I turn on the AC. It boiled over out of the overflow tank.

 

This post is of great interest, However jfb, you have me totally confused.....

First, addressing robblk90's problem.....I was under the impression that the cooling system would purge itself of air after a couple warm-up/cool-down cycles. In other words, heating it up and the temp going up pretty far which allows the pressure in the system to blow off the excess air into the overflow bottle, then letting it cool down so the system could draw back the fluid from the overflow bottle back into the system. Thus purging itself. This can be helped by removing the cap until the thermo opens up which allows the air to escape, but capping it as soon as it starts to "surge" out the filler opening.

jfb....According to my repair manual, the CTS feeds the ECM which signals the main fan which is on the left side in a 1990. The right or lower fan is controlled by the switch on the A/C tubing just under the fill tank. I have not been able to find any reference to the temp settings of 228 and 238 and have looked all over between my #1 and #3 cylinders for a second sending unit. Help me out here.

robblk90.....Since you changed the thermostat and the (left ?) fan does not come on, my impression would be that everything is OK unless the guage is reading some real high temps. The fact that the A/C operation operates the (lower ?) fan also indicates that everything is operating properly. According to my manual, that's the way it should be unless I'm missing something here.

jfb....65Z01...Rich B.....I know you guys are a lot smarter than I am, how about setting my straight on this? :confused:

 

Couple things.

The 90 does not have the secondary fan sensing switch located between cylinders. Believe 89 was the last year for that feature. Later years featured fans with different speeds. Cooling system designs are very year specific.

For those with Factory Service Manuals, MY 90, page 6B-5 lists inputs for fan(s) operation. AC condenser, coolant temps, oil temps, throttle position, vehicle speed, ECM status. Balancing these factors is explained elsewhere in the manual. More on this later.

Generally, with A/C off, in neutral, the fans, primary and secondary will do the temp thing (226F&235F respectively). If you let your 90 idle, the fan(s) will start but you may not see the secondary fan come on cause it doesn't get hot enough. My primary fan failed this spring, and it DID get hot enough, and the secondary fan came on (around 235) with primary fan off. This sent me finding out why the primary fan didn't come on, (bad motor).

At idle, hit the A/C, both fans should come on.

The definitive discussion of the MY90's fan control can be found in section 6E3-C12-1, Driveability and Emission (you would expect it to be in the cooling section but not so). This sections lists and explains the various criteria used by the ECM in selecting fan off and fan on operation. It's 3 pages. Too much to type. Typically, vehicle speeds of 35 and 50 are used in speed sensing.

The problem listed in the original post SUGGESTS the ECM is not seeing the correct coolant temp. From what I can determine, the relays are good as they ground properly when ECM sees A/C on request. As stated, a scanner would be very helpful. The thermostat is near where CTS is located. Perform a visual inspection of the area near the Coolant Temp Sensor(CTS). Are the connections good? Are you absolutely certain the thermostat was installed correctly, the bypass port not blocked, correct orientation, etc. Not sure if there is a chance an air pocket could be surrounding the CTS. You can also remove the CTS, perform a bench check (Ohms vs ambient temp) and visual inspection of the coolant contact area. Since it's out, I would just replace the thing. Only around $12 list from GM. This would be my area of interest.

Let us know if you need any more help. Hope you get your car fixed.

dlmeyers 90 coupe zf6 3 speed shocks

 

Thanks DLmeyers. I agree with you about the relays, the fans do come on when I turn on the AC. I let the car run and neither fan turned on and the temp hit about 240 according to the gauge. I will take another crack at it tomorrow. My question also is if the CTS is damaged would it throw a code or light a dummy light in the DIC?

 

Don't have my manuals handy but believe there are only two codes associated with the CTS. Temp too high or temp too low. This would have to be on the outer edges of the CTS range.

Basically the CTS is a negative temperature coefficient thermistor. As the temperature increases, the resistance decreases and the ECM sees the voltage change on the return circuit and associates a temperature with this voltage. It is the electronic choke (low temperature-long injector pulse by ECM) and the fan activation device (high temp) The check is to measure the resistance across the sensor, making sure the unit is unplugged from the ECM, and compare this resistance with the chart for ambient temp/ohms located in the factory service manual. For a 10 year old car, not a bad idea to just replace the darn thing. My fans were not coming on at the correct temp and the CTS was a little off. Trouble is, the resistance check for ambient may be OK while the 200F measure could be off. That is why the scanner is so helpful. If the analog gauge is way up there, and the CTS/scanner readout says its 190 degrees, no fans.

As a note, if you do lots of work on your cars, have both a GM and another brand, a good USED Snap on bidirectional scanner with descent cartridge, hookups, case, can be had for around $550 from a Sanp on truck. Sure, the laptop route is cheaper, but a descent cartridge will cover GM, Ford, and Chrysler. Software can add up and the Snap ones are easy to upgrade. Just an opinion. NOT a good option for everyone.

Hope you get your car fixed.

dlmeyers 90 coupe zf6 3 speed shocks


[Modified by dlmeyers, 5:37 PM 10/16/2002]

 

If I am waiting to put my new Blower Moduler part for my AC will while not operating will it cause my Coolant to boil in the coolant overflow bottle?

 

The temp sender the ECM uses is the one in the very front of the intake manifold. The one in the head(s) is for the in-dash gauge and the secondary fan.

And yes, it is supposed to purge itself of air bubbles if everything is working properly (cap especially).

 

Was just curious. I replaced my coolant temp sensor on my 86 Corvette and I noticed there is another type of sensor just to the left of the coolant temp sensor. It's a little bigger. Can anyone verify what that is?

 

That's the "thermal time switch". There is exactly one mention of this in my FSM, in a picture, that's it.

 

Thanks for replying on this. Do you what it's function is?

 

"The thermo-time switch limits the injection time of the cold start valve dependent on the engine temperature.

When the engine is cold the thermo-time switch is closed and therefore the cold start valve injects additional fuel. But when the motor is already warm when starting it the switch is opened - no additional fuel is injected.

In addition to that the maximum injection time is limited. Otherwise the fuel mixture would enrich too much and the engine would be flooded"

--- borrowed from the internet. Believe at your own risk

 

Ahhh. Thank you for the info Bill. That was the only sensor out of the many on this vehicle that I wasn't sure about.

 

I recently flushed out the old coolant on my 1985 C4. Recently acquired the car and just did this due to regular maintenance. There was what looked like soap suds that flushed out with very dirty water/coolant. After flushing the system by filling it with distilled water and driving a bit, I pulled the bottom radiator hose to drain the system and the liquid was still rusty and dirty after about 6 times of repeating this process. On the 7th time, it looked clean so I added pure coolant. After going for a test ride, I saw the temp gauge go up to about 190. Prior to the flush, it stayed around 182-185. I can hear the smaller fan or fans come on at around 195, but the large fan in back of the radiator I never saw or heard come on. Just yesterday, I removed the relay between the battery and Master cylinder. It was packed with grease and oily debris. After cleaning out both the connector from the wiring harness and as well the 4 pins of the relay, I'll see what happens. I had to use Q tips and tooth picks with a few squirts of wd40. I was going to use tuner spray but the new can I had didn't spray out anything. My question is, the large fan behind the radiator, when does it turn on? I may have fixed the problem by cleaning the 4 pin terminals, I'll soon find out. I am curious as to why the car runs hotter after a good coolant flush and new pres-tone concentrate anti freeze. I also removed the overflow tank and cleaned it out . It looked like a chocolate cake inside so I used small pebbles and warm water and shook it back and forth until it became clean. (This is a trick used to clean out antique bottles). I could be all set now, I'll find out on my next cruise. It just seems funny how the smaller aux. fans came on first and the large fan in back of the radiator not at all. Again, I could have remedied the problem by cleaning the relay contacts. From reading posts, I'll also try turning on the AC and seeing if all fans come on.

 

The cooling fan comes on about 228 and off at 235. They should also cycle on when the a/c or windshield defroster is used.

Gary