I did this also except my switch is 195 0n and 180 off. It works great and I can't believe how much cooler my car runs.

 

185 Ohms @ 210F, 3400 Ohms @ 68F, 7,500 Ohms @ 39 F but don't ask me how to check it.

One thought to easily check would be to unplug the main fan, let the car come to temp, and see what temp your aux fan comes on? If that comes on way low, you know the temp is just not being displayed properly. If that comes on at normal, 235 I think, then it looks like your ECM is turning on the main fan at 150.

Perhaps you have a modified EPROM? Can't imagine anyone setting it to 150 but that would cause the behavior you see.

 

Well, I think I've finally got my head on straight.

Yesterday, I found the relay for the main fan on the driver's side wheel well, just as everyone else's is. I tied my heated 02 sensor there so that it only heats with the igntion key in the "ON/Run" position.

And, yes, the relay on the fan shroud is for the Aux fan, but since I;ve completely by-passed it, that relay isn't being used at all. It's just sitting there waiting for something to do.

To operate my new Derale 'AUX' fan and my oil cooler fan, I installed a dash mounted toggle switch.

Remember, my original AUX fan threw a blade last year.

So now that I've got this fan business finally straightened out, I can move on to more important things, like re-grounding my digital dash display. Starting yesterday, my dash is completely blacked out, making night driving a bit of a challenge. LOL

Both fuses are good, so the Shop Manual is directing me to a ground fault.

Always something with these jokers, right?

Jake

Jake

 

That's a good idea. That would let me see that the other fan comes on. The eprom is stock.

Thanks

 

Back to the start of the thread. The issue with the car all of a sudden not idling turned out to be the IAC. Replaced that (and cleaned the throttle body while I was there), back to idling ok, so I'm back to the cooling fan issue.

Following the diagnostic process posted by Agent 86 (thanks by the way):

I ground the ALCL and the fan comes on. Run car til it overheats (which it obligingly does), disconnect cooling sensor, ECM faults and fan runs all the time, diagnostic says replace coolant sensor. Already did that and still having the problem. Possible I have a faulty coolant sensor, so I'm going to try that.

 

Make sure you were sold the correct coolant temp sensor too. There are three different ones and I've had the AutoZone guy try to sell me the wrong one.

Since the fan does operate, that eliminates the fan motor as a possible cause.

I'd also try swapping relays with another one. For ther 86 the main fan relay is screwed onto the driver's side fender, sort of above and inboard of the tire.

Many of the relays are interchangable, so you might be able to swap, say the aux fan relay with the main fan relay to see if that makes a difference. I'm assuming you have the aux fan option.

In any event, relays are pretty cheap even if you have to resort to buying one. The parts store guy should be able to tell which ones are interchangable just by looking at his part numbers and cross-referencing.

Keep us posted.

Jake

 

This continues to sound like an ECM problem. If you don't have a scanner, it's a little difficult to diagnose, but you can check the accuracy of the coolant temp signal by purchasing another sensor making sure that the replacement has a new harness (most do). Then, read the resistance across the terminals of the new sensor and note that number. Unplug the harness to the installed sensor and plug in the replacement harness and using the leads of that harness, read the resistance across the terminals of the installed sensor. On a cold motor, both sensors should read about the same and have a resistance close to ambient temp - for example, at 70 degrees ambient, resistance should be 3400 ohms (resistance increases with lower temps). If they're close and it isn't spitting out a 14 or a 15, the ECM is probably shot, but if you want to, you can verify the wiring and the sensor signal by checking voltage at ECM Pin C10. With the sensor disconnected and the ignition on, the wire should have the signal voltage or 5 volts. Remeasure with the sensor connected and note that voltage (it will be lower). Then, unplug the harness and plug it into the replacement sensor and again measure the voltage. If both of the voltage measurements are close, the installed sensor and the ECM wiring are ok. Again all tests should be done on a cold motor, preferably after it has sat overnight.

 

Man, this thing is seriously testing me.

I got the new sensor (2nd), fan would now turn on around 255, off at 230, not good. After thinking it over a bit, I decided to try a different brand. Tried Napa but not open so I go to Pep boys and pick up a Borg Warner (Autozone, where I got the other TWO, sells Wells). Pop it in and bingo, on at 228, off at 215, I almost fainted.

Conclusion, don't use Wells coolant sensors (at least if they're sold to you by Autozone). Autozone also sold me a ****ty rad cap that didn't hold pressure, even though it said 16lbs, it would blow all the coolant out at temperature. On the other hand Autozone sold me a perfectly good IAC and throttle body gasket set, so who knows.

So I do a final coolant refill process (cap off, run it up, top it off), and then let it sit for a bit. Come back after a couple hours to take it for a drive, get it out on the highway and it won't cool down, running at 225 (coolant temp) or so. Get off the highway and it runs up even higher. Pull it over, fans are both whirling away, blowing cold air, no heat is coming off the radiator at all, despite the fact that the thing is running 250 or so. Limp it home and contemplate driving it off a bridge.

(Now I had a premonition that something else might be coming, as I was driving to the NAPA earlier in the day, I pulled the connector to the main fan, so that it would run all the time. While on the way to NAPA it was running pretty hot, even though the fan was running nonstop. I thought it was odd but ....)

1. There are no coolant leaks. 2. I had replaced the thermostat when I did the IAC since I had the throttle body out of the way. It worked fine, so unless it had a sudden failure.

So now I'm on to either radiator blockage or water pump. I'm thinking the stress of running it up to high temps is causing marginal components to fail, and it is painful. (don't even mention head gasket)

SunCr, when I had the IAC problem, I actually agreed with you, I thought it was the ECM, so I pulled and replaced it. That didn't fix the idle problem, so I moved on to the IAC and replaced it to get rid of the idle problem. So its a new ECM in there.

 

Would it be ok if I connected the primary and secondary fan in a parallel cricuit? Isn't the primary fan behind the radiator and the secondary one in front?

 

jrockw01

You said both fans were whiring like crazy (cool air) but no heat coming off radiator. This, plus you said you replaced the thermostat, makes me wonder if the thermostat is opening. Is the radiator roasty hot or luke warm? After you run the car for awhile and it heats up, turn it off and immediately touch/check radiator, top, middle and bottom. Do you feel temp diff i.e., hot, warm, cool? Is top hose a burner and bottom hose cool? The main fan pulls and the aux, I believe, pushes so if both fans are running you gotta feel some heat coming off if you are getting full circulation through rad. If thermo is staying closed you are only getting a trickle. I know all of this may sound kinda basic but, I am just trying to help.

p.s. I am having the same on/off sensor problem you are. Sounds like Rick has his working good with same setup, but I continue to have some problems and may just put the stock sensor back in.

 

I'm assuming you haven't recently changed water pump and put in the wrong one.

I'm also assuming that you've down all the basic stuff, like making sure there is no obstruction or buildup of leaves, paper, etc., in front of or between the condensor and radiator.

I'm also assuming the stat was installed in the correct manner and not upside down.

Let's go back to square one. Did you change anything just before this condition started?

Is the engine running as it always did, other than the gauge readings you're getting?

You can remove the radiator cap with the engine warmed up (careful now) and you should be able to see the coolant actually flowing.

You can put the stat in a pot of water on your range with a meat thermometer next to it to see at what temperature it begins to open. If you're running, for example, a 180 stat, it should BEGIN to open when the temp reaches 180 on the thermometer.

Does the engine show any other signs of overheating, OTHER than what the gauge is telling you?

Are the radiator hoses getting hot? What about the heater hoses? The hose to the throttle body?

Are we sure the gauge is right? You can screw in a manual temp guage to verify that the dash gauge is correct, or at least in the ballpark. Up until now, you've been swapping the coolant temp sensor in the intake manifold that controls when the main fan turns on/off, right? Have you checked the gauge sensor/sending unit?

There's a sensor in each head; one's for the Aux fan and the other one is for the dash gauge. GM switched them from one side to the other on certain years.

One my 86, the one for the dash is in the passenger side head between 6 and 8. Your 89 should be the same. Have you checked the one in the passenger side head?

The one in the driver's side head between 1 and 3 is for the Aux fan. I've even seen a Vette Manual that had it wrong.

Hops some of this helps.

Jake.

 

Just to complete this post.

I took it in to a reliable shop. It wouldn't overheat on the way up or in the couple of drives they took it on.

I had em do the chemical test on the coolant. Changed colors indicating combustion gases in the coolant, head gasket or cracked head. Sort of makes sense with the symptoms I've had. Intermittent overheating, no circulation consistent with getting air gaps into the coolant system.

Now I've got a tough decision to make, which I'll put in another post.

 

Don't make any expensive decisions until you thoroughly checked this out.

If combustion gases are getting into the coolant, you should see a lot of bubbles in the radiator, with the radiator cap removed and the engine idling; stat open.

Stop by AutoZone and borrow - they lend them - a coolant system pressure tester. Screw it onto the radiator, pressurize the system and see if it holds pressure.

Jake

 

My fan stopped working and it was the fuseable link.

when I replaced the link I checked the fan and it was drawing reasonable amps so I expect the fuseable link just got old and tired.

S'nut

 

StingrayNut - I appreciate the feedback, but if you read back through the posts you'll see that the original fan problem has been fixed.

Jake - the shop did a coolant pressure test, it passed, but from what I've read, this is not definitive - especially given the intermittent nature of the problem. Whereas the combustion gases in the coolant is pretty definitive?

Trust me, I'm all for finding that the head gasket is fine, in this case I think if it quacks like a duck, walks like a duck ...

My only other thought is could be the cat? I would think if it was the cat though, that I would see the overheating every time I went on the highway, as opposed to suddenly and intermittently.

 

Someone said they assumed you did etc.etc.-but I've got to ask the question-have you pulled the radiator schroud and checked between the radiator and condensor????

 

I didn't actually pull the shroud off, but I got in there with a hose and blasted it out from both sides. Used the little weep hole down on the right side to work any crap out of. Used a flashlight to peak down in there, I'm pretty sure it got pretty well cleaned out.

Again, if it was a blockage like that, it wouldn't do it intermittently I would think, it would be all the time.

When it's not going through the roof, the temp is fine. I can go out at 75 on the highway forever and coolant is 195, oil 205. Stop and go it will edge up but the fan will go on at 228 off at 215 and never overheat. It's just occasionally I get these spikes.

 

I have a 1989 corvette hatchback the smaller front cooling fan comes on without fail at 185 degrees.

But my large cooling fan behind the radiator in front of engine which I think is the main fan correct? does not come on even with engine temperature up to 235?

at wits end afraid the drive the car when its over 80 degrees outside cause it gets to 230 degrees or more

**************************************** *********************

it has a black wire and a black wire with red stripe going to it, I cut these 2 wires and hooked up some of my spare wires touched to battery terminals and fan still did not come on.

that would be with direct 12v current to it?

so looks to be bad fan motor?/ any thoughts?