I've read about 15 or more posts and articles on this topic and most of them have brought me to the theory I am about to share. but I'm hoping that you guys can verify or support what I think the problem might be or am I missing something that I didnt consider.

1987 IROC 305 TPI 5spd with dual fans. NEITHER fan comes on, ever. I know the circuitry for the primary fan is intact because when I jump ALDL into diagnostics and put key at on/not run mode the primary fan kicks on. But while in run mode the temp gage can go to 260 and no fan. Since circuitry works its got to be either a sensor problem or an ECM problem correct? Highly unlikely to be the ECM but as Sherlock Holmes would say, "if you've eliminated all other possibilities then what is left must be the answer. So in effort to arrive at a bad ECM I need to eliminate the sensor. Before I change the sensor to a new one let me ask this. Anyway to test the sensor when its installed? If not then wouldn't this question also apply? if it is the CTS wouldn't that make the car run crappy? Doesn't the CTS provide data to the ECM to manage AFM? Your thoughts?

As far as for the secondary fan. If I understand that one correctly, the only thing that really controls it is the sensor between #6 & #8 cylinders correct? Or the relay, which can't really be tested doing the ALDL key-on test, correct? So basically if the secondary fan doesn't kick on its either bad circuit, bad sensor or bad relay? (or bad fan). Your thoughts?

I must be missing something here. I mean what are the chances that I have both fans not working at the same time? Is there some other commonality between both fans that I have missed?

 

Yup...

The temp gauge.

Go get yerself one of these http://www.harborfreight.com/catalog...ult?q=infrared (yeah it's a complete POS, cry me a river) and see what the temp REALLY is.

Nothing in your post indicates that you have any kind of a problem, other than maybe your dash trim package is ... what it is.

 

I hear that. I should have noted that I've got an infrared test gage (that exact one) and yes I did check the temp at the therm housing as well as the TB coolant passage housing AND the block near the secondary fan sensor. The readings match the gage. When I say the car got to 260 degrees I meant it. The lower hose burst. In fact I think I might even have fried the head gasket or worse case the head. Which is a different issue.

Any other thoughts? Im still leaning towards a bad CTS for the primary fan and a bad sensor for the secondary. It just seems highly improbable that they'd both go bad at the same time. The only other explanation seems to be a bad ECM for the primary and a bad relay for the secondary. Swapping relays for the secondary is easy but how do you test it without getting the car up to 240 plus degrees which I am hesitant to do based on what I said above about the head gasket.

 

OK, heres something that seems very odd to me. I thought for sure I had a bad CTS so I replaced it. No change. As noted, I had previously hit the thermostat housing with an infrared scan tool and the thermo housing matches the temp gage. When the housing reads 220 so does the gage. But then I scan the intake manifold right at the CTS fitting and it reads 180. 40 degrees cooler than the thermo housing that's about an inch and a half way and also sitting directly on the manifold. No wonder the primary fan is not kicking on. The CTS is not hot enough. Wouldn't the entire front of the manifold be nearly the same temp? The only thing I can suspect is that the coolant passage that feeds up to the CTS is clogged?

Any thoughts?

 

So you are pointing the Inferred temp gun at the base of the Temperature Gage sending unit ?
Mine looks like this and is between the #3 and 5# cylinder plugs in the head.
The wire slips on from the side using what looks like a female spade connecter.


Also if you ground the wire going to the Fan Switch in the head the fan should come on.

 

Blockage maybe; or a badly installed hose ??

@ 260 the valve in the radiator cap should have given way and the overflow bottle pumped with boiling fluid,.......... WAY before any hose should have failed. I've over-heated many cars in my day, but never blew a hose off !!

From the 87 shop manual:






Start at the top of the chart; check the Fusible-Link and Fuse first !

 

No I pointed the infrared gun at the base of the coolant temp sensor that is located just below the throttle body. That is what signals the ECM to turn on the primary fan. You might already know this but just for the others out there, the pic of what you sent is just simply for the in dash gage and does not actually have anything to do with the fans. The switch on the passenger side between cylinders 6 & 8 controls the secondary fan and that has not worked either so I am changing that switch. I have ordered a Jet switch part #60600 that turns the secondary fan on at 195 and off at 185. As opposed to the factory switch which I am told kicks on at closer to 235 degrees! I'll report on how that goes as soon as it gets installed.

But for now it appears that my primary fan is not kicking on at 220ish when that gage reads 220ish (or even as high as 235) because the temp at the actual coolant temp sensor is 40 degrees cooler. And that makes no sense to me.

 

UPDATE: wouldn't it be great if every initial poster gave us their updates so we could see if problem solved and how?

So here goes. I verified both fans motors work by grounding them. As noted above I knew the primary fan worked when I jumped the ALDL in to diagnostic. I also knew both relays worked by swapping them with one another while in diagnostic. But still no primary at 220 and of course no secondary either. I still have no idea if the primary system works because I cant seem to get the car warn enough AT the CTS sensor to kick it in. And here is why I believe that might be so.

I put in a new radiator and as soon as I did that the car immediately ran cooler. Which leads me to one of my favorite rants. NEVER PUT ANYTHING BUT DISTILLED WATER, coolant and maybe an additive in a radiator. NEVER put tap, hose or well water in a radiator. That water will cause deposits in the system due to electrolysis and the whole system will work less efficiently.

Anyway. New radiator; now I cant even get it warm enough to kick the primary on at 220ish. But I did install a new fan switch for the secondary. I used the Jet 60600. It ain't cheap at almost $50 but it comes with a new pigtail. Now the secondary kicks on at 195 and off at 185 and it works perfectly. Ironically now the secondary comes on before the primary. I drove the bejesus out of this car on stop and go city streets and it never went over 200!

The radiator cost $90 and fan switch cost $50. I might even do a new thermostat. But the point is that you can probably get your car cooling really well for around $150 and the stuff is super easy to install. Just remember. DISTILLED WATER. Don't ruin your new radiator.