My 85 Z51 is running hot (236 degrees in slow conditions) My question is: at what temp is the Aux fan supposed to come on and should it be on at all times if the air conditioner is on? My main fan comes on at 175 degrees. I should mention, the main fan comes on with the air on but not the Aux.

 

Normally my aux fan comes on at 185 degrees. I seem to recall that the main fan comes on around 226 degrees. If your main fan is coming on at 175 has it been wired up to the aux switch?

If it is running warm then you probably need a new rad. Fans only move air. If the rad can't get rid of the heat then it is time for a new one. Trust me...once I changed mine the heat levels dropped.

 

Thanks for your input and you are probably correct on the rad. replacement. However, I'm still wondering what the parameters are for the Aux fan to operate. We put 12 volts to the fan and it worked.

 

Not unusual temps ( stock)

238 ish
Aux fan on early cars runs independent of ECM or A/c
See below

Stock is 228F so you have non stock fan settings in ECM
or a lower temp mechanical switch wired on the main fan

As it should.
Aux fan has standalone mech switch on your car

 

So what triggers the Aux Fan to come on and at what factory temp?
Thanks JW

 

AS I NOTED ABOVE

By a standalone mechanical temp switch mounted in the head.
238F

 

The auxiliary fan is controlled by a temperature switch in the driver's side head between spark plugs #1 and #3. It turns on at 238 degrees. You can test the circuit by unplugging the connector and shorting it to ground (I used a screw head on the alternator). There is no relay for the auxiliary fan, it connects directly to the temperature switch. Ignition switch must be on...

You can buy aftermarket temperature switches that turn on at lower temperatures.

 

On a 85 switch is on the pass side ; sender on drv side is for temp gauge

 

As noted above, your L98 should be running in the 195 degrees range. If you seem to be running alot hotter than that when you are moving, you should consider cleaning out the debris between your radiator and your AC condenser. Also make sure you clean any debris out from in front of the condenser (look under the front end at the air dam intake)

see attached.

 

Rad nice and clean so thats not the problem. Should mention, when I bought the Vette in Jan., the main fan ran at all times with the key on. Wiring not harwired and no switch. So, had the ECM changed out and a Hypertech stage 2 chip installed. Result, main fan worked correctly but comes on at 175. I understand the ECM can be programed to allow the fan to come on at higher temps. At this point I'm wondering at what temp should I be worrying about harming the engine?

 

I think it's weird that my '86 FSM doesn't even show the auxiliary fan at all in the circuit section. The main fan is shown on page 8A-31-0, but the auxiliary fan circuit is not even mentioned.

I checked out the schematic section and found out I was wrong about the auxiliary fan (option VO8) not having a relay. Now I'm wondering where it is...probably near the driver's side horn is my guess.

 

On DS of rad.

 

You could also have a thermostat that is sticking or the lower radiator hose could be collapsing. Either one can reduce the flow of coolant. While SBC water pumps typically last a long time, it's possible that the impeller is rusted to the point that it won't push enough coolant.

Have you done a flush of the cooling system and refilled with a 50-50 mix of good ethlyene glycol (green) coolant and distilled water? Is the pressure cap new and the correct pressure rating?

Also, if the radiator is original to the car, the tubes that the coolant goes thru are so thin from wear that the radiator has lost a lot of efficiency. You should check both the radiator and A/C condenser to make sure the fins have not been bent to where they are not allowing proper airflow.

The HyperTech chip you installed does have programming to turn the main fan on at a lower temp. But to change that signal, you have to get another chip burned with a different value. The early ECM can't be "programmed" like the later C4's.

236 degrees is high especially given we haven't had hot air temps yet. With a cooling system in good condition, you should see 185-195 as a normal temp range. Sitting in traffic even with the A/C on temps can go higher but between the main fan and the aux fan, temps should drop once they start up.

With my 87, I have never seen coolant temps above 228 even doing track days at Portland with air temps around 100 dgreees.

 

Relay for Aux Fan.....On Drivers side firewall up under the Master Cyl.
Relay for Main Fan....Drivers side Wheel well
Temp switch for Aux Fan....Pass side block under the spark plugs

ECM controls main fan.....temp switch controls aux fan

If your main is comming on at 175. You might have a aftermarket chip in the ECM or you have a aftermarket Temp Switich and it has a jumper wire attached to that.
Stock Settings...( I Think) Main Fan on-225 off-205...Aux on-238 off-215

 

[QUOTE=Cliff Harris;1580806591]I think it's weird that my '86 FSM doesn't even show the auxiliary fan at all in the circuit section. The main fan is shown on page 8A-31-0, but the auxiliary fan circuit is not even mentioned.

It was either in '85 or '86 that it became a Dealer installed option outlined in Chevy's Service Bulletin for owners who were either high altitude users or autocrossers. It was never meant for much more than that (well, it was a 100 Buck option that probably never generated a warranty claim) and up until '90, more Vettes were ordered with the HD radiator with just the single fan - even when it became a Factory option. If it's coming on, there's usually some cooling or electrical issue - like an overtaxed alternator which if you run both fans long enough, becomes an issue because the CS130 can't keep up with the load.

 

The auxiliary fan was part of the Z51 option (which I have) and also available separately. That's according to the '86 sales brochure.

 

The auxiliary fan was also included with all 86 convertible pace cars along with 9.5" wheels.

 

Looks like the Bulletin was issued in '84. Z51's included it in '85 (makes sense given the suspension and complaints from autocrossers noted in the Bulletin). By '86, it was also an option along with being included in the Z51 package. Looks like a little more than half of the production (Z51's, Coupes and Convertibles) got the option. A higher number came with the HD radiator and 1 fan. By '90, the aux fan was re-engineered for R134 refrigerant which was coming and needed substantially more air flow to keep it at around 100 degrees and that remains the case for everything being made today - some use 3 fans for this purpose. A single fan - or any increase in air flow - should maintain coolant temp. Assuming the system is clean and the shrouding is intact, block air flow, turn off the a/c and note the temp that you aux fan comes on. The initial Bulletin describes a 238 degree threshold. Later Years are 228. Also check alternator output, CTS temp (you'll need a scanner) comparing it with your Display and any resistance in the wiring after 25 plus years is going to skew temps, performance. Schematic may have been an oversight or addressed in a Service Bulletin (and there's a handful for other mistakes, misinformation in the Manuals).

 

I started this thread and want to thank all your help. I think the answer is to replace the rad and the stat. However, I have a few more questions. I find it odd that the aux fan waits till 238 to come on. My Mechanic who charged the a/c checked the pressure in the a/c system and found it at 200 psi (should be 180 or so). After putting a box fan in front of the nose we found the pressure came down to normal. Also, did the Z51 come with a HD rad or was that an option? Don't want to get the wrong rad.
Thanks. JW

 

This is where the Hypertech is set to turn on the main fan...If you are going to continue to run this chip you will needto use a..(dare I mention it) 160* thermostat which is not the best idea for your engine.If you run this chip with a 195 stat you WILL burn the fusible link that controls the main fan....