jrmott

I am at whits end here replaced every componet on the electrical side of fan system. I went as far as changing out ECM with no results. I have the factory GM trouble shooting chart and down to only checking the ground wire from ecu that controls the main fan. What puzzles me is that i can not pull codes from computer with a light on when key is in run position. You should also have the main fan operable when grounding the ALDL connector. This does not happen? I have tested relays and replaced both checked fuseable link and hot wire to relays fans are working and new. Also changed sender that sends signal to ECM to tell ECM engine is hot. Where to go from hear car is all original with only 15,000 miles and stays in garage. The front pusher fan will come on with fan switch in passenger side head replaced switch also. All i can fig. Is it is a wiring issue from here.

ANY IDEAS would be AWSOME!!!

gliderdad

I just went throughthe same battle with my 87 as it kept blowing one of my fusible links:
Some things that I did:
1) Replaced both cooling fan sensors- 1 between #1and #3 cylinder and the other between the #6 and #8 cylinder.
2) You say you replaced both relays? 1 on the redaitor support on the drivers side and the other by the battery
3) Did I understand you correctly that both fans are new?
4) Double check your fusible links. Mine had 2

After all of the component replacing I found that the main fan was pulling too many amps and wiping out my fusible link. I replaced the fan motor ($24.00) and everything works great

Hope something here helps...

leesvet

To the OP..
You can easily test for power and trace back to the last place it exist. Fan motor to relay to harness.

Does the fan turn on with the a/c? it should after a few seconds.

Does grounding the green wire at the sensor start the fan? it should. The fan is managed by ground path, NOT power circuit.

Is the relay hot anywhere?

do you have the FSM drawing to trace the wires?

Wire corruption is COMMON in C4s. Insulation breaks from drying out, grounds, shorts.

Track the power. test at the fan motor 1st and work your way back. if you continue to throw parts at the problem without knowing what the problem is....get some OT in..its gonna be a long summer. The FSM set is cheaper...only $100 new and you can use it more than once..and you will.

when you ground your alcl do you get the codes at all? any 1....2 ?
a code 12?

yes, the fan should come on when you ground it for diagnostics. If a different ECM is doing all the same things and the alcl is acting funky, that suggest harness issues...or the alcl is not right. Does the car run well with NO SES lite? Does SES cycle when the key is turned initially?

AGENT 86

85 cooling fan control is wired like no other year. My fuzzy brain recalls that late in the 85 build, they were wired like 86, but not 100% sure.

Can you scan and post the wiring schematic from your GM service manual ?

jrmott

using the power probe tool i can activate either fan from their relays one is behind the battery. the other is on the fender well on drivers side. I do not have the factory manuel only the flow chart showing what steps to take to test for componets.AS far the alcl when key is turned to run SES light is on. I can not ground the alcl and pull any codes SES light will not flash and fan will not run It appears that the fan relay is not getting any ground signal from ecm to fan relay to energize the main fan. At this point i want to test from ecm to the main fan relay ground to see if their is a break in the wire. Have not had much time this week to really dig further into this thing. I do think this is going to be a wiring issue. I am trying to help a friend out who owns the car and knows nothing about mechanicals. The A/c is not operational due to low charge. compressor seal is bad I have all the parts to repair system just wanted to get cooling system right before i repair A/C system. All of this is the results of using a HD battery charger trying to boost battery to start the car. i was not there and do not know what the owner did to cause issue .Thank everyone for your responses and time helping me.

gerardvg

The main fan is turned on and of via the ecm, the booster fan (in front of a/c condenser) which is optional is controlled by the temp switch between spark plug's 6 and 8 when that is grounded the booster fan activates. See diagrams from my 85 shop manual




Hope this helps you trace the fault, the fan relay is a common failure (located on driver side hood latch support beam that connects to frame). If it is the ecm you can always get a reco ECM or fit a thermostat switch that has a capillary to fit into the radiator hose and wire that to B on the relay and the other side of the thermostat switch to ground .

Good luck

vetteoz

A one year only setup

1985 was the 1st year with ECM fan control but with a 84 style mechanical switch as a backup / override
Maybe GM were hedging their bets on the ECM controlling the fan correctly ?

vetteoz

Neither of which have any effect on your main fan operation
Pass side head is the sender for the temp gauge
Drv side head is the switch for the Aux fan in front of the radiator.

gerardvg

My 85 works as the diagram i posted because i have the dual fans, if you ground the temp sensor wire (from the temp sensor between cylinder's 6 and 8) only the booster fan operates.

The original poster JRMOTT stated his pusher fan activates, so he should go by the "dual fan" diagram. Looking at terminal D on the cooling fan relay to see if there is power on the fusible wire to the relay, and that the ecm grounds the relay at terminal B from 226f minimum and before the booster fan activates at around 238f (via the temp switch in the head for the booster fan relay).

65Z01

With ignition On jumper the relay contact from the ECM which should bring on the main fan.

If that works open the small connector in the A/C pressure line and start the engine; the ECM should bring On the main fan.

If not, check voltage at the ECM connection to the main fan relay to verify that the ECM has indeed pulled that line to ground.

If not, drop the ECM and repeat the above test at the proper terminal of the ECM connector.

jrmott

Thank you for the diagram. I will keep the post up to date it will be next week before i can get back on the car i only get a couple hours in the evening to look at the car. I beleive that what ever has the ses light on has to be contributing to the issue. All componets are new at this point thank goodness most of them were very cheap ECM was under warranty. In all my years of working on cars i have never came across a problem like this one I do love a challenge though. I am ashamed to even say i am a ASE mechanic. Wiring is just not my thing.

leesvet

Not to worry...

as much as you train, the Corvette is a different animal.

It has several pioneering systems that ONLY Corvette and Cadillac offered for many years...top shelf stuff in the 80s and 90s. Once this technology was proven and deemed reliable, it was used on the rest of the GM line.
You have to train for Corvette in order to be successful when working on a Corvette...
The FSM set is your key to understanding the systems that interface. ex: fuel permissions given by the dist, bank fire inj, all ground path control...many innovations that are not common to other cars.
You'll get it. Just do NOT look at the Corvette and compare it to any other car and how it works...cause its not the same and thats where even good techs get screwed up.

Cliff Harris

I believe those sensors were on the opposite sides before 1986.

jrmott

pass side is for pusher fan and drivers side is to temp gauge on this car.

jrmott

update today i have found that relay is not getting ground signal from ECM almost sure its a damaged wire between ecm and relay going to try running new ground wire from ecm to relay and see what happens tomorrow.

leesvet

uhhhh

thats one of the grounds that bolts to the block by the oil filter. ALL the engine controls ground wires all bolt to the same place down there. These grounds come out of the big trunk line on the firewall under the wiper motor. The splices break easily...so a broken ground is a very good catch. Look at the 5 on the bolt...one of those may be broken off...

jrmott

Thanks for the heads up i double checked again and went through the whole system and removed harness from ECM and used my power probe to send 12 volts through dark green wire /white stripe ck335 and used volt meter on ECM end no power at all. Then took ohm meter to check for continuity on each end nothing. Looking at wiring Diagram it only shows wire to go from ECM through Bulk head to the relay is this correct? If it intersecs at oil filter and is broken this could be my issue. This may also be why i cannot ground alcl and pull codes and ses light stays on when key is in run position .Grounding issues makes for a tuff repair.

leesvet

before you go any further...
you should understand and try to change your thinking regarding electrical...on a C4 anyway.

Most things have power and ground. Ground is there and power is sent or not to make it run.
Corvettes are backwards. THIS IS WHAT MAKES THE GROUNDS SO IMPORTANT !

On a Vette, POWER is everywhere. Everything is hot all the time. There is power to the inj, every sensor, every servo. It is ALL managed by simply adding a ground path thru the ECM.
The ECM has many grounds to frame. The ECM finds an internal path for whatever system it wants to operate,, and supplies that ground.

If you follow the FSM drawings you will see that the power comes from the battery jumper pole, thru fusable links to the ECM, all the engine controls and efi harness. These power wires are labled "hot always". Thats the hot wire from the jumper thru the link to the ECM, for ex. There will be a hot wire to the fan relay as well. Its hot all the time. When the CTS sends the temp to the ECM that says its time, the ECM will then add the ground. Grounding ANY switch or device gets operation...fans, switches, blowers,. a/c clutch etc. ALL done internally.
Now these grounds are everywhere. The battery ground is the LEAST important of all. As long as there is a cable from batt to frame its ok. The HARNESS grounds are what matter...
The ones that you CARE about in this case exit the harness under the wiper motor on the firewall. There are 4 or 5 blk wires that are different ground circuits that all MUST be to frame...or something under the hood will NOT WORK.

These wires drop from the wiper motor area behind the rt side cyl head to a bolt on the bellhousing, OR a bolt in the block by the filter, or sometimes they are stacked under the oil temp sensor in the block. They ARE there somewhere in that area. Those grounds are the most important of all.
IF you can remove enough stuff to get in the harness and check the splices, you should. These are cheap splices that can come apart very easily and ruin your day within the harness sheath.
Its the most common "fix" of all....clean the grounds. Fix the grounds. ADD grounds. People chase problems and when told to clean the grounds they mess with the battery cable and then ignore all pleas to fix the right ground wires....until they've run out of things to guess at. I've seen starting problems cured, temp problems, misfiring, serious MAF issues, all "fixed" by simply cleaning the harness grounds and reconnecting them properly. Do the same for the jumper post and you will have a different car to drive.

Remember that these ECMs get their info in the form of resistence from sensors and they use reference voltage that is very specific. IF the connections are dirty, corrupt or broken, the ECM has no data to use so you get poor performance or a no run, no start condition that nobody can explain.
Maintaining the elec system on a Corvette is just as important as the lube the mechanical parts need.
Good luck. you have the problem ID'd. just fix it and you;ll be good to go.

vetteoz

I was replying to the owner of the 87 in post # 2 ;
not the OP with his '85

vetteoz

You are correct , the fan control wire runs direct from the fan relay to the ECM ONLY where it is grounded by the ECM through one of it's ground paths.
Obviously , the lack of continuity means you have a break or bad connection in that wire somewhere