vetteLT193

This is an exact symptom of only 1 fan operating. There should be 2 fans behind the radiator. When only 1 runs and the AC is on you will overheat in traffic. Turn off the AC and I bet it runs fine.

If you put the car into diag mode through the OBD port you can see if a fan is broken. Both fans will turn on when in diag mode. If only one comes on then you are dealing with a broken fan or relay.

johno504

:Got stuck in heavy am traffic, temp goes to 248( i had never let it get that high before) second fan kicks in temp quickly falls to 225. will have the ECM set to a much lower "on" setting. my FSM states that there is a dash warning lite that will display @ 260 !!!. never seen that warning, hope i never do. open to suggestions for on/off temp.

WVZR-1

**You posted while I was thinking answering some of the following!

You need to know that the shop understands what you're interest is --

I've a friend who likely wouldn't touch this "check out" unless it was left with him "overnight" so that he could observe the data/information from a "cold start". I'd think that's maybe a good idea.

OP - I asked where you're located and if you had a correct FSM (Factory Service Manual), you didn't answer. There could maybe be someone right around the corner with a good scanner or data acquisition software that could help you. I'd think someone close-by would maybe jump at the opportunity to observe/learn and more importantly HELP YOU!!

Correct fan operation in both stages and "real temperature"! Does the AC work?

johno504

located in new orleans, yes the a/c works. trouble for me is tuner i planed to use i a 200 mile round trip ,so leaving car overnight won't happen. i'll check with dealers to see if i can have reprogram done in city. stand by for that answer . yes i have FSM, looks to me like just about every diagnostic has been done at 2 previous shops. second fan "turn on"@ 248 is a good/bad situation for me, still plan on change to mechanical pump.

RichS

You keep saying second fan turn on. They do not work that way. They both should run together on low speed then both together on high speed. If they are not there is a issue. If one is turning on and off while the other is running the motor is probably bad and cycling on and off on thermal overload.

You could send the computer out to any tuner and they can adjust the fan on temps if you don't want to leave it. My last mez pump lasted 10 years with no issues. The car runs a constant 180-190.

johno504

ahhh, thanks for the info. left side fan is always on , a/c is always on due to temps here in the deep south. i have never seen right side fan run, but it must have come one when hi temps fell so quickly. i know for a fact that both fans did not run at same time in the past . when i was pulled over on side of road and temp was 240, only left fan was running.car has 3 fan relays,2 fans, what is purpose of third relay?

WVZR-1

The '96 FSM is a very different format than the earlier years but here's some suggested reading maybe in this order, 6-731 is an explanation of operation, 6-639 diagnostics, 6-158 for the wiring diagram.

You don't need a "tuner" yet - if you read and understand those sections of the FSM and can get either of the previous shops to accommodate you maybe if they have an appropriate scanner have them do the diagnostics. If you doubt they can you just confirm with a local shop that they understand the procedures and have them check. You only need "diagnostics" I'd think at this point. What works - what don't - how has wiring been modified

With OBD 2 there's references to DTC codes for FC Control so maybe you check for codes also. History codes could be useful I'd think.

I've a couple friends that have a EWP and I doubt any of them would revert back to mechanical given your set of circumstances.

BlowerWorks

Sounds to me like you need your PCM re-programmed. A well equipped shop can so do. As an alternate you can send me your PCM and I will bench top flash to what you need.

I strongly suggest to forget changing the "pump" until you resolve the fan turn on points, i.e., PCM program. greg

johno504

CALLED local chevy dealer to see about lowering fan turn on temp, was told " we don't work on cars older than 10 years due to insurance liability " ASTOUNDING !!! they are more than happy to sell me all the parts i can stand. found any indy shop in town that can reprogram PCM, scheduled service first thing tomorrow.

johno504

will re read FSM sections called out , thanks.

WVZR-1

I'd want to know how everything is working before I handed it over to a "tuner". An experienced "tuner" should certainly be able to do the confirmations and I'd think would want to confirm the operations before even starting. I'd make sure the guy understands the issues with the car.

How long have you actually owned this car? How many of the 90K miles are "yours"?

vetteLT193

On my 93 and 96 only the left side fan would run with the AC off unless it got stupid hot (stupid hot is 238 degrees to be exact) However with the AC on and car under 35 MPH BOTH fans should run.

I think the issue you have is not the second fan, it's a wiring issue with the AC. The fans don't 'know' the AC is on so they are operating under the Non AC parameters. I can't remember if there is another relay in line with the AC or not or what triggers the fans to run on AC. I seem to remember 3 relays, one tied to the AC, the other 2 not. Investigate what turns the fans on with the AC and you'll find the issue.

Another option, which is probably easier but not 'right', is to swap the aux. temp switch to the lower degree part. I think it's on the passenger head. The factory installed the 238 degree switch there, you can drop it to a 200 and it will turn the secondary fan on then instead of 238. It's not 'right' because the second fan not only helps cooling but also helps with AC. So anytime the car is below 200 and you are in traffic the AC won't work as well until it hits 200. That shouldn't take more than about 14 seconds in Arizona though. LOL

RichS

Both fans on a 96 run "together" all the time in either high or low speed. If they are not you either have a bad motor or relay, or someone rewired it. The computer controls the fans from the input of the coolant sensor on the water pump, the sensor in the passenger side head is for the analog gauge and is not an on off type sensor. You can not replace it with a cooler sensor since it reads resistance. I'll have to look at some diagrams to 100% verify if the water pump sensor is for the PCM and digital gauge.

johno504

at this point, i don't know what to think. will go to shop to program/check fans and suggest they change both temp sensors in the am (i have both). it is my understanding that water pump switch is for the signal to ECM, switch in head is to the analog gauge. the digital and analog readings seem to be off by at least 25 degrees,so i just ignore the needle.

vetteLT193

Found the info I was looking for... There is a fan control switch mounted on the AC high pressure line. That is likely the issue. When it hits 233 psi it should tell the ECM to ground circuit 335.

I was wrong earlier, both fans should operate together in either low or high speed mode. If it isn't going over 240 on the digital readout you might be worrying about nothing anyway. These cars run hot.

I installed a 160 degree T stat in mine and while that didn't help it in rough traffic it did help some because I had a lower starting point. Instead of starting at 185 or 195 ish (depending on year and current stat) I'd start below 180 which bought me more time.

johno504

[QUOTE=vetteLT193;1595423077]Found the info I was looking for... There is a fan control switch mounted on the AC high pressure line. That is likely the issue. When it hits 233 psi it should tell the ECM to ground circuit 335.

I was wrong earlier, both fans should operate together in either low or high speed mode. If it isn't going over 240 on the digital readout you might be worrying about nothing anyway. These cars run hot.

I installed a 160 degree T stat in mine and while that didn't help it in rough traffic it did help some because I had a lower starting point. Instead of starting at 185 or 195 ish (depending on year and current stat) I'd start below 180 which bought me more time. I want to thank all who responded to this post. took car to competent C4 mechanic this am, he found blown 40 amp fuse in fan wire system. secondary fan did not turn on. mechanic did his ECM MAGIC and set fan on @ 95 C / off 90 C. never saw temp higher that 220 on ride home. glad issue seems to be resolved, but for $1,100 it was sure an expensive "fix". i will now guzzle down 6 pack of Kronenbourg and take a nap(it's what old men do). Thanks to all...