haps

iTrader: (4)

I hope. I've done my homework and I don't see any topics with this issue, so here we go. The primary cooling fan on my 91 (5.7 TPI)won't come on. The secondary comes on when the temp is around 255 and works fine. It doesn't come on when the a/c is switched on, but I'm attributing that to the a/c system not being charged. I've replaced both relays and both sensors. (the two pronged one in the manifold that sends a signal to the ecm and the single pronged one in the passenger side head that controls the secondary fan) Both fans spin when 12v is applied. This is the interesting part. When the connection to the intake manifold coolant sensor is removed, the ecm throws code 51 AND the primary fan comes on when the terminals are jumped in the aldl. My next step is a new ECM unless you guys can tell me what I'm doing wrong. TIA.

haps

iTrader: (4)

Ok, I found this thread and it appears that the ecm turns on the primary fan with the aldl terminals are jumped(paper clip trick) Still having difficulty understanding why the ecm won't signal the relay when the engine is at temp. I rang out the wire from the sensor in the manifold on terminal E16 at the ecm and it appears to be fine. I also rang out the ground to neg battery terminal.

haps

iTrader: (4)

A little more puttering and I find that the ECM isn't grounding out the primary fan signal wire (green/white) to the relay. It has constant battery voltage throughout the temperature range. With a new manifold sensor giving a signal to the ECM, I can only deduce that the ECM is bad. Anybody ever had this happen? Thoughts?

rapid rods

I have a similar problem in that the main fan,it stays on all the time even with the fan switch disconnected.( new fuse plus 2 relays. (87 iroc heavy duty cooling) I had a new chip burned for upgrades,wonder if it damaged the ECM !!!!!

rapid rods

I will borrow a replacement ECM , from GM SALVAGE, in San Jose, and test the system.

haps

iTrader: (4)

Update. I've replaced the ECU while using the same stock EPROM and the primary fan turns on. However, the gauge in the car indicates that the primary fan comes on at 255-260 degrees. My next step is to replace the sender and attempt to rig up a probe to get actual engine coolant temps. I've seen the radiator caps with thermometers in them, but I'd question their accuracy. Any thoughts?

haps

iTrader: (4)

New temp sender still indicates that the primary fan is coming on at 260+, but at least it's coming on. I've replaced everything in the circuit except the EPROM and the gauge. Bad gauge? Bad EPROM?

Lucid

iTrader: (4)

First of all, dont trust the gauge on these cars, get a thermometer and dip it in the coolant just inside the radiator. The thermometer will be more accurate than the gauge. I have seen different gauge readings with different brands of temperature senders too, not to mention the actual gauges themselves are notorious for being inaccurate.

At 260, it sounds like you should be boiling over by then, is it?

You could burn your own chips and set the fan temp to come on when you need it to come on. I dont remember if both fans are ECM controlled in your year.

When you believe the fan is supposed to come on tap on the fan relay, these are known to stick.

haps

iTrader: (4)

Thanks for the reply. The fan relay is new, so I hope it isn't sticking already, and I didn't see any boiling over. I'm guessing it's the gauge. I intend to get a chip burnt to address this issue and a couple others. I'll attempt check the temp with something more accurate. Thanks.

haps

iTrader: (4)

Continuing an old thread with the same issue. I believe I have some success. Primary fan comes on first, however it's at an indicated temp of 260 degrees. Could the sender and the sensor be 40 degrees apart?

ACCLR8N

iTrader: (8)

I think you'll find they are only 25° apart. The stock ECM activates the fan 230-240°. The sensor for it is in the front of your intake manifold, while your gauge sensor is in the driver side cylinder head. The secondary fan switch in the passenger head comes on even hotter. The factory L98 cooling fan set up runs very hot.

abray1

The factory system is enginered well IMHO but the temps are too high. I have switches that turn on both fans on the console. The factory gauges are not accurate at all and I have bowtie gauges in the pillar pod to let me know whats going on. I made an aluminum air dam to replace the factory plastic one and it helped a lot along with turning on both fans early with the switch. I also found the plastic piece that bolts from the front spoiler to the bumper support. I think it is called a baffle and directs air into the front of the radiator. I also use a 170 stat which is best all around especially in a warm climate like Arkansas. The more control you have over it the better you can keep it cool. Purple ice and water wetter will drop it 10-15 degrees as well. I also bought a be-cool radiator and that helped a ton and it fit great as well. I am going to mount some reliable gauges under the hood to keep an eye on things also.