87IROC-DAN61

Hello All!!

I was wondering if anybody tried on an ECM controlled single fan 305/350 TPI, to "trick the ECM" into thinking that the engine temp was 220 degrees, when it was actually running at 190-200 in order to get the fan to come on? Having seen in the service manual that the ecm controls the fan via the CTS, and that the CTS has a resistance value associated with the temp reading, is there a way to trick the ecm into thinking that it is seeing a higher temp than it actually is via lowering the reistance in that circuit?

Chevy8588

iTrader: (1)

when you change the temp that the ECM sees, you are the ecm is going to change the fuel trims and such, it will probably run like crap. your fan SHOULD come on because of a switch mounted in the passenger side head, if NOT and it is ECM controlled, you SHOULD have a backup switch in the passenger head, in which case i would get a lower range switch for the passenger side head and wire that in, bam, instant fan without messing with the ECM readings, if there is no switch there, there is probably a plugged hole for one, so i would still get the switch and wire the relay the that instead of the ECM

91interceptorZ

iTrader: (3)

if you look in summit or jegs, they sell a adapter that plugs in between your temp sensor and the ecm and does just what you're asking, it tricks the computer into thinking it hotter than it is, turning the fans on at a cooler temp. I believe that JET is one of the brands that makes them. They are very inexpensive and easy to install.

87IROC-DAN61

Hello Chevy8588!!!

Thanks for looking here!! I changed that switch back in 1993 from the original 235 degree, to a switch that was rated at 203 degrees, and it did not make a difference, as I found out later that this switch on the single fan 305 TPI is a redundant switch, and no matter what temp switch is installed here, the ECM still controls the fan for 235 degree running. That is why if you can lower the resistance in the CTS circuit it should work, the trick is how to do that?

87IROC-DAN61

Hello 91interceptorZ!!!

Thanks for the info!!! I'll look into that!!! Have you got a part number, just in case I can't find it????

87IROC-DAN61

Hello Again 91interceptorZ!!!

I just checked out Jegs, and found what you were referring to, close but no cigar All the JET brand fan switches are for TBI, or dual fan TPI. This won't do diddly squat with an ECM controlled single fan set up. Now If I converted to a dual fan set up, (using all original parts of course) I could use the JET switches, which is probably the best route.

Chevy8588

iTrader: (1)

well the answer to this is QUITE simple my friend, how can the ECM controll the fan if the excite wire is snipped? though i do not quite think you have the correct switch installed, or properly wired, becauase that switch is a failsafe for the ECM, the ECM can not command off the relay if it is popped on my that switchso what i would do is try grounding the wire to the switch with the key on and see if the fan kicks on, if not there's your problem, lol. i would figure out why the switch is not working, or you can simply re-wire the system to run entirely off of that switch. your choice. don't trick the ECM into thinking the engine is warmer, you'll have a metric @ss load of problems if you do.

deadbird

There is no fan switch for his year car with single fan.

Chevy8588

iTrader: (1)

well if thats the case then all he needs to do is install one and route the fan's relay to startup from that then!

91interceptorZ

iTrader: (3)

Well that sucks, that would of worked out good. That's kind of strange though, because I believe on the dual fan setups, the driver's fan is controlled through the computer and the passenger fan is on a switch type circuit. I'll do some lookin' and see what I can find. You could always put an aftermarket thermostat controled fan switch in. I'll get back to you on that....oh and then there is of course reprogramming the computer or buying an aftermarket chip that might change the temp control....

91interceptorZ

iTrader: (3)

here....try this link about single fan mods.
https://www.thirdgen.org/electric_co...pec_mod_single

87IROC-DAN61

Hello Chevy8588!!!

I believe that you are right that this is supposed to be a fail safe for the ECM, however, I believe that it is a fail safe for high temp only, as in more than 235 degrees. Up until 235 degrees the ECM controls the fan, after that the fail safe switch will kick in. I'll have a look at some info that I found and try to find where I seen that this is a redundant switch. Just so that you know, on my car, the driver's side cylinder head has a sensor which is the gage temp reading, and the passenger side cylinder head has this fail safe redundant sensor, that I changed which gave me no change.

87IROC-DAN61

Hello 91interceptorZ!!

Yes, the dual fan set up does work that way, one is controlled by the ECM, and the other is controlled via the sensor in the passenger side cylinder head. I've looked into the PROM chip thing, and the aftermarket fan stuff, but was wondering if it could be done via a sensor, and some wire. I do believe that the really smart way would be to do a dual fan set up, controlled like it should be via the ECM etc. Having a lot of extra switches/lights/and other whatnot is not my style. I'd prefer to do it looking stock, you know, the old "sleeper" tricks.

87IROC-DAN61

Hello deadbird!!

I think that the "fan switch" is the one that is on the passengers side cylinder head. This "switch" was a 235 degree switch, with corresponds to the ECM temp fan turn on, also known as the redundant switch. I had changed this to a 203 degree switch, and nothing changed!!!

Maybe I'm wrong, and this is for the A/C, also known as a A/C head pressure switch???? I don't, nor have I ever had A/C on this car, but the wiring does exist in the harness!!!

deadbird

I know what it is, there shouldn't be one with the single fan setup as the Helms (factory service) manual shows only the 2.8 v6 having a redundant fan switch. It's 238° for any year. But, '87 was the kind of a crossover year for wiring so, that isn't concrete either. '86 shows no sign of a redundant fan switch either for an F code engine.

A non-a/c car will still have the wire for A/C request to the ECM. The blue wire for the switch should be grounded to prevent the fan from running at all times.

If you've already installed a lower temperature switch and nothing happened, then that would indicate a wiring/relay problem as chevy8588 stated.

87IROC-DAN61

Hello deadbird!!!


I wasn't trying to "school" you on the switch, I was merely saying that "I" think....anyways This is my set up, drivers side cylinder head has a sender to the gage for the coolant temp reading, passenger side cylinder head has a redundant "I" think switch factory rated at 235 degrees, changed to 203 degrees in 1993 that seemed to make no change to the fan turn on temp, there is also a coolant temp sensor in the intake manifold beside the cold start switch, this is a yellow wire that leads to the ECM, "I" think. This was the circuit that I was originally wondering about lowering the resistance in, to trick the ECM into thinking that the coolant temp was higher in order to get the fan to turn on sooner.

Now that I have completely baffled/confused the heck out of everybody, myself included, I'm sure my best bet would be to go with a factory set up changing from a single fan to the dual fans!!! That would probably be easier, as this hasn't gotten the answer that I was looking for!!!



Maybe it was just a idea!!!

Thanks all anyway!!!

Chevy8588

iTrader: (1)

You don't neccissarily need to convert to dual fans, but I would, if you want to make this work with your single fan, trace your fan harness back to the fan relay, cut the excite wire from the ecm from it, then run a wire from the fan switch in the passenger head directly to the excite terminal on the relay, this will give control of the fan to the switch rather than the ecm, problem solved, 15 minute fix

deadbird

Sorry to come off as an -ss. I know you didn't mean anything by what you said.

The thing is, chevy8588 has given you the answer and what to try to make sure your fan circuit is working. Seemingly, you don't care to use his advice and want some other answer.

If your fan isn't working by any means of temp control, there is a circuit problem.

This has to be fixed first. Be it wiring issue or a relay problem.

Chevy8588

iTrader: (1)

thanx for backing me up on that one deadbird, I kind of got that hint too. if he wants to make his ECm think the car is warmer than it is, it's going to be difficult seeing at the resistance drops as the engine gets hotter, there are plenty of cheap easy alternatives that wont potentially damage his engine by making the computer think its 20 degrees hotter than it really is and leaning out the mixture becuase of it.

87IROC-DAN61

Hello deadbird, and Chevy8588!!!

I got what the both of you are saying, it's that I just couldn't get my head unwrapped from that "redundant" switch, and I wanted 'the fix' to not involve cutting up my wiring harness. Sometimes it's a little difficult to get the point across trying to put it on electrons (forum posts), as a picture is worth a 1000 words or so.

Having said all that, I'll try using the 15 minute fix that Chevy8588 suggested yesterday.

Chevy8588, in your post above you said,"You don't neccissarily need to convert to dual fans, but I would, if you want to make this work with your single fan, trace your fan harness back to the fan relay, cut the excite wire from the ecm from it, then run a wire from the fan switch in the passenger head directly to the excite terminal on the relay, this will give control of the fan to the switch rather than the ecm, problem solved, 15 minute fix" Yesterday 11:20 PM

Can I do this using a fused jumper wire as a temporary fix to test this out, or will that cause some sort of problem?

Chevy8588

iTrader: (1)

yea, you should be able to test with this, just know that a fuse should not be necisarry, you are working with grounds.

87IROC-DAN61

Hello Chevy8588!!!

Thanks for the confirmation, the fused jumper was just insurance against anything happening to my ECM via me doing something stupid, Murphy's Law states something to the effect that, if it can go wrong, it will, darn I hate that Law!!!

deadbird

Even with a single fan, you can hook multiple switches to the relay ground to control the fan w/o worry.

Again, if neither the ECM is switching the fan on nor, the fan switch, there is a wiring/power issue.

Any way, to control the fan (relay), it is basically this (colors are fairly accurate)....

Chevy8588

iTrader: (1)

Excellent diagram. Pretty accurate too, when I built a new fan harnness in my 87 setup I think the stock excite wire was bright green

87IROC-DAN61

Hello deadbird!!!

In your post above, it seems to me, that you think that I have no power, or that the fan is not coming on at all. Actually, I do have the fan coming on, it was just not coming on at the indicated temp that I wanted!! Maybe that was a source of confusion, and what led to you, and Chevy8588 into thinking that I wasn't taking your advice.

Any ways, I just wanted to clear that up!!!

Thanks for all the help, all I got to do now is try your, and Chevy8588's solutions out!!

Also, thanks for the diagram!!!