Ran fine last time went out for a cruise. Decided to go for a cruise today and shortly afterwards both fans went on. Had put my "performance" chip back in couple weeks ago after Emissions check as NOX gets higher. I have switches to manually turn on both fans separately when using stock chip.
The temp gauge did not show the high temp to turn both fans on automatically. Went home and installed stock chip. The first fan went on and with my AutoXray it showed 235.4* but gauge was around 200*F but there was a quick drop of temp to 182* then back up to 235* with the AutoXray reading the ECM terminal. Checked on line to find one to order but just had the spade type and not the clip on that I had (right side between 6 and 8 plug). One other symptom; when I got on it one time loss of power then next time okay.
Did a search and thanks to billmcdonald with a pic and Hooked on Vettes with GM and Delco p/n's I know what to get.
QUESTION... am I right in my diagnosis?

 

..... Make sure your radiator is full of water .............

 

I already popped the rad cap and its full and the overflow was about 3/4" over the cold line where I like to have it.

 

If you're saying the ECM coolant temperature reading is wrong causing the fans to
come on, the Engine Coolant Temperature Sensor is probably bad.

It's screwed into the front edge of the intake manifold and has a two wire plug
going to it. A yellow wire and a Black wire. The engine coolant temperature sender
is screwed into the passenger side head between the 6 and 8 spark plugs. The
sender is only used for the dash cluster temperature gauge and not by the ECM.

Here is a picture of the ECT sensor. 2nd picture shows the two connector pins.

AC Delco 25036979

 

...and be sure the replace the CTS on the front of the intake manifold under the throttle body. That's the one that the ECM uses.

The CTS in the head is for the dash temperature gauge.

 

Hooked on Vettes and Cliff Harris thank you for the info. Yes the dash gauge shows okay but the scan shows hot and once the temp dropped for a brief second to 182* and then back up on my AutoXray read out. Im well into retirement and the one on the head would be a difficult chore for the ol body but the front sensor on the intake would be easier for me.
Thanks again for the help
Bill

 

"UPDATE"
Bought the new intake manifold sensor and installed it. All went smoothly with the install.....BUT it went screwy again. I had installed the a/market chip to have the primary fan kick in but at 137* on the scanner it jumped to 235*. It then started to go down and stopped running fan about 190*. Thought it was okay but started the jump in temp again. Note...the engine was cold and this problem started about 2 minutes into run time. The analog gauges for temp showed low and even the oil temp gauge showed low with high oil pressure just like a normal cold start up.
After letting in cool down and starting up several times over the next couple of hours I put in the stock chip. It ran the temp on the scanner and analog gauge just fine. Problem solved I thought as at 215* both showed the same. All the testing were done at idle as they were going to resurface the road in front of the house and could not get out for a test ride.
I was a happy ol guy and a few hours later tried it again. At 137* after a couple of minutes it immediately jumped to 235* and the primary fan went on while all analog gauges showed the engine was just starting to warm up. When it got to just below 220* the fan cut out and at 215* it jumped down to 176* on the scanner and the analog gauge was at about that mark and it was holding and slowly climbed to 182* and the analog gauge followed suit.
Im lost now....is it the ECM? Note ... Vette has 22,913miles on it.
Need your ideas Please.
Thanks
Bill

 

Here's a chart showing the Engine Coolant Tempeature sensor
resistance and what the ECM temp. would be.

Lower resistance on the Yellow wire to the ECM = a higher engine temperature(lower voltage at the ECM Yellow wire).

In your case you say the Autoxray shows the temperature jumping high which indicates the Yellow wire from the sensor is showing less resistance than what it should.

Could be something is partially shorting the Yellow wire, the ECM is bad or something is wrong with the coolant system.

The Yellow wire from the CTS goes to the Green connector at the ECM pin C16. The Black wire at the CTS is sensor ground and goes to the Green connector at the ECM pin C10.

The same Black wire is a common ground at the ECM is also used for the engine oil temperature (used by the ECM), engine oil temperature gauge and the Throttle Position sensor. A shorted Black wire wouldn't cause your problem since it's ground. A open Black wire at the CTS would cause a high resistance = a lower temperature reading and not a higher one.

To eliminate the sensor and the Yellow wire I'd cut back the insulation at the ECM green connector pin C16 and use a volt meter to monitor the voltage. Should be less than 5 volts with engine cold and slowly decrease as the engine warms up.

If the voltage doesn't jump around and the Autoxray does, it would point to the ECM as the problem.

 

Thanks ...will tackle it today.
Bill

 

UPDATE AGAIN...
Started the Vette up and within a couple of minutes the fan kicked in. Was not prepared to check .. caught me off guard.. So I let the car cool down and did the procedure that was recommended. I turned the ignition on ONLY and got the following readings by using the yellow wire at pin 16 on the green connector with my multimeter and scanner.
TEMP ... VOLTAGE
208* --- 1.828
180* --- 2.280
160* --- 2.825
153* --- 2.962
143* --- 3.174
139* --- 3.271
134* --- 3.389
At this point started the car up and the voltage .... scanner ... and analog gauge were progressing normally until about 205* and the fan went on showing 238* but analog gauge was where it should be.
238* --- 1.651 with fan on
then the fan shut off shortly after at 204*
204* --- 1.821 fan off
205* --- 1.810 fan off
202* --- 1.877 fan off
197* --- 1.960 I turned on my manual switch for the primary fan
I guess this means a new ECM?????
QUESTION .... can I still run the Vette with this intermittent problem with primary fan going on and off with no driveability problems?????
If not where can I find a ECM?
Took this data off the ECM

Serv. No 01227727 ........ ANHW
865101M93400268 .... same # as on bar code lower on sticker
16125101
Prom I.D. 5101 from scanner
Need help here as to where to find (if possible and still available) and what will be the $$$$ cost.
Thanks for all the help (Hooked on Vettes) to get me this far

Bill

 

Did this problem appear after you did something?

The ECM does use the engine temperature as one one of the
parameters to determine the proper fuel mixture.

That's a very common ECM used in many GM vehicles. It appears the
Mem Cal is the only thing different for each car model.

In the US from an auto parts store, it sells for around $100 plus
a core charge/exchange.

Looks like the shipping from the US to Canada is a killer. Around
$45 US dollars.

Used ECM's are around $50.00 and up.

http://www.ebay.ca/sch/i.html?_odkw=...27727&_sacat=0


http://www.ebay.ca/sch/i.html?_odkw=...27727&_sacat=0


http://www.ebay.ca/itm/New-1990Chevr...d=520467328558

 

Never did a mod change but put my a/market chip in a few weeks ago as I had the stock one in for the emissions check. Car ran great for that time then the trouble started.
Thanks for all your help and will start looking for one.
Thanks again
Bill

 

Funny that all this occurred on or about the time you changed memcals (chips)? I would put the stock one back in and see if you can create the same problem. Always disconnect the negative when doing anything with the ECM, or memcal. Look to see if possibly you may have bent a pin.
The ECM 1227727 can be found on Ebay as cheap as $39. The memcals are a bit harder to find, but most programmers should be able to provide a replacement, or a piggyback adapter with a program based on your year, and broadcast code.
Whenever I have an issue, I try and stop to think what I've changed or done differently. Just a thought.

 

Ive switched back with both chips and have the same results. The a/market chip was in for years previously but it ups the NOS in emission checks by 245 as my 1990 has speed density and not Maf which will adjust but Speed Density will not so I put the stock back in for emission checks. I always disconnect both + and - terminals to the battery when I switch chips. Basically the a/market chip I like only for primary fan kick in at 200* on and 190* off even though I have manual switches to turn on the primary and secondary fans separately at my discretion. They are rocker switches mounted in the blank space between the seat adjusters on the consul.

 

Good thing is that eliminates the memcal as the fault. What I don't understand is the smog test issue. Both chips running in closed loop should be modifying the fuel tables to adjust to a 14.7 afr. It's possible I'm not understanding something though, but MAP vs MAF they should both adjust. Map is more sensitive to slight changes though, and should be logged and tuned more often tha maf.

 

The Map on Speed density will not allow for changes as for instance it cannot recognize or adjust for intake and exhaust if mods are made to intake for air and free flow exhaust like Maf. SP goes to a calibrated table and that's it. With both e proms I thought I had the air /fuel ratio good with my BLM and Integrator readings but the a/market chip was a problem. It surprised me as I bought a new Formula 350 Firebird with the L98 back in 1988 with Maf and used the same methods and same supplier of a/market chip with much more extensive mods (eg headers) and it still passes the emission test (son has that one now) but after 19 years playing with the Formula and 6 years with the Vette there is such a difference and I do not know how to ReMap the system for changes and have learned a lesson. Im over 65 and the few minor things I have done to the Vette is good enough for me. Had to redo the several changes I made to pass emissions. About the biggest change that still exists though is going from 2:73 to 3:54 rear gearing and that's a keeper
The emission check here in British Columbia is more realistic (moved here this past June and moved in with our son) than in Ontario where they are just as stupid and strict as California. For all the years with the Formula and Vette I closely watched how the max limits in all categories dropped every few years.
Bill

 

Sounds like your ECM is gone; not the memcal or chip. Basic test is to check CTS on your Scanner after it's sat overnight. Should be ambient or close to it (the CTS doesn't have to be installed to check ambient temp return). Start it up, and it should rise to the thermostat and then cut back a degree or two when the thermostat opens before rising to the fan threshold. You've put a new CTS in it and it's all over the place. When mine went, it would start toggling between 230 and 250 making cold starts impossible and the fan was running well below it's threshold. You can trace the pin out's to verify the 5 volt reference (CTS disconnected) and check for excessive resistance (ECM and CTS disconnected). Reference voltage will drop as resistance decreases meaning less volts at the ECM with it all hooked up, but it should hold steady if the gage is steady. I have no idea what to look for on any circuit board so after going through all the tests, I knew the ECM was the only thing left

 

Totally agree with you but mine would jump at times from a cold start at say 137* and go right to 235* after a couple of minutes from a cold overnite start then drop down as primary fan would kick in and the analog gauge was where it should be. Just got back from ordering a new ECM. Total frustration at times figuring out this situation but all will be good now once the new ECM arrives.
Bill

 

UPDATE
Got a remanf. ECM in and tried it out with a/market chip. Kicks in every time at 201.6* (200* advertised). Have to take it out when its not raining but I am confident all will go okay. The scanner readings on other areas are somewhat different from the old ECM but better to my liking especially the BLM and Integrator readings . Will scan thoughly when the roads are dry.
Bill