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Hi guys
I have a small issue with my 1989 V6 firebird. The coolant temperature given by the ECM is lower than it should be (by around 40°F). It doesn't seems to impact the car behavior except the fact that the cooling fan starts to run quite late (around 230°F on the dash indicator) probably triggered by the security switch.
I've checked the sensor it gives the right value. The harness is fine. When I measure the voltage between the yellow cable and the ground I found 5V as expected. The ground is OK.
Do you have any idea of what could be the issue? Bad ECM or PROM? I've checked the ECM (1227302 with HNT PROM) and all the connections and everything looks fine.
Thanks for your help
Hi Sierra6
you probably right. Looking more carefully to the schematics I agree that the fan should be only triggered by the switch and not the ECU.
But I'am still puzzled by the fact that the temperature given by the ALDL scan is different from what it should be. If this is what the ECM sees it could be an issue for control of the injection and others..
you say the reading ecu is low by about 40 degrees. is it possible the gauge is reading 40 degrees too high somehow? 230 degrees - 40 is 190, which is about where the stock fan should come on I believe. stock thermostat is 195 degrees.
I've checked the gauge and it's fine. When the thermostat opens at 198°F, the gauge is given the right temp but the ECM tells a temperature around 160°F
It is a possibility that the CTS connector and pigtail for it aren't making good contact. Any additional resistance in the connections between the CTS and ECM will cause the ECM to read/report a lower then true engine coolant temperature.
Note that the ECM also has control over the fan. In parallel with the over-ride head temperature switch.
It is a possibility that the CTS connector and pigtail for it aren't making good contact. Any additional resistance in the connections between the CTS and ECM will cause the ECM to read/report a lower then true engine coolant temperature.
Note that the ECM also has control over the fan. In parallel with the over-ride head temperature switch.
RBob.
I have checked all the connections and everything is fine. Used cleaning fluid in all connectors just in case. I am wondering if it's not a bad information from the aldl scan ?
I've tried to use a calibrated resistor in replacement of the coolant sensor to see where the problem is coming from. With a 240 Ohms I should read a temperature of 195°F where I have in fact 160°F. There is definitely an issue either in the connections (but I have 5V arriving on the connector and the ground is fine with less than a ohm) or with the calibration curve in the aldl scan.
RBob, does that mean the fan switch only comes into play if the ECM doesn't click the fan on itself? If so, at what temp?
According to the service manual the fan should start when the temperature reaches around 223°F and stay on for 30s.
If the ECM fails the override switch will start the fan.
I've down a lot of test today and it seems for me the scan tool has a calibration error.
What I've down :
- check of the electrical continuity of the circuit from the ECM (C10 to D13)
- check of the resistance at the ECM connector (C10 and D13) : around 200 ohms at 210°F which is very close to the theoretical curve of the sensor and eliminates the probability of a parasitic resistor.
- check of the voltage at the ECM : 1,85V at 210°F
- Fan starts at 230°F on the dash gauge. This corresponds to a voltage at the ECM of 1,47V
- Service manual says that the voltage in normal operation should be between 1,5 and 2V which is the case.
- For all the above test the scan tool is always giving a lower temp than expected.
Does anyone checked the temperature using the scan tool on a V6 ? I'm using the aldl Bluetooth scan from 1320 electronics and the A020 file.
When I plot the curve from the file and compare to the theoretical curve there is clearly a mismatch.
Last edited by flechebleue; 06-09-2019 at 11:02 AM.
If ECM is reporting too low, and you are convinced sensor is reporting correct, recommend jumper connector wire between sensor and ECM to eliminate possibly that resistance in circuit between sensor and ECM is reason why scanner is reporting low numbers.
While you’re at it recommend using IR thermometer on outlet hose to upper radiator to compare numbers between IR thermometer and dash gauge. I get wide variation on mine. Typically dash gauge reading a lot higher than IR thermometer.
Hi have disconnected the two large connectectors on the ECM and look for bad contacts. Everything is fine, used some contact cleaner in case. Still the same issue.
In any case if there was a parasitic resistance somewhere in the circuit it would change the behavior of the measurement and the voltage will never go down to zero. If a replace the sensor by a simple wire I can read 400°F on the scan tool which means that the voltage is going down to 0 then I assume that there is no resistance on the circuit.
I already checked the temperature of the coolant with a thermometer and the gauge is pretty accurate (it dépends how you interpolate the scale!)
Last edited by flechebleue; 06-10-2019 at 01:36 AM.
It seems you have a concern that the ECU is losing the message in the translation from the coolant temp sensor, and is causing your fan to turn on late.
As I see it, you have three choices.
1. Live with it as it is - albeit frustrating to you in an intellectual sense - the car may be less concerned.
2. Try a different ECU, and see if the different ECU handles the report from the coolant temp sensor differently.
3. Re-write the PROM chip to tell the fan to kick on at a lower temperature.
Hi WEG
you're right my concern is that the ECU is not using the right temp. Not only for the fan but also for the injection parameters (opening time, timing…).
I will try probably try a remanufactured ECU to see if it solves the inconsistency between the scan and what sees the ECU.
By the way what material do you need to burn the PROM?
I see in your signature that you also have a V6. When you use the scan tool your reported temperature is fine?
As far as a prom goes you need a pocket programmer or equivalent.
Your ecm uses a 2732 prom. You can solder in a adapter from moates.net and use a eeprom and a burn 2 programmer. For software you can use tuner cat or tuner pro $3a mask.
As far as a prom goes you need a pocket programmer or equivalent.
Your ecm uses a 2732 prom. You can solder in a adapter from moates.net and use a eeprom and a burn 2 programmer. For software you can use tuner cat or tuner pro $3a mask.
I don't remember the temperature being an issue for my car.
I can tell you that the instrument cluster that came with the car (the car I traded a $300 rifle for) was shaky. The temp gauge rarely moved, and when it did, it was all over the place.
I replaced the V6 cluster with a different cluster from a V8 car. Now the speedometer and the tachometer read a bit low. The oil pressure gauge reads pretty normal, but at the top of the range because the correct V8 oil pressure is (oddly) lower than the correct V6 pressure. The temp gauge reads pretty much in the middle when I've got it floored going down the highway. I drive with if floored most of the time because its a V6. When I'm stopped in traffic the needle goes high. The fan kicks on when its in the red. I've IR-gunned the top radiator hose when its in the red, and the IR reading is about 30 degrees lower than the dash-gauge reading.
So long as your water pump is operating properly, and your thermostat is opening properly, I wouldn't worry about whether the dash gauge is reading higher than you'd like. The factory motors in these cars aren't going to blow up or burn up if the needle goes to the red when there is no significant load on the motor. We are talking low compression - and let's face it - LOW PERFORMANCE - motors.
Obviously, if the motor is blowing steam, you have a real problem. The temp gauge showing high at a traffic light wouldn't worry me so long as the fan is coming on.
Tuned Performance is your man if you have questions about re-programming your PROM.
I have the Moates cables and TunerPro RT (free download) loaded on an ancient laptop running Windows XP.
I got into the TunerPro thing mostly because I wanted to see what the ECM was seeing. I had a hellacious "stuttering" problem. Basically for no apparent reason the motor would cut-off, and then it would instantly re-fire (a-la-drift-start). This ON/OFF "stutter" thing would happen in machine-gun fashion, and quite frankly made for a brutal drive. Once I figured out how to get all the configuration files loaded on TunerPro, and then connect to the ALDL port and the laptop, and drive a stickshift while the laptop bounced in the passenger seat, I realized something was really wrong when the screen showed me this:
Well, obviously my V6 car wasn't going any 254 mph.
Something was reporting to the ECM that it was going 254 mph though. My conclusion is that it was the little yellow box on the back of the V6 gauge cluster (Vehicle Speed Sensor - aka "VSS"). Jason Fritz was offering a Z28 gauge cluster on Facebook for $65, so I jumped on it. I really only got the V8 gauge cluster because I thought the 145 mph speedo was cool, and I figured for $65 it was worth a shot that it would fix a gauge or two - which it did more or less. Now the tachometer and the temp gauge both at least move. But the real bonus was that when I put the V8 cluster in the car, the stuttering stopped completely. It hasn't done it once since I swapped the gauges.
And, for the money-shot, with the new gauges, I'm not getting crazy mph reported to the ECM. With the Z28 gauges in the car now, TunerPro always reports mph that is completely reasonable (very close to my actual speed). All I can figure is that the replacement VSS is keeping the ECM from going absolute bat-****-crazy on fuel-trims now. I can actually stand to drive the car now.
So yeah, get TunerPro, and a laptop, and a hundred dollars or so worth of Moates cables, and finally see what your ECM sees. I'll warn you now, unless you are already familiar with old-school tuning software, you might be pulling your hair out the first week as you try to get the right BIN files and ADX files. Be patient. Read the GM how-to page on the Moates site. Its not pure hand-holding material, but it will definitely get you started in the right direction. Ask questions here if you run into a wall. You won't be the first. I'm not on here every day, and I'm definitely no expert. But, at least I've been to war, and lived to tell my story. There are a few guys here who are supreme experts at this business of TunerPro and knowing all the oddities of the various ECM's in these cars. You seem to write well (coherently), so you'll probably get along fine in this forum. Not everybody is cut out to do the reading and thinking required to have a useful understanding of this old-school EFI tuning.
Without even spending any money, you can do a lot of useful reading about using the Moates equipment at http://support.moates.net/obd1-gm-getting-started/
That page is basically the Bible School 101 for OBD1 GM systems that teaches you about prayer, even if it doesn't do a perfect job of expaining what you should say in your prayers, and what you should expect. The Moates guys are great too if you need help. I couple times I really thought my head was going to explode, because this stuff doesn't "come natural" to me. Fortunately, I do know how to speak English, and I know how to ask questions in a sufficiently detailed and specific manner, that I was able to get up to speed to the point where things were really working for me. I even got brave, and soldered on the motherboard of the ECM to fit it for PROM chips that are easier to manage than the original factory-type. I didn't really need to go that far, but I got excited once I figured enough of it out.
I'll shut up now and let you chew on it for a while. Thanks for checking in with us.
Hi WEG
thanks for your long and detailed answer I will definitely go through some of your links and especially the one from Moates which seems to be clearly the bible on everything concerning OBD.
I agree that there is no risk for the motor with the current behavior. The car is running fine and I have no overheating. However from an intellectual point of view I will be happy to understand why there is a discrepancy between what the sensors says and what the ECU is telling.
I will try another ECU to see if it changes something.
But also in the meantime i will try to see if there is something wrong with the scan tool. I am using the ALDL bluetooth converter from 1320 electronics which seems to be very close to the moates cable except that the final link is through BT and not USB. For the analysis I am using either the original software from 1320 electronics or ALDL droid. I already exchanged with the guy who wrote the andoid software and is using the same ADS file than others (so in principle the issue is not coming from threre). I will ask also to the guy who makes the aldl device if he has any idea about the problem.
In any case many thanks for your help. I will keep you posted if I find the solution
If somebody has any idea on how the voltage read by the ECU is translated in counts (0-255) I would be happy. I assumed a linear conversion (0V=255. V=0) but not sure this is right.
Hi all
I've finally bought a remanufactured ECM to make some test and try to solve the wrong temp reading. Unfortunately the new ECM gives exactly the same values !
I don't have any issue on the car except that :
- the fan starts quite late (around 235°F) where it should start around 223
- sometime i have the SES light flashing (most of time at startup when hot conditions). I just restart the car and it goes off
- when the car is hot sometimes the RPM stall below 500 but most of the time the IAC correct it.
I made a 20min drive with the scan tool connected to see if there is any issue. For me all the parameters seems normal. It could be helpfull if somebody could have a look on it to confirm
Thanks
Hi six shooter,
I'm using the aldl Bluetooth scan from 1320 electronics and the A020 file. Whatetever the software on the phone (aldl droid or aldl scan tool) I read the same value.
Knowing that app is important. Possibly ALDLDroid?
If it is ALDLDroid, it's EXTREMELY possible that the formula entered for the CTS is wrong, or the ADX you are using is not correct for your application, and is actually displaying different (non CTS) information.
Hi Six Shooter
this was my assumption (see earlier in the post) but there is a large doubt. Strange thing is that the app is using the A020 adx file which is in principle commonly used.
The app in itself is not the issue as I've also used the one delivered with the electronic and I have exactly the same result.
06-06-2019, 02:05 PM
I will definitely go through some of your links and especially the one from Moates which seems to be clearly the bible on everything concerning OBD.
I will be happy to understand why there is a discrepancy between what the sensors says and what the ECU is telling. 
