How to test ECU coolant sensor??

[moorcroft]

I have just fitted a new coolant ECU sensor to try to fix a small cold start problem. On the first start (from cold) my car will fire ok, but will try to tick over at 800-900RPM, fairly lumpy but will run. If a apply a little throttle to pick it up to 1100rpm for 10-15 seconds then release the throttle it will tick over ok at about 800-900rpm. I thought the sensor was knackered at not telling the ECU the car is cold. Now I want to test the wiring to ensure there is still current there. How would i do this and what sort of figure would i be looking for?? On all warm starts there is no problem and the car runs well, just the little problem on cold starts. Thanks

[GreenV8S]

Disconnect the plug from the sensor and measure the resistance across the sensor. The resistance should vary with temperature, showing roughly 5K Ohms at 0C and roughly halving for each 20C rise.

Then reconnect the loom, disconnect the ECU from loom and make the same measurement across pins 7 and 25 on the connector. This should show the same resistance as when you measured directly across the sensor. (If not, either you have got the wrong pins, or there is a connector/wiring fault, or the thing you measured previously isn't the coolant temp sender!)

[moorcroft]

Thanks GreenV8s, Is it not possible to remove the sensor's connector and test across the connectors pins to see if its receiving current?

[blitzracing]

Just check for 5 volts with the connector disconnected and the ignition on. This will show a good connection to the ECU, but not test the sensor. A quick test with a multimeter will verify the ohms value. Much easier than grovelling around in the footwell!

[blitzracing]

moorcroft said:

I have just fitted a new coolant ECU sensor to try to fix a small cold start problem. On the first start (from cold) my car will fire ok, but will try to tick over at 800-900RPM, fairly lumpy but will run. If a apply a little throttle to pick it up to 1100rpm for 10-15 seconds then release the throttle it will tick over ok at about 800-900rpm. I thought the sensor was knackered at not telling the ECU the car is cold. Now I want to test the wiring to ensure there is still current there. How would i do this and what sort of figure would i be looking for?? On all warm starts there is no problem and the car runs well, just the little problem on cold starts. Thanks

Second thoughts check the stepper is winding right back when you turn the ignition Off so the engine can get get enough air in to start the engine correctly. Disconnect the stepper when the engine is warm and ticking over and then let the engine cool. Remove the stepper but leave it connected, then get someone to cycle the ignition on then off. As the ignition goes off the stepper should wind back fully.

Mark

[Slacey]

Mine cold starts exactly as you have described. I've tried a new stepper and recently fitted a new ECU coolant sensor, with no change. If you do suss this out I'd be interested to find the cure.

[blitzracing]

The fueling can be very rich on start, and only corrects after 20-30 seconds once the lambda probes get hot, (Im assuming this is a catalyst car)and the change in engine note is quite obvious, from lumpy rich, to smooth idle. Its possible that the mixture control on the side of the AFM is used for a base point for the mixture before the lambda probes come into play. All my tests so far show this setting with the engine running have have no effect on catalyst car but Mark Adams says otherwise, so I can only assume it may be only an intial value thats "read" when the ECU is first powered up then ignored otherwise. Yet to verify this one. Anyway just to be sure it needs to be set to 1.8 volts between the red / black wire and blue/red wire on the side of the AFM (engine off, ignition on). The voltage range is around 0 -3.5 volts, and the control has 10 turns, but simply stops having any more effect at its limits, although the screw will keep turning.

Mark

Edited by blitzracing on Sunday 15th February 10:51

Edited by blitzracing on Wednesday 18th February 19:44

[moorcroft]

Thanks, how would i go about taking a reading on the AFM without disconnecting the connector? Where abouts is the screw on the AFM? Thanks

[blitzracing]

You can force a couple of small nails or the like into the back of the connector, where the wires go in as test points. From memory they need to be at least an inch long to get a good connection. Make sure the heads dont short out however. The screw is either a cross head or hex head, deeply recessed in the side of the AFM. There is the odd AFM or two out there without one, and the hole is just blanked out, but I think these are jag conversions.

Mark

Edited by blitzracing on Sunday 15th February 16:48

[moorcroft]

blitzracing said:

The fueling can be very rich on start, and only corrects after 20-30 seconds once the lambda probes get hot, (Im assuming this is a catalyst car)and the change in engine note is quite obvious, from lumpy rich, to smooth idle. Its possible that the mixture control on the side of the AFM is used for a base point for the mixture before the lambda probes come into play. All my tests so far show this setting with the engine running have have no effect on catalyst car but Mark Adams says otherwise, so I can only assume it may be only an intial value thats "read" when the ECU is first powered up then ignored otherwise. Yet to verify this one. Anyway just to be sure it needs to be set to 1.8 volts between the brown /orange wire and blue/red wire on the side of the AFM (engine off, ignition on). The voltage range is around 0 -3.5 volts, and the control has 10 turns, but simply stops having any more effect at its limits, although the screw will keep turning.

Mark

Edited by blitzracing on Sunday 15th February 10:51

I have just tested across the brown/orange wire and the blue/red wire and seem to have a voltage of 9.96V!!!! Can this be right?

Edited by moorcroft on Tuesday 17th February 17:48

[moorcroft]

Blitzracing, Right, if i test between the two wires you mentioned I get 9.96V at ignition on and 12.26 when idling. If I test between blue/red and ground (chassis) I get 1.22V ignition and about the same idling. Does this sound about right?? The 'bible' mentions figures lower then you suggested, 0.25v-0.50v ignition on and 0.5v-1.5v idle? I think these are for a 350i TVR, not sure what difference a 500 Griff may be. Any help will be appreciated thanks.

[blitzracing]

Yikes !! my Boo Boo. The wires are Brown/orange 12 volts. Red/black Ground. Blue Green-airflow out. Blue/Red CO trim.

So the trim value should be across red/black and Blue/red.

Ive corrected the previous post as well- sorry about that.

Mark

[blitzracing]

Heres a bit by Mark Adams on the voltages. Its more acurate than my offerings !


Here's a quick fault finding guide.

Note that you can drive without an airflow meter in case of emergency (i.e. airflow meter disconnected), because the system will drop into a default (limp-home) mode based on throttle opening.

Most airflow meter faults will cause the engine to run excessively rich. However if the airflow meter remains connected whilst defective then the vehicle will probably not run. In most cases the output from a defective airflow meter will be in the range 2.0-2.5 Volts, which is a viable value. This represents a moderate load and will cause heavy over-fuelling without setting a fault code.

Testing is performed in the following manner. Peel back the rubber boot on the airflow meter connector and leave it plugged in to the airflow meter. Set up the digital multimeter to read voltage. Insert the negative probe into the Red/Black wire (sensor ground), and the positive into the Blue/Green wire (Airflow signal).

Turn on the ignition, but do not start the engine. The meter should immediately indicate a reading of approximately 0.3-0.34 Volts. Most defective airflow meters will overshoot to 0.8 Volts or higher, and take at least 2 seconds to come down to the correct voltage.

Now start the engine, and the reading should rise to 1.6 Volts (3.5 Litre engine) to 1.75 Volts (5.0 Litre engine).

The next test is full load, and as with the fuel pressure test it will require use of a rolling road or a steep hill in the same manner. Under full load the voltage should rise to 4.45 Volts (3.5 Litre engine) to 4.95 Volts (5.0 Litre engine).

On this injection system, the idle CO mixture adjuster is provided on the airflow meter. It is located in a boss on the top of the airflow meter, pointing towards the engine. Leaving the multimeter negative probe in the Red/Black wire, move the positive probe to the Blue/Red wire.

Now turn on the ignition but do not start the engine. Observe the voltage. The normal adjustment range is between 0.0 and 3.6 Volts, with the higher Voltages producing higher idle CO values. There are approximately 20 turns of the adjuster screw to cover the entire range.

Annoyingly, the adjustment may be clockwise or anticlockwise to increase the value, and this varies from meter to meter! For this reason it is always preferable to have the multimeter connected in this manner when adjusting idle CO, so that you see can something is actually happening.

Typical Voltages that would be found at this point are between 0.9 to 1.4 Volts for non-catalyst cars. This Voltage is always factory pre-set to 1.8 Volts for catalyst vehicles. A value near to 3.5 Volts will generally produce an idle CO value of 9-10%. These Voltages may be used as safe initial values particularly if no CO measuring equipment is available.

[moorcroft]

Blitzracing, thanks alot for the information, thats great. I have set the co trim already to 1.80v. ill check the other voltage tommorrow.

[moorcroft]

Well i seem to have solved the hunting idle from a cold start!!! First I checked the AFM co trim figure, mine was reading 1.22v. So I adjusted this back to 1.80v. This did have a little effect on the idling when cold, but more importantly seemed to have more effect on closed loop driving. The car felt more response and lively below 3000 RPM. I then found a that the diaphragm on the vacuum advance was leaking slightly, I could draw air through the rubber pipe. So i decided to disconnect the advance and bunged the rubber pipe up to stop air being drawn in whilst idling and upsetting the stepper adjustment. Lastly cleaned the stepper again, however this was clean so don't think this made too much difference!! Now when I start it from Cold the idle is rock solid, no more hunting and no more need to assist it with a little throttle. I think the leaking vacuum advance was uspsetting it more then anything. Great!!!! Well pleased

[netdream]

Hello All
I am struggling to understand the wiring around my ecu coolant sensor. My early griff 91 4.3 precat had a fusebox meltdown and I have been rewiring the car when I get the opportunity and with covid I have more time on my hands.
I pinned between the sensor and pin 7 and found no continuity so took the dash out and found a broken connection on a block. ECU side of the block connector pin 7 is using an Orange / black trace and engine side to sensor is a black wire.
On the black wire is a broken resistor ( Green/Brown/Brown/Gold - i think 510 ohm).
I have no idea why this resistor is there. No searching on google is giving me a clue and no wiring schematic I have found shows this resistor - 510 ohm resistor does ring a bell but I think it is tune resistor)
I have owned the car since 1992 and know that the resistor is not my doing but.....
Any of you knowledgeable folk able to tell me if the resistor should be there or has TVR Arkley put this resistor in situ to circumvent some other issue ?
Thanks

Edited by netdream on Saturday 25th April 12:57

[Steve_D]

netdream said:

Hello All
I am struggling to understand the wiring around my ecu coolant sensor. My early griff 91 4.3 precat had a fusebox meltdown and I have been rewiring the car when I get the opportunity and with covid I have more time on my hands.
I pinned between the sensor and pin 7 and found no continuity so took the dash out and found a broken connection on a block. ECU side of the block connector pin 7 is using an Orange / black trace and engine side to sensor is a black wire.
On the black wire is a broken resistor ( Green/Brown/Brown/Gold - i think 510 ohm).
I have no idea why this resistor is there. No searching on google is giving me a clue and no wiring schematic I have found shows this resistor - 510 ohm resistor does ring a bell but I think it is tune resistor)
I have owned the car since 1992 and know that the resistor is not my doing but.....
Any of you knowledgeable folk able to tell me if the resistor should be there or has TVR Arkley put this resistor in situ to circumvent some other issue ?
Thanks

Edited by netdream on Saturday 25th April 12:57

Yes it is 510 black wire goes to earth at top of nearside tunnel. orange/black goes to ECU pin 34 and its function is to fool the ECU that thinks its in a Range Rover and cant find the auto gearbox gear selector.

So nothing to do with temperatures at all.
Coolant temp should be a Green/blue wire from ECU pin 7
Fuel temp should be grey/white from ECU pin 32
They are the same sensor type so measure the resistance from each of those ECU pins to earth should give the same results with the engine off.

Steve

[netdream]

Thanks very much Steve - I better check again !
I had labelled pin 7 as orange / black from the ECU - obviously incorrectly. - I was slightly confused as everywhere I read suggested pin 7 was as you suggested. - Green / blue - better solder a 510 ohm resistor back inline on the orange / black. and find the correct pin this time !

[Steve_D]

PM me if you want a circuit diagram.

Steve