New here: Temp gauge failure

Hi all, I am new here but not to the P38. I have had four of them.

Regardless, I have been looking all over the internet the last couple of days with no luck, have searched here as well as posted on RR.net.

I have an 02 P38 4.6 in the U.S. It has 66k miles on it.

Last week was driving along and it indicated overheating, which it has never done before. I immediately pulled over and popped the hood, it did not seem hot at all. When I got back in the car and gave it gas it went back down to the middle of the gauge.

I went to my local independent LR garage and they recommended I change the coolant temp sensor, which I had them do. After picking it up, I noticed that the gauge now does not move off of the blue line. It does move up to the blue mark, but not to the middle. When I put the ignition key in it also lights up the red overheat lamp in the gauge, which it is supposed to do.

Figuring maybe a faulty coolant sensor, they replaced with another brand new one. Still have the same result.

Was hoping it was perhaps a faulty gauage itself, but looking over various threads that seems unlikely. It does seem to point to the BECM?

Just angry because this is the SECOND becm I have had to change.

How should I proceed? I am mechanically proficient, but I can't use an electrical tester-volt meter- to save my life. The shop can't get back in for another 5 days or so.

Also, had a good look at this thread http://www.landyzone.co.uk/lz/f10/no-temp-gauge-198158-11.html
And he seemed to have the exact problems I am having, unfortunately that is on a diesel and I have a gas "petrol" engine

Unlikely to be the BECM IMO as the Gauge originally registered an overheat proving the BECM transmits the signal from the gauge. There are several different sensors according to engine type and use, it's likely the garage have fitted the wrong type. On the diesel for example the temperature sensor for engine management is different to the one for the gauge, I presume the same would apply to the V8.
It's very easy to prove the gauge and the sensor with diagnostics, buy yourself a nanocom.

I had trouble with the Temperature gauge indicating overheating.
Turned out to be a bad earth.
Check whether the gauge moves when you turn headlights, heater & heated screen on at the same time.

Hi, as Datatek says there are a choice of sender units available depending on year etc.
It was my thread you referred to ,and i found i could still drive the gauge from the nanocom for testing purposes,which was utimately how i found out-with the help of the guys on here- that i had the wrong sender fitted.

Nanocom is the answer and will help immensely with the diagnosis.

Good luck with it
Ed

Thanks for the replies...I'll have to wait until Monday when it can go back to the shop and I'll have them play around with the system they have, I think it's autologic.

One other thing I forgot to mention...when you plug diagnostics into the OBD port, you can get an accurate reading. I was showing anywhere between 93 and 97 degrees C with a handheld device, which indicates engine management is getting some sort of reading...why it isn't showing up on the dash is the question.

stevejg said:

Thanks for the replies...I'll have to wait until Monday when it can go back to the shop and I'll have them play around with the system they have, I think it's autologic.

One other thing I forgot to mention...when you plug diagnostics into the OBD port, you can get an accurate reading. I was showing anywhere between 93 and 97 degrees C with a handheld device, which indicates engine management is getting some sort of reading...why it isn't showing up on the dash is the question.

Click to expand...

There are two sensors...one drives the Gauge and one is for the Engine Management System....

Saint.V8 said:

There are two sensors...one drives the Gauge and one is for the Engine Management System....

Click to expand...

I'm not sure if that's right, I have asked multiple people that and apparently that is only on earlier models. I have a 2002 Bosch 4.6 v8

stevejg said:

I'm not sure if that's right, I have asked multiple people that and apparently that is only on earlier models. I have a 2002 Bosch 4.6 v8

Click to expand...

Nope, you're right - only one on '99 onwards...hmmm

If the Diagnostics is showing correct temp reading to the Engine ECU, it can only be either a duff gauge on the Instrument Pack or the BeCM isn't driving the gauge correctly...

I would drive for a duff Gauge rather than duff BeCM...

Saint.V8 said:

Nope, you're right - only one on '99 onwards...hmmm

If the Diagnostics is showing correct temp reading to the Engine ECU, it can only be either a duff gauge on the Instrument Pack or the BeCM isn't driving the gauge correctly...

I would drive for a duff Gauge rather than duff BeCM...

Click to expand...

before I get access to another instrument pack to test the gauge, are there any grounds I should be cleaning in particular?


Also, how common is it for a gauge to fail?

stevejg said:

before I get access to another instrument pack to test the gauge, are there any grounds I should be cleaning in particular?


Also, how common is it for a gauge to fail?

Click to expand...

I have never seen a duff gauge reported, more likely to be wiring, connectors etc.

Are there any grounds I should be cleaning etc. while I wait to get it into the shop on Monday?

stevejg said:

Are there any grounds I should be cleaning etc. while I wait to get it into the shop on Monday?

Click to expand...

Check RAVE for all the connections and earth points.

Well, spent a little bit of time cleaning grounds...and not one bit of difference

With my luck it will be the BECM. It's aggravating...if it had been the fuel gauge it would have been less annoying, but workable.

I can't even drive the thing without knowing what temperature it is

If the rest of the dash works fine - unlikely to be a Duff Earth or a Dodgy BeCM....

I am betting on a duff gauge....

See if RAVE gives you the resistance values of the Sender, and then emulate that with a resistor or even a variable resistor and see if you can get the gauge to follow the resistance value changes.....

Not sure if it would be possible to open the instrument panel up and physicaly move the gauge needle to see if it is stuck or if manual movement will free it up!

Saint.V8 said:

If the rest of the dash works fine - unlikely to be a Duff Earth or a Dodgy BeCM....

I am betting on a duff gauge....

See if RAVE gives you the resistance values of the Sender, and then emulate that with a resistor or even a variable resistor and see if you can get the gauge to follow the resistance value changes.....

Not sure if it would be possible to open the instrument panel up and physicaly move the gauge needle to see if it is stuck or if manual movement will free it up!

Click to expand...

It is possible to open the cluster, however I did not want to even touch the gauge because I fear ruining my cluster...seeing as I have low mileage I don't want to ever lose that should I decide to sell in the next couple of years.

I'll check Rave, and I will have the shop swap in a new cluster Monday either way...just hoping that swapping in a new cluster won't give me an odometer error when I put the original back in. Apparently it IS possible to swap the temp gauge from one cluster to another, so that might be the solution if the new cluster works.

Caution....the Mileage is recorded in both the BeCM and the Cluster...

Swapping over will force either one to adopt the highest value....

So if the cluster is of a lower mileage, the BeCM will force the cluster to adopt the BeCM's higher reading...likewise if the Cluster is higher, it will force the BeCM to adopt that higher mileage...which means on refitment of your original cluster the BeCM will force the Cluster to adopt the new higher mileage!

there is a magic trick

Saint.V8 said:

Caution....the Mileage is recorded in both the BeCM and the Cluster...

Swapping over will force either one to adopt the highest value....

So if the cluster is of a lower mileage, the BeCM will force the cluster to adopt the BeCM's higher reading...likewise if the Cluster is higher, it will force the BeCM to adopt that higher mileage...which means on refitment of your original cluster the BeCM will force the Cluster to adopt the new higher mileage!

Click to expand...

Well heres the weird thing...I had to have my original BeCM swapped to a junkyard unit which surely had higher mileage. Upon installation, the cluster still reads the correct mileage, however it says "odometer error" every time I start the car.

Are you saying that even though the BeCM has been swapped to a unit with higher mileage, swapping the cluster to a higher mileage unit to check and then swapping back in the original will force the BeCM to adopt the mileage of the second unit?

Just odd considering it doesn't do that now...

Steve ,don't know if you have tried this yet,
unplug the temp gauge sender plug and jumper the plug connectors on the loom end.
Switch on ignition and the red overheat light will illuminate and the message centre will say something like "temp gauge fault" or something very similar.
that lets you know the wiring from the sender plug in and out of the BECM is good.
If you can someone with a Nanocom ,you can drive the gauge from it, and that will point you back to the sender unit.
Do you know the year of your replacement BECM, as early cars had a different sender i think,and the BECM might use different values as the senders are different.
My sender was indentical to look at ,but as my car had an engine change before i bought it ,the Omega engines sender put out values that the BECM could not translate.
HTH
Ed

rodntod said:

Steve ,don't know if you have tried this yet,
unplug the temp gauge sender plug and jumper the plug connectors on the loom end.
Switch on ignition and the red overheat light will illuminate and the message centre will say something like "temp gauge fault" or something very similar.
that lets you know the wiring from the sender plug in and out of the BECM is good.
If you can someone with a Nanocom ,you can drive the gauge from it, and that will point you back to the sender unit.
Do you know the year of your replacement BECM, as early cars had a different sender i think,and the BECM might use different values as the senders are different.
My sender was indentical to look at ,but as my car had an engine change before i bought it ,the Omega engines sender put out values that the BECM could not translate.
HTH
Ed

Click to expand...

I have not tried that. I apologize, but let me re-phrase that...do you mean disconnect the coolant sensor plug, and jumper the wires-use a paperclip or something to cross all four of the lines?

As far as the replacement BeCM...interesting theory. However the replacement has been in the car a couple of months now and this problem only surfaced last week

AND THANK YOU TO EVERYONE HERE THIS INFO IS INVALUABLE!!