View Full Version : Fuel Gauge Problems After LPG fitted

31-05-2008, 03:06 PM
Hi all,
I have a:

Prado GXL
Late 04 Model
6 Sd Manual (The very first of the 6 spd's in 120 shape)

I have had my Prado converted to Gas since Dec 07. The whole thing works flawlessly - except the fuel gauge. As they have taken the rear tank out, the gauge just doesn't know what to do. The 90L light has never worked since and the problems with the gauge are as follows.

Installation was by The Gas Man - Sydney.

A 50 ohm 5 Watt resistor was placed across two of the wires on the loom which normally goes to the gauge in the rear tank. This resulted in the gauge measuring half full when the tank was FULL. This also means that the fuel light stay's on for ages before you actually run out and 1/4 full isn't really 1/4 but more then 1/4 etc. So it was all screwed up.

I took it back and they put a variable resistor in place. After setting it at Full, with a full tank it worked till half tank and thats where the fuel ran out - and stranded the miss's miles from the servo. As you need petrol to start the engine, before it switches to gas !!...

Took it back again and they have put a 5 ohm 5 Watt resistor in. Now the gauge shows just above half - and I know how much fuel is in it, so in reality the fuel light should be coming on and gauge reading almost on empty and I should only have about 100k to go... not just over half a tank.

So in short - The Gas Man doesn't seem to know what else to do and nothing seems to fix the issue. Does anyone here have any knowledge on the inner workings of the electronics in the gauge systems on the Prado?

I really need an accurate gauge as it is a safety concern on longer trips miles from anywhere etc. No doubt you should always carry extra fuel but a working gauge is a must...

Any electronic guru's out there willing to take a punt at this ??

The annoying thing is that there is something different about my model because The Gas Man has converted heaps of the 120's and has never had this issue, however, they did contact a few other converters as well as EMCO (Gas systems) and they have heard of rare issues where the usual resistors don't work on the gauge.



10-06-2008, 11:26 AM
Bump !!


No one here know anything about the workings of the Fuel Gauge? - No Auto electrician's in the ranks ?

10-06-2008, 07:19 PM
Please place your location in your profile per forum rules.

I wuld suggest a use of the search button to find who hass gas already and then PM them to ask their fuel gauge experience, there IS a couple of members who have it done. Stan, Harvey (cellmed) etc.

10-06-2008, 07:47 PM
Just a stab in the dark...

The rear tank is actually the second (Aux) tank, so the resistor should be of the value of the aux tank sender unit reading empty. (So the gauge switches over and only reads form the main tank and I imagine the 90L light would be on all the time)

Wild guess as I haven't looked at the wiring diagram.... Someone may shoot me down in flames.....


10-06-2008, 10:05 PM
I imagine the 90L light would be on all the time

Interestingly enough, the 90L light is not on. It seems to have gone into retirement since the conversion.

I am wondering if there is an earth that is missing somewhere? Also - I have contacted a number of Gas installers on the off chance that they might know something. The best info I have so far is that they do know of "a few Prado's" that exhibit my problem, however, no one has the fix yet. Either they have not come across them personally (just heard of a mate of a mate etc.. ) or they have the same problem I have with one of their customers. I spoke to someone who did have this same issue - but the installer said the customer didn't care about the inaccurate gauge - so his problem was solved!

I might measure the resistance across the empty gauge. Then try a resistor of that value - although im sure I will find it is either 5 ohm or 50 ohm ( The two resistors the Gas place has already tried.)

There seems to be very little information relating to Gas conversions - although strictly this is not a Gas issue but rather a complication as a result.

I think I'll need to be off to a good Auto electrician..

18-06-2008, 06:21 PM
mate i had the same problem with the gauge and no body could fix it,i disconected the battery for two hours to do a 40k service and when i reconected the battery she was all good.Best of luck. The gas bloke did this but only for about 5minutes and it didnt work.

18-06-2008, 10:17 PM
i disconected the battery for two hours ... when i reconected the battery she was all good.Best of luck. The gas bloke did this but only for about 5minutes and it didnt work.

Hi Tezablue,
I actually did try that when I first got the Gas put on. The Gas place did mention that sometimes you need to do this - it resets something in the computer... however, I must be extra special as it has no effect at all when I left the battery disconnected over night. Although the gas place has tried a couple of different things since then so perhaps I should try the disconnect again ?

On a good note - hopefully - I am taking it to the Auto Electrician's tomorrow. They have already had it for one day but failed to find a cure for the problem. They have apparently gone away and obtained a bit more information from Toyota as well as some industry user groups. They want it back tomorrow to try some of the options out. I just hope something works as if they can't get it going I guess i'll have to either live with it or try to get someone to fit a custom gauge and sender unit ?? Nearly out of options I think.

I'll let you know the outcome. Thanx for all the reply's so far. I know this isn't a popular topic (Rare issue) as most of you will never have to worry about this problem..

23-06-2008, 06:42 AM
Here's a quick update,

It looks as it this problem may have to stay unresolved. The auto electrician has had the car for 3 days and I picked it up on Friday avo. I had 1/2 tank of fuel in it and the gauge was reading 1/2. However, over the weekend the car ran out of petrol with the gauge reading 1/4. If you fill up the tank the gauge reads 3/4. I believe the electrician installed a 22 Ohm resistor somewhere in the wiring coming from the old sender - which is no longer there of course. It would seem that in my situation... the problem is that no matter what value resistor is used in the wiring off the Aux fuel tank, the only thing that is achieved is to change the position of the "empty / Full" mark. I have now had 5 ohm, 50 ohm and 22 ohm resistors in the system and all they do is change the start/finish position of the gauge. It seems the gauge will move the equivalent of 2 "quarter" marks on the dial. This range will consistently represent empty to full. i.e.

5 ohm Resistor, Gauge range = 1/2 (Empty) FULL (full)
22 ohm Resistor, Gauge range = 1/4 (Empty) 3/4 (full)
50 ohm Resistor, Gauge range = EMPTY(empty) 1/2 (full)

One thing to note is that the fuel light (using the 5 ohm) will come on with just under 1/2 physical tank of fuel left... Which is annoying because you never really know when you will run out. I guess when the gauge reads just under the E mark you would expect to run out totally. But if were to settle for this compromise I would have to get use to the fuel light been on constantly.

Anyway this is all the information I have - but I though I would share it with you all as there appears to be a number of members out there with this exact same problem. As it stands I have further issues with this as "The Gas man" who installed the system is telling me that I need to pay for half the bill of the auto electrician. He is at a point where he is saying that this is no longer his problem so it looks like I am on my own. This is very annoying as over all I have been extremely happy with the gas installation. The Gas system is awesome and has given me no issues at all. However, I personally think that if I deliver the car with a working fuel gauge - I would expect that when I get it back the gauge would still be working. After all, the gauge is part of the systems that were modified by The Gas Man during the gas installation job.

I'm going to call up the Auto electrician this morning to see if they have any further idea's. But to be honest - if they can't fix it in 3 days - Im not sure what else can be done.

23-06-2008, 07:05 AM
OK ... I know bugger all about the gauge systems ..... and I'm only guessing .... but here goes anyway .....

With the two fuel tanks in place (assuming both are full) the fuel gauge begins its journey ..... from the FULL mark to HALF ..... then the light comes on and the gauge then resets and travels from FULL to EMPTY.

SO it sounds like the sender that you have remaining in the car ... after the gas conversion ... is the one that only travels from FULL to HALF ... and that you can put a resistor in line to change the position of the two stops .... but not increase the range.

It may be really over simplifying .... but is it possible to use the sensor from the other tank (the one that has the full range of motion .... from FULL to EMPTY) even if it means disabling the green warning light ???? AT least then you could get the full range of motion from the gauge.

:?: :?: :?: :?: :?: :?: :?: :?: :?: :?: :?: :?: :?: :?: :?: :?: :?:


23-06-2008, 11:27 AM
Do you still have the tank/sender they removed?

I'm thinking strip the factory level sensor out of it, lock it in the empty position and plug it back into the loom.


23-06-2008, 05:51 PM
I had my 2004 prado put on gas and have the same problem. I still have the sender / float from the rear tank , I have plugged it back in and moved the float up and down . Makes no difference . The gauge goes up and down but no green 90 light . With the float up / (tank full ) the resistance is 50 ohms ,, half way half full 22 ohms ,,, and at the bottom empty 5 ohms . I would have thought plugging the sender back in everything should work as before the conversion but I have not seen the green 90 light since been on gas. I am beginning to think it not a gauge problem but in the computer , I , we don't know were the gas system is wired into or whats been altered. I will keep at it though like a dog with a bone.

23-06-2008, 05:58 PM
OK I've looked at the wiring diagram and can see a possible solution.

Do you have the factory manual on CD? Look at page 55 of the body wiring supplement (covers the combination meter and inputs.)

If you look at the fuel sender inputs (Bottom left of diagram) you'll see it overlays wiring for 3 and 5 door models with and without sub tanks.

Take note of the bridging wire that is in the non sub tank model.

Only three wires are in the fuel gauge circuit to the combination meter(dash). The two senders are in parallel and are shown as variable resistors so I assume the 90L and low fuel light are activated by resistance values by a circuit within the combination meter (dash).

If you don't have the CD, PM me.


26-06-2008, 09:10 PM
Hi All,
Interesting twist... I have had my Prado on gas for 2 years, fuel gauge working fine... However, in the last 3 months the 90L light has gone on holidays and now registers 1/2 tank when full.

Had it in with the gas guys today who called around and no one seems to have any idea how to rectify this issue. Resitstors were replace with no luck.

Any further info on how to rectify would be great...


26-06-2008, 09:15 PM
Hi All,
Interesting twist... I have had my Prado on gas for 2 years, fuel gauge working fine... However, in the last 3 months the 90L light has gone on holidays and now registers 1/2 tank when full.

Had it in with the gas guys today who called around and no one seems to have any idea how to rectify this issue. Resitstors were replace with no luck.

Any further info on how to rectify would be great...


Just shows there is a solution. If it worked for two years I'd say something else has let go in your case.


26-06-2008, 09:26 PM
I had never even heard of the issue, but what I am now experiencing in my 03 Prado is exactly the same as tkstrong.

Will keep you posted on how I go... I have been dealing with the guys at Steens in Perth for 2 years and they have always resolved all my issues.


27-06-2008, 04:21 PM
I have been to Steens , Clive had a play with my gauge , still reads half when full. Good place though at least he listened and had a look.

06-07-2008, 12:19 AM
An update for all.
I just got back from holidays in Darwin - Very nice up there... I should still be there but finances won't allow such luxury's.

Anyway the Prado has been back to the auto electrician's once again. This is their 3rd go at fixing it.

They have put a 47 Ohm resistor somewhere near the sender unit of the main tank, under the 2nd row seats. As this was done on friday just gone its a bit hard to know exactly how accurate things are as of yet, however, I believe that when the fuel light comes on it does mean that you need to fill up. This is in contrast to previous, where the light came on early and you had ages before needing to fill up.... However, the gauge only shows just over 1/4 and I have around 50 liters in the tank. So clearly I still have the problem of a full tank only showing as 1/2 full on the gauge.

At this stage I am not sure the people working on this problem (Auto electrician's) know what they are doing. I certainly don't know - but that goes without saying as I"m first to admit I'm a novice at electronics.

I do, however, have an extract from the manual showing the wiring of the Sender unit. I think I'm starting to understand some of the "workings" so may start some experimenting myself. (Hope I don't break it for good)..

One point to those with similar problems, My 90L light has never worked since the modification (Gas install). It appears that those who have working gauges and have Gas installed have a functioning 90L light ?

- - - Would a blown 90L bulb effect the gauge in any way - It is a bulb isn't it - not an LED. Just guessing.. Has the bulb blown ?

Also - Anyone reading who has Gas installed and a working gauge - please confirm that your 90L light is always on ?

** Also - if possible, the installer of your Gas system.. As I am at a point where I am willing to ring / talk to anyone who has successfully converted the Prado to Gas and has kept the Gauge working.

06-07-2008, 07:06 AM
Hi tkstrong
I had my gas installed Oct 07 and the petrol gauge works as it should (indicates full when full and empty when empty).The 90L light is always on.Mine was installed by The Car Clinic in Brisbane.

08-07-2008, 06:58 AM
Gas is now installed and fuel gauge is working as it should :D . The tech that did the install had already left so I couldn't get the information on the mod that was performed to get the guage working. I will ring them later today and I've a talk with him and see what he did.

09-07-2008, 07:34 AM
Ok - Just another update:

Had a chance to test out the car fuel gauge. It seems that even though the Auto electrician feels the issue was fixed, it is not.

Somehow we are back to the original configuration.
Empty = Empty
1/2 = Full.

Currenty there is a 50 Ohm resistor being used.

I also dug out the old fuel tank and took some readings. "raydes" has already done this and my findings were just the same - as one would expect.

Original Tank says..
5 Ohm = Full
50 Ohm = Empty

Where to go from here I'm not sure. I performed some further tests as I purchased a whole bunch of resistors. I tried a 60 Ohm resistor which simply reduced even further... the range of the gauge.
For example, I know I have 1/2 a tank of fuel. 60 Ohm resistor showed 1/8th of a tank... which gave me 1/4 = full and empty = empty.

IMHO - and it is just a guess.. I think "raydes" is correct in saying that the issue is possibly pointing to the computer. Is the computer registering a fault code that is causing this issue?

From what I can tell, others on this forum have had "working" gauges and after a time, for no apparent reason, the gauge has stopped working and exhibits the same problem as I am experiencing... hence (computer fault?) Also - from what I can tell, my setup is the same as others - i.e. Resistor is the only thing used to correct fuel gauge.

Well - I'll keep working on it. Not sure where to go from here. I have no way of looking for computer fault codes..
I'll call back the Gas Man and Auto Electrician today..
Wish me luck..

09-07-2008, 04:54 PM
tkstrong you are having no luck there,mine has the 60 ohm installed and works fine :shock: . Before you go down the computer track, might be worth while tracing the wires back and make sure they didn't get damaged during the install. It is a long shot but you never know sometimes faults like this turn out to be something simple in the end.

20-08-2008, 07:50 AM
no news tkstrong??

07-12-2008, 11:37 PM
I had the battery out of my car today ,for about an hour I also had the earth lead off the front inner guard . I refitted the earth lead and battery and took a few fuses out and refitted them while checking to see which are live with the ignition on . I had the car back together and started the motor ,reset the windows and put the code back in the radio and noticed the green 90 litre light was on ( it has not been on since gas was fitted in Feb ) the gauge also was reading 3/4 full ?????? Who knows, could have been just a bad earth on the guard .