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fuel gauges are all over the place

Peewee

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HZJ79.... fuel gauges are all over the place. Main is full but gauge moves between full, half then back to full. Sub says just above empty but it's at least half. I have removed the sub sender and it reads full when it's out of the tank...... Are the two gauges on the same circuit? Is there a low fuel warning light on the sub tank? I have 3 wires to the sub sender but only 2 coming out..
 

nick_the_fish

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Hi PW,

First thought is a bad earth if the gauge is all over the place. Does it move about quickly when moving from full to half to full?

I have removed the sub sender and it reads full when it's out of the tank
do you mean it reads correctly when removed - i.e you move sender to full position and the gauge says full / move sender to empty position gauge reads empty?
 

PeterLC

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Hi PW, I don't know anything abouts HZJ79s but I can give you some general ideas regarding fuel gauges. I'll assume that you have two senders and one meter and one problem. That leads us to look to the meter in the first place. These meters usually incorporate a low pass filter so the needle doesn't move as the fuel moves around in the tanks as you drive. Older meters will have some electronics filter circuit, newer ones will have digital filtering. If you have a bad solder or defective component in the filter circuit, that would result in the behaviour you describe. As you have a sender out, this is easy to check. The meter should not react quickly.
 

StarCruiser

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It could also be that the resistor has worn through on the sender unit. Unlikely as you are experiencing the problem with both senders. If as has been said your gauge reads either tank, there will be a change over relay of sorts somewhere. The fault could lie there.
Iirc someone else had this problem so it may be worth doing a search on here.

Worth testing each component to rule each out. Also worth wiggling wires to see if the gauge moves, this can be a very speedy way to find loose connections.
 

Peewee

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Both gauges move slowly. When I move the sub tank sender to the full position it slowly reads full. Disconnect the wire and it reads empty. If I put a wire across the sender gauge side, (unplugged) it reads full.
 

Peewee

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I just looked and the main tank said full and the sub says half..... Great. However after I turned on the ignition the sub gauge went to near empty....!?
 
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StarCruiser

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Check your battery charging voltage and report back. Anything less than 14.4v after start could indicate a problem. You could also try unplugging the plug from the back of the alternator and see how the gauges behave then.
 

Peewee

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Check your battery charging voltage and report back. Anything less than 14.4v after start could indicate a problem. You could also try unplugging the plug from the back of the alternator and see how the gauges behave then.

Charge rate is 14.2 but I couldn't unplug the alternator without fear of breaking it.
Guess it's one of those 'once you know it's easy' things.

However think I might have fixed it.
Cleaned all the connections, including behind the dash, with 'Electronic Circuit Board Cleaner' and an ear bud.
Looked clean but the ear bud was black after.
Now it appears to be working.

Main tank gauge still drops to halfway on starting but creeps back up to full, where it should be.
I will clean the connections on the main fuel tank next time I have the interior out.

Thanks for the help.

Pete
 

nick_the_fish

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Glad you got it sorted Pete

Iirc someone else had this problem so it may be worth doing a search on here
That was probably me Rich. If you remember I took the truck apart looking for that Bloody Relay. Still didn't find it. Wasn't the same issue as mine tho. Mine was a fault (i suspect) with the meter which was an acknowledged bad design by Toyota and one which they subsequently replaced with a better meter in later vehicles. I already checked and Pete does't have the same meter.
 

StarCruiser

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I knew I remembered something similar but the details had escaped my one functioning brain cell. Glad you got yours both sorted Nick and Pete. It sounds like Pete has two fuel gauges and you have one (iirc) Nick hence a relay to change meters, but unlikely on Pete’s.

The dash area can sometimes fall foul of water damage either from a windscreen leak or condensation. This can in turn cause corrosion to terminals. Add dust and constant heat/cool plus vibration and you can easily see where cracks, bad connections or dry joints can present themselves. It’s a pretty harsh environment.

A pencil rubber is pretty good for a light clean on copper edge type contacts. One other problem that can occur is the tracks on the flexible circuit boards can break, plus capacitors tend to degrade over time particularly if any high voltage soaked have been received, such as can occur with jump starting without suitable care.
 

Peewee

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I spoke too soon..... I have 2 gauges on the left side.
Both tanks are full at the moment but the gauge for the main slowly dropped so low it looked more like a broken wire.... then I turned off the sub tank button to feed from the main tank and it creeped back to where I would expect it to be.

Sub tank still shows full but I have only done 20kms...

I will have another go at unplugging the alternator and see what happens.
 

StarCruiser

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TBH the alternator is a long shot and given your charging voltage is about right I would take that one as ok to leave it.

It’s a bit tricky knowing that the gauges move slowly but it seems like you may be on to something with what you did earlier. Moving wires around and watching the gauges is worth a shot. Post up some pics of your gauge layout and the reverse if you have them back out. I’m not familiar with your truck. The meters have previously been mounted with screws that make the connection. If this is the case, check these aren’t loose.

I’m not sure if @Firewout has any manuals for the 79 but a circuit diagram would be useful. As stated, it seems like a common connection to both is poor. This maybe around the tank wiring, a ground connection maybe, or in the dash.

Have you checked fuses? Look for any corrosion or poor connection.
Is there anything that was done or happened to the truck that coincided with this fault appearing?
 

Firewout

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I only have the circuit diagram for the hzj75 which has just one fuel gauge.
upload_2019-9-22_8-56-5.png
 

Peewee

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Given that there is an intermittent fault I'm thinking it's a poor connectiosome some where. Next time I have a free weekend I will pull out the seats and take a look at the main tank.

Back of the gauge cluster is a printed circuit on a plastic backing.

15691446728311193942346548763244.jpg
 

Peewee

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I saw a Facebook post with random warning lights on the gauge cluster. Most of the comments suggested checking the alternator and associated wiring
 

StarCruiser

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Hence my post about the alternator which we have found is putting out the correct voltage. If you don’t have the alternator light on then that also suggests it’s working fine.
 

Peewee

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Bugger.... I'm falling out of love with this car... fuel gauges both went to just about empty at the same time.
If I short out the sub tank it reads full without the engine running.
Alternator voltage with the engine at a tad above tickover after starting is 13.4v and no warning lights

Any suggestions?

1HZ79
 

Peewee

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Finally ..... the sender on the sub tank had a 'gradual dead' spot so would read different if it was on the flat, slope, going up hill etc...

Ran a wire across the main wiring sender to check the gauge.... showed full

Checked the rear sub tank sender for continuity and ohms resistance.... about the midpoint it gradually 'lost its connection' only to improve the closer it got to full or empty.

Replaced the sub tank sender and, as if by magic, it works.

$154 pick up from Toyota, same day.
 
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