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1998 Rodeo Fuel Level Sender Problem...Maybe Gauge?

18K views 37 replies 12 participants last post by  Buchanfarmer 
#1 ·
First off, I have searched and Googled, and searched again. So my wife's 98 Rodeo's gas gauge hasn't worked in a while. First, the gauge cluster had gone on the fritz a while ago and ended up getting it replaced. The speedo and fuel gauge didn't work on that one. On the new one, the speedo worked again but the fuel gauge still didn't work. So I assumed that it was the notorious fuel sending unit problem. I held off getting it fixed and finally ordered the whole new fuel pump/ sending unit assembly. I dropped the tank hooked it all up, and the fuel gauge still does not work. (And yes it has gas in the tank) I'm not sure where to go from here. I have another cluster that I am going to try tomorrow and see if it is maybe just the gauge. If that is not it, does anyone know where the sending unit wires go to, to find out if it is a wiring issue? Thanks
 
#2 ·
The PCM provides a +5VDC reference voltage to one side of the fuel tank sending unit which is a variable resister with a resistance range of 80 to 290 Ohms, the return side of the sending unit is connected to the PCM.

The +5VDC sending unit reference voltage supplied by the PCM is connected to a pull-up resistor in the PCM, so electrically the sending unit (variable resistor) is in series with the pull-up resistor and the return in the PCM. This configuration creates a voltage divider so the voltage the PCM fuel gauge circuit senses varies with the fuel level. As the fuel level decreases the sending unit resistance increases causing more of the + 5VDC to be dropped across the resistance of the sending unit. When fuel is added, the resistance decreases and less voltage.

When this circuit is open due to broken wires or an open in the sender DTC P0463 will be set and the CEL will be ON. Since you do not have this DTC set, you probably have a problem related to the gauge cluster.
 
#3 ·
Its simpler then that. Some rodeos have to have the PCM flashed with an upgrade program in order to read the new fuel sending unit. Most often its '98s that need to have it done but not all do. If you cant find an isuzu service garage in your area, any honda dealership can do it since the passport is a rebadged rodeo and has the same issue.
 
#4 ·
So I switched out gauge clusters today. Now with the float all the way up the gauge moved up to about 1/8 of a tank and the fuel light went off, but it still isn't working properly. I made an appointment at a former Isuzu dealer here in town for tomorrow. Do you think that the PCM just needs reflashed and should they be able to do it?
 
#9 ·
Hey Buster 28 or anyone else that might be able to help.... where do i check the ohm range on the new module? i had to replace the fuel module assembly and now my fuel gauge is not working. I carried it to the local isuzu dealer to get it reflashed after reading this post but they said it was the wrong part or installed incorrectly. I thought i took my time when installing the new one and have rechecked my wires but its still not working? Any help will be appreciated.
 
#10 ·
You would measure the resistance across the two leads that go to fuel sending unit, difficult to do with the tank in place and the resistance you measure would depend on the amount of fuel in the tank so that is not much of a test. You could measure the resistance at the PCM as well, but you would need a schematic to figure out which pins.

Did the gauge work before you replaced the sending unit?
Why did you replace the original fuel sending unit?
What replacement brand did you use?
Is the CEL ON?
Does the low fuel indicator work?
Does the gauge show any reading at all?
 
#12 ·
Based on your information I think you wired the unit correctly, if the wires from the PCM to the sending unit were open the CEL would be on, a DTC set, and the low fuel indicator would not work.

I think the actual problem with the 98 model is that the algorithm in the PCM firmware that controls the fuel gauge is not compatible with the new fuel sending units and a new version of the firmware needs to be loaded into the PCM You could try a Honda dealer, a 98 Passport would have the same issue so they will have the necessary equipment.
 
#13 ·
I have called every possible Isuzu/honda dealer around the Allentown Pennsylvania area and no one has the equipment to reflash. I was referred to dealers in Harrisburg, Reading, and Scranton with no luck. Anyone have a idea of somewhere that can still do the reflash of my 98 Rodeo PCM?
 
#19 ·
I just sent mine to Par-Tech, http://isuzusource.com/v2/service_techn ... cement.php , and just got it back on Monday.

This the form I used, http://techinfo.honda.com/rjanisis/pubs ... 20Form.pdf , but I think you will be able to use the link above and pay online.

It cost $125.00, They received it on Thursday and I got it back on Monday.

It was awesome to see the gas gauge go to full!

PS. I purchased a PCM of ebay ($35.00) and installed it in my passport while I shipped my OEM to them for programming since I still wanted to use my truck while it was shipped.

Let me know if you have any questions.
 
#20 ·
I pulled the resistor out of the old assembly and swapped it into the new assembly. I wish I could have found a dealer willing to reflash so the new parts would work without modifications. They are lying about not having the equipment, something along the lines of buy a new car or live with the broken gauge, aka no help from us.
 
#21 ·
Net-Tech said:
I pulled the resistor out of the old assembly and swapped it into the new assembly. I wish I could have found a dealer willing to reflash so the new parts would work without modifications. They are lying about not having the equipment, something along the lines of buy a new car or live with the broken gauge, aka no help from us.
What resistor did you pull?
 
#22 ·
The sending unit itself is a resistor. new units have a different range from full to empty. I carefully removed/unsoldered the old one and swapped it into the new one. The old one had what looked like soot on it which I removed gently with a pencil eraser. The gauge was weird about displaying the last quarter of a tank of fuel from day one, but now works perfectly.
 
#23 ·
Net-Tech said:
The sending unit itself is a resistor. new units have a different range from full to empty. I carefully removed/unsoldered the old one and swapped it into the new one. The old one had what looked like soot on it which I removed gently with a pencil eraser. The gauge was weird about displaying the last quarter of a tank of fuel from day one, but now works perfectly.
The original sending unit resistance was 17 to 120 Ohms and the replacement is 80-290 Ohms.
 
#25 ·
The fuel level potentiometer is not directly connected to the fuel gauge, it is connected to the Powertrain Control Module (PCM). The PCM converts the fuel level potentiometer resistance value into a signal that drives the fuel gauge. The most likely failure is an open circuit in the wiring between the fuel pump assembly connector and the PCM.
Click on schematic to expand.
 

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#26 ·
Buster28 said:
The fuel level potentiometer is not directly connected to the fuel gauge, it is connected to the Powertrain Control Module (PCM). The PCM converts the fuel level potentiometer resistance value into a signal that drives the fuel gauge. The most likely failure is an open circuit in the wiring between the fuel pump assembly connector and the PCM.
Click on schematic to expand.
Ok. Where's the approximate location of this where they fail?
 
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