I have tested everything and ohm reads right however the fuel gauge shoots way past F on run and when off it dwindles down to FULL, I have changed pumps, senders, fuel buffer ,pcms and for each instrument cluster I put on the needle flickers for a sec then shoots past F does anyone have thoughts about this?

 

What year is your vehicle?

where did you measure “ohm”? What value did you get?

So more than one gauge cluster gives the exact same result?


George

 

Sorry for the vagueness, I have a 97 Chevy Blazer 4 door LS I have checked the ohm readings at, the pump to harness, fuel buffer, PCM, and instrument cluster connector. It happens with each cluster I have put in. I have also disconnected each connection at a time and with everything disconnected besides the instrument cluster it still reads way past F

 

So are you checking the resistance of the open wires in the harness, disconnected at both ends or are you reading the value of the fuel tank sending unit resistance and/or its wiring and what reading are you getting and measured where?

George

 

While I am waiting on your answer regarding what you have checked so far, here is a review of the circuit and what can be wrong.

The fuel sending unit is in the gas tank. It is a passive (no power) variable resistor. The resistance value varies with the fuel level. Typical ranges are 0 - 90 ohms or 33 - 240 ohms. They can be positive or negative coefficient, IOW a full tank can be the top or bottom of that range. I don't know the specs on your truck but an online subscription to Mitchel1 DIY or Alldata will give you access to that information - at around $25/year. Alternately a new fuel pump assembly that is returnable will give you the information directly. You would disconnect the correct purple wire at the fuel level buffer module and measure the sender resistance to ground to see if it makes any sense, so to speak. If the value is wrong it can be the wiring, the sender or the sender ground. You would first repeat the measurement at the sending unit output then test the ground.

That value is read by the Fuel Level Buffer Module which smooths out the values from a sloshing fuel tank and presents that processed value to the IPC. I believe that the buffer is disconnected in neutral or park. If you stop quickly and then put it in park do you see the gauge bounce around with the fuel level bouncing around? I don't know if that buffer translates the sender value or just smooths it and passes it through. Again that online subscription can likely tell you this. If that module has failed it can be the module, one of its two power sources, wiring or its ground. You always test your power and ground levels before replacing something so you don't waste your time and money. On older vehicles the wiring is the problem at least half the time due to age and previous hacks.

If all that checks out then its either the other purple wire from the buffer to the IPC or the IPC. I think that you have ruled out the IPC.

You check a wire by disconnecting it at both ends and testing continuity, short to power and short to ground.

Let me know if you need help with any of this.

I'm not a trynostic parts replacement guy, I like to diagnose a problem and do a targeted repair. If your plan is to swap parts instead I would go for the buffer module with one from a junkyard. Its quick, easy access and cheap. I would rather work with you to figure out the problem if your up for it.


George

 

Yea I have replaced all parts including fuel buffer and still no change with any of the replacements, I believe it has something to do with the power to the gauge itself. It stays at full then when turned on it goes pass F then when shut off dwindles back down to F, this has happened with all module connections and buffer completely disconnected as well.

 

Well then its power, ground or wiring. Let me know if you want to walk through the process. You will need a digital multimeter and some back probes. If you still have the old fuel pump, measure the sender resistance at both ends of the travel.


George

 

I would like a walk through the process if possible, I believe its the power because the continuity between pin #29 PURPLE and #11 FUEL SEND GRD in the instrument cluster connector is ok. However im open to any ideas

 

OK lets start with the two power sources to the Fuel Level Buffer Module. With the module connected to the wiring harness as it normally is and the ignition in the run position, back probe the two wires (one at a time) on the connector end that are pink (pin A) and the other is gray (pin B). There should be four wires on that end before the no connect pins. Positive meter lead back probed into the back of the connector, black lead on battery ground, meter on 20VDC. Report your two DC voltages.

You always probe voltages with the load connected so that if the wiring is marginal you will see the marginal voltage levels that result. If you probe an open connector with no load a proper reading does not tell you much if wiring is suspected.

George

 

and on the IPC, ignition in run:

12V should be on the three pink wires on pins 15, 29 and 31 on the IPC. Measure with everything connected (back probe the wires from behind the connector).


George