dodored

I wanted to reach out to the forum as I am stumped to find a solution to my fuel gauge wiring issues. I have a 1987 22RE pickup that is unmodified from stock. I have researched the threads looking for an answer and have been unsuccessful in isolating what I need to do. The issue is that gas gauge registers 1/8 tank and that is all it will do. This is what I have done to figure out the issue:

Put new factory sending unit in tank which was ordered from a Toyota dealer based on the VIN
Installed new gas tank (unrelated but just in case there are rust issues)
Tried three different gauge clusters from same year truck with all the same results
Tested the fuel gauge itself on all three cluster and all showed correct ohm resistance
Tested grounds on tank for continuity - all good
Checked engine fuse at fuse panel to see if voltage was present - it is
Changed 15 amp engine fuse on panel just to be sure
Checked voltage at the fuel sending unit connector with a jumper cable connected to the negative of the battery and my meter connected to the hot wire of the harness at the tank and that I where I got confused.

The strange part was when I check voltage to the sending unit at the fuel tank connector, it showed voltage jumping up and down. It would go from .04 volts to 6 volts up and down very quickly. I tried to isolate where these voltage spikes were originating from, but I could not find any such jumps at the fuse panel, gauge cluster, or at the connectors that go into the gauge cluster. I inspected the wiring harness leading from the fuel tank to the inside of the cab and could find no damage that could be causing a short circuit. I switched the wires with a jumper to the sending unit harness plug and this wire and voltage and illuminated the "low fuel" indicator on the cluster, which would have been correct as the fuel tank was empty. The sending unit I ordered do not have the third wire that would match up to my truck's harness for low fuel indicator light.

Based on RAD4runner's suggestion on a PM, I followed the factory service manual and hooked a test light to the plug on the harness at the fuel tank and the other side to a ground. The light pulses with the ignition on.
Do you have any suggestions as to what I need to check next to isolate this problem?

Thank a million for your input.lem?

dodored

Jimkola

Well, I went through both my FSM and EWD manual. As you saw from using the manual, they had specific values at each terminal.
The FSM did mention to replace gauge if out of parameters, which you've done(and tested to be within spec)
The next culprit was the sender, which you replaced.
The FSM states if both the sender and gauge have been replaced then start looking for a wiring issue.

without knowing the history of your issue I'd recommend starting at the driver's kick panel fuse block and slowly work my way back. I say slowly because the biggest crimes against electrical wiring seems to be usually under the dash.

Co_94_PU

iTrader: (-1)

Unplug O1, it's under the seat or near the kick panel. Check the voltage on pin 4 or 10. If it's not bouncing/blinking here you've isolated a short in the rear harness. If it is still bouncing/blinking you're going to work back towards the cluster.

Connector #59 should be one of the first two branches off the cowl/dash harness. Up high and rearwards. Should be a single wire the same color as the one you checked at O1. It's connected to a condenser (capacitor) to smooth the fuel guage. Unplug this and test again. If it's not bouncing/blinking you've isolated the problem, check the condenser is firmly mounted to a clean ground. Plug it back in and try again, if it's still bouncing/blinking run a wire to a good ground and try one more time before replacing the condenser.

If it still bounces/blinks move closer to the cluster. Unpin 5/6 from plug A. Verify you have the correct wire by testing continuity between unpinned wire and fuel level sender plug at the tank. Probe the plug A tab, this is the output from the fuel guage. If it's still bouncing/blinking call a preacher because it's possessed!

2ToyGuy

Yes, as mentioned start measuring the voltage at the driver's kick panel. Check both sides of the "engine" fuse. Is the voltage pulsing there?
Find out where, in the circuit, the pulsing ISN'T, and work your way to where it IS pulsing. Whatever component is between IS and ISN'T, that's your problem. Key, fuses, fuse block connectors, kick panel ground, etc etc.
Something is causing that peculiar pulsing. Find the something, you've found your trouble.


I wish you all the best. Problems like this can drive you buggy...
Pat☺

dodored

I have checked the Engine Fuse and there was no pulsing. The connector you are talking about is on the passenger side? When I put the floor covering in I cleaned it and made sure the contacts looked fresh, but I did not look at voltage there. I will pull up the covering tomorrow and have a look.

dodored

I think this is the connector you are talking about. True?

Co_94_PU

iTrader: (-1)

Yeah that is O1 on the pickup (it's routed a little different on the 4runner).

..
This is a basic RC circuit between the guage and ground made up of the condenser (capacitor) and fuel level sender (resistor), that keeps the guage from bouncing around due to voltage changes.

13Swords

I might be off-base here but I've had a similar symptom in the past. It might be the "fuel condensor" marked "E" in the diagram. This is a capacitor that "smooths out" the voltage coming from the sender, to provide a more steady voltage to the gauge. If that capacitor is out of spec it might be providing the "swinging voltages" that you are reading. I couldn't find an OEM one for my '78, so ended up using a ceiling fan motor start capacitor, of the same original spec, works fine.

dodored

Where is the Fuel Condensor located? Is it in the gauge cluster?

13Swords

I don't know about the '87, but on my '78 it is on the gas tank. It has a metal tab on it that grounds it to the tank, thru one of the screws for the fuel pump flange.

Jimkola

I have the Toyota FSM and the 1987 Toyota Electrical Wiring Diagrams Book. The schematic for the fuel sender and gauge is identical to the one shown above, except for a condenser. Maybe it’s integrated into another component, but I think all the likely suspects have been replaced.


From 1987 EWD manual

13Swords

Here is a pic of my original part from my "78.

dodored

Is there a Toyota part number on this condenser?

dodored

The other thing that I would mention is that when the engine was installed by the shop that built it they indicated that the fuel gauge was actually working. The original fuel tank was rusty so I had them install a new replacement tank and that is when they tested the new sender I had installed.

dodored

As a follow up, I took apart the gauge pod for the unteenth time. Here is what I have determined:

The voltage that feeds the gauge cluster from the "Engine" fuse on the driver's side kick panel shows 12 volts and does not fluctuate.

The harness plug in that feeds the fuel gauge shows 12 volts in, but shows the voltage fluctuation at the yellow wire with the red stripe that goes to the sender

Of the four studs that bolt in the fuel gauge to the gauge cluster, the bottom stud on the gauge cluster shows the fluctuation.

From this I presume that the harness that goes from the gauge pod to the sender is intact.

The fuel tank show .2 ohms of resistance to the battery ground, so the tank is adequately grounded.

I had three complete gauge pods and substituting them out had no improvement on the fuel gauge accuracy either as a unit or disassembling them and swapping them around.

Grounding out the yellow wire with the red stripe to battery ground, at the harness plug in on the gauge pod, will not peg the fuel gauge to full at the cluster.
Taking apart one of the three clusters, I could find no evidence of a voltage capacitor other than an odd soldered connection on the speedometer that had some sort of glass tube. With the key on and the harness plugged into the cluster this showed zero voltage.

Filling the fuel tank had no effect on the fuel gauge.

I am fast running out of ideas, so any suggestions you can throw my way would be appreciated.

Co_94_PU

iTrader: (-1)

Don't short guage wires to ground it is a known cause of damage in multiple systems.. You want to use your test light, the book specifes a 3.4w bulb. Also keep in mind some of these gauges are fluid dampened and will take up to "90 seconds" the books say to respond. (I expect the fluid dampened guagea are not using the RC timed circuit and won't have a condenser)

I digress.

The book shows the #59 connector and condenser located above the ECU, as I mentioned before it's one of the first two branches as it starts to descend.

You didn't isolate the guage from the sender side of the circuit. This is important to isolate as it proves the problem isn't in the cluster and is in fact somewhere between. Hence me asking you to unpin the FU yellow-red wire from the connector and checking voltage on the cluster side.

Jimkola

I dunno if this will help. This is from the book that Toyota gave to techs that attended their Body Electrical technical training course back in the late 80's-early90's. I just used my phone to take images, if you need better resolution I can scan and repost. I was just being lazy.

First, as I mentioned before in other threads the FSM is a little hazy when it comes to diagnosing electrical issues, especially anything to do with gauges. Now, the tendency bitd was to have tech condemn a perfectly good gauge based on the readings(especially anything based on ohms readings). But it wasn't too often it was the other way around; giving the thumbs up on a bad gauge.

The last picture is from the 1987 FSM. I find the wording "vibrates" interesting. They're assuming you're using a analog meter to test, not a digital. I wonder what the needle would be doing, as opposed to the digital readout jumping up and down? When I was diagnosing my oil pressure gauge per FSM I had a issue with using a digital meter when testing the sending unit. Just a thought.

dodored

Jimkola:
Maybe the idea of "vibrates" lines up with what I am seeing with the voltage fluctuations. It did look like it was centering around 4 volts now that I think about it. Thanks for the pics. I would love to have a copy of that manual if you might know where it can be found.

Co 94 Pickup:
What you are saying is that there is a capacitor on my truck above the computer on the passenger side kick panel? This is for the 1987 model?
In grounding out the gauge, what I did was to just touch the ground to the harness wire to see if the gauge would respond in any way rather than just hooking it up to see if it will peg. Maybe the 90 second rule was in effect, and that is why I saw no movement.

Jimkola

The manual could be a tough find, but every dealer tech attending the factory class was allowed to keep the book, so you might see it show up on eBay from time to time.
The Publication #STI018E (or in lower case sti018e)

I've thought about getting a analog meter just so my readings are more in line with the FSM