HELP -1988 GT V8 Swap - Oil Pressure, Water Temperature, and Gas Gauges Not Working (Page 3/5)
olejoedad JAN 04, 11:52 AM
Tach and fuel are on connector C3 (vertical 18 pin connector on right side of dash cluster), and water temp is on connector C2 (horizontal 12 pin connector on left side of cluster).

The contact fingers on the flex circuit board sometimes (many times) come loose from the substrate and short to their neighbors.

The sensor you linked earlier is for the ECU (PCM), and typically uses a 5v signal.
I do not know for sure if the sensor has the capability of operating with 12v, and am unsure if the resistance values are the same as the stock Fiero sensor.
If it does meet the above criteria, the black wire on the plug would need to be grounded to the block to complete the resistance circuit to drive the gage.

The stock Fiero temp gage sensor is also two pin, but two separate circuits - a resistance circuit for the gage and a switch circuit for the HOT lamp. The sensor grounds through the threads, and uses a rectangular plug with part of two sides on the sensor missing. There is also a sensor with a similar plug to the sensor you referenced that is sold as a replacement for the stock Fiero gage/light sensor.
One way you can determine if the sensor you have for the gage is correct is to check for resistance between the sensor threads and one of the pins on the sensor.
V8Steve JAN 11, 02:06 PM
Good break in weather today so I worked on the tach and fuel gage.

Tach - The white tach wire had always worked so it was a simple matter to remove the MSD Plus 6 box to gain access to the tach signal. I plugged in the tach wire and it all worked so I set the RPM cutoff limit, tidied up the wire and put everything back.

Fuel Gage - Following the FSM service manual I disconnected the plug under the car and then turned the key on. The FSM says if it goes to FULL, repair or replace the sender. Well, it went right up to FULL and stayed there after I turned off the key. Next I disconnected the battery and reconnected the fuel connector under the car. Then I removed the entire cluster from the dash.

If the above test with key on goes to full, the FSM says the sender is the problem and to check the pink wire for high resistance on #7 of the C3 pinout test. It should normally be 0 ohms for empty and 90 ohms for full. I got around 15 ohms and I know the tank is pretty low. The gage cluster is on the bench and the fuel needle has gone to around 5/8. Before that, it wanted to stay on FULL. I'm not sure why it didn't just return to zero. Something doesn't add up because it says one symptom of a bad sender is high resistance but mine was in the normal range.

I'm not quite sure what to do next. I'm inclined to add a few gallons of gas to the tank to see if the pink wire resistance increases. On the other hand, if I'm looking at removing the tank, it's a lot easier when nearly empty. The sending unit, pump, and float are all new.

Below are photos of the plug under the car as well as the gage stuck on 5/8 while siting on the bench. One of the plug wires is cutoff and bypasses the plug. With my V8 swap, that wire powers the fuel pump and is controlled by the Holley multiport ECU. That cut off wire has been that way since the first V8 generation of this car in 1999.

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[This message has been edited by V8Steve (edited 01-11-2020).]

olejoedad JAN 11, 03:22 PM
If the gage has a 15Ω load, the gage should respond, provided it has power and ground. From your SM test, full gage travel indicates this.

In reviewing your thread, I didnt see where the fuel gage problem was described, other than 'not working'.
Could you provide more info?

And if it is something with the gage, Forum member JGunsett repairs and recalibrates OEM Fiero gages. You can reach him via PM.
V8Steve JAN 11, 03:50 PM
This was my first pass at the fuel gage. It always worked before and during my V8 swap right up until the ZZ4 quit. Once I got the car running last month I noticed the water temp and the fuel gage were both not working. The fuel gage was always on EMPTY and the temperature gage was always pinned to the left. For the time being, together with the warm weather, today was a good day to get under the car and check out the fuel gage issues.

Unless someone points me in a better direction, I plan to check the pink wire ohms again, add fuel, and check again.

[This message has been edited by V8Steve (edited 01-11-2020).]

olejoedad JAN 11, 05:03 PM
That is a good plan.

Check the contacts on the laminate, they are a know problem for causing crossed circuits.

I cant remember how many I have fixed.
V8Steve JAN 12, 11:50 AM
Today I checked the ohms again on the pink wire and got different readings, in fact it seemed to wander. A poor ground was responsible so I connected the meter right to the battery ground and checked again. The reading was 2 ohms and stayed that way. Next I put around 2 gallons of gas in the car with no change in the ohms, even when I rocked the car a bit. No change in the 2 ohms.

Next I checked the resistance from the pink wire on the cluster plug to the same wire under the car where the 3- connector is located. The resistance was near zero so I know I have connectivity from that plug all the way to the dash cluster. Next I checked to see if there was any resistance across the ground and pink on the other end of the plug coming from the tank sending unit. I could not get the meter to record anything across those pins on the tank side. I put a probe in the ground with the other probe grounded on the block and go connectivity. The meter was on the 200 ohm beeping scale.

So this causes me ask why is there no resistance across the sender itself? The tank is roughly 1/4- 1/2 full but no reading of any kind across the plug. It's a brand new Fiero store sender that went in the tank at least two years ago. The car sat with no power until recently.
olejoedad JAN 12, 12:00 PM
Time to drop the tank and take a peek inside.
V8Steve JAN 12, 12:44 PM
I agree. The warm weather is leaving and winter is coming back. Next, I'll go back to the water temperature gauge and go through the same process of tracing wires. Hopefully I can solve that one and then return to the gas gauge and remove the tank. I've done this several times and it sounds worse than it really is. Since the fill and vent hoses are all new, coupled with new underside tank clamps, it should go fast.

While the car is up on the air on jack stands and 8X8 pieces of timber framing, it's a good time to remove the alternator and tend to the walking serpentine. That problem has been traced to a tensioner that is plumb and true before you put the belt on. Once it winds up the clock spring inside to load the belt, the tension arm and pully go out of alignment. I bought a LSX manual tensioner to replace it.
olejoedad JAN 12, 01:15 PM
Its good to have a plan.

[This message has been edited by olejoedad (edited 01-12-2020).]

FFIEROFRED JAN 12, 02:48 PM
On my sbc swap I have all aftermarket gages. For years I was having problems with the temp gage, and getting the FiTech TB to ground. Turns out the powder coated intake was not grounding. I had SS washers under the intake bolts to protect the PC. I had to run a ground wire to the gage sender and to one to the TB stud.