FJ40 gauges not working after engine swap. (1 Viewer)

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Dec 8, 2012
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Recently finished a LS 4.8L/4L60E swap in my '72 production date FJ40. Used a PSI wiring harness for the engine. I can't get the factory gauges to work correctly since. In particular, the oil pressure gauge, using the OE sender, pegs then drops to 0. The sender is tee'd with a mechanical gauge so I know I have 40+ lbs. psi. Don't expect the water gauge to read correctly because of a mismatched sender. Can't get the fuel gauge to read at all, needle doesn't budge off E. Any suggestions? Am I missing some resistance somewhere since I no longer have the voltage regulator or ignition coil resister due to the stand-alone engine wiring?
 

reddingcruiser

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Okay, I'll bite and get the conversation rolling. However, it seems we need to clarify a few items.

Right off the bat, any senders that aren't matched to the gauges are already one strike against you.

Do your OEM Toyota gauge sender wires still run through the '72 Toyota factory harness direct from the sender to the gauge, or through the PCM? I checked the wiring diagrams courtesy of @Trollhole, and the OEM Toyota oil, temp and fuel sender wires go directly to the gauges. One additional temp sender for the '72 goes through the Spark Control Computer, which I assume you abandoned.

How is the engine grounded? Frame? Tub?

If the fuel sender wire was undisturbed during the swap, that makes this issue a bit more intriguing.

Is you engine harness completely separate from the chassis electrical? On my builds, the only thing I share between the two on an LS swap is a 12 volt power source and ground. Separate fuse panels and wiring entirely.

Next!

 
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Expanding a bit on Dicks message above:
1. You can adapt your stock temp sending unit to the GM engine using Toyota #90404-16160 (if still available), or McMaster Carr #9151K96.
2. You can use any 1/8" NPT fittings to adapt your stock oil pressure sender to the GM engine. 1/8" NPT will not screw into the Toyota metric engine, but your metric sender will screw into 1/8" NPT, just use some thread sealant on Toyota sender.
 

Dynosoar

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It sounds like you grounded the oil pressure wire Yellow with black tracer. it needs to go to the center pole on the sender not the weird tab welded to the body of the sender.

Dyno
 
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Dec 8, 2012
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Recently finished a LS 4.8L/4L60E swap in my '72 production date FJ40. Used a PSI wiring harness for the engine. I can't get the factory gauges to work correctly since. In particular, the oil pressure gauge, using the OE sender, pegs then drops to 0. The sender is tee'd with a mechanical gauge so I know I have 40+ lbs. psi. Don't expect the water gauge to read correctly because of a mismatched sender. Can't get the fuel gauge to read at all, needle doesn't budge off E. Any suggestions? Am I missing some resistance somewhere since I no longer have the voltage regulator or ignition coil resister due to the stand-alone engine wiring?
Engine harness is independent of factory harness except for power to the battery through the + starter post. As I said, I don't expect the factory
Okay, I'll bite and get the conversation rolling. However, it seems we need to clarify a few items.

Right off the bat, any senders that aren't matched to the gauges are already one strike against you.

Got that. As I said, I don't expect the temp gauge to work because of mismatched sender, using a mechanical water gauge.

Do your OEM Toyota gauge sender wires still run through the '72 Toyota factory harness direct from the sender to the gauge, or through the PCM? I checked the wiring diagrams courtesy of @Trollhole, and the OEM Toyota oil, temp and fuel sender wires go directly to the gauges. One additional temp sender for the '72 goes through the Spark Control Computer, which I assume you abandoned.

Through the OE harness. Oil pressure worked on the 350 I pulled. Using the same factory sender and same wiring that worked on that using OE factory wiring.

How is the engine grounded? Frame? Tub?

Grounded to both frame and tub.

If the fuel sender wire was undisturbed during the swap, that makes this issue a bit more intriguing.

Is you engine harness completely separate from the chassis electrical? On my builds, the only thing I share between the two on an LS swap is a 12 volt power source and ground. Separate fuse panels and wiring entirely.

Engine harness is completely separate. Only connections are grounds and alternator hot feeding voltage to the battery through the + starter terminal.

Next!


water temperature to read
 
Joined
Dec 8, 2012
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Expanding a bit on Dicks message above:
1. You can adapt your stock temp sending unit to the GM engine using Toyota #90404-16160 (if still available), or McMaster Carr #9151K96.
2. You can use any 1/8" NPT fittings to adapt your stock oil pressure sender to the GM engine. 1/8" NPT will not screw into the Toyota metric engine, but your metric sender will screw into 1/8" NPT, just use some thread sealant on Toyota sender.
Thank you, I'll make note of those part numbers.
 
Joined
Dec 8, 2012
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It sounds like you grounded the oil pressure wire Yellow with black tracer. it needs to go to the center pole on the sender not the weird tab welded to the body of the sender.

Dyno
I will re-check my wiring to the oil pressure sender (again) to make sure it doesn't go to ground or pick up something hot anywhere. Thanks.
 

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