Idiot Lights to Gauges

[Bmstd]

1985 Chevrolet Fleetbed SB project.

I am converting an old C10 Custom Deluxe 4.3 V6 to a 350 V8 and want to swap from the original idiot lights to a stock gauge (no tachometer).

I have swapped the correct sending unit for oil pressure and water temperature into the truck. When i tried to plug in my old sacraficial gauge set nothing worked as I hoped it would. the temp and oil both pegged out and the blinkers are definately not right.

Can anyone lend some insight to sorting out the wiring for this swap?

Thank you,

[ray_mcavoy]

See post #3 by hatzie in this old thread --> http://67-72chevytrucks.com/vboard/s...d.php?t=491032 That should help you figure out the wiring differences between the warning light vs. gauge clusters.

[Bmstd]

Thank you for the link. This will get me where I need to go!

[motornut]

I went from 78 idiots to gauges with tach,you will have to repin the plug
even the location of the light to gauges changed
yes the blinkers wires need to move, as do the neg/pos for the cluster
this is K78 idiots and K79 gauges only.

[Bmstd]

OK, i have re-pinned all the wires and added the one additional positive and negative pin to the plug.

However, i am experiencing the oil pressure gauge is wanting to peg over past 60psi at the 3:00 position. When i unplug the sending unit on the engine, it will at least stop trying to push the gauge the any further.

Has anyone else experienced this? I used a new Standard PS154T (single terminal) sending unit and am questioning that this is any good or if i may have further issues or possibly damaged the gauge. Just as FYI, i have another gauge set i have tried as back up to validate the set i am using.

Thanks,

[ray_mcavoy]

Originally Posted by Bmstd

However, i am experiencing the oil pressure gauge is wanting to peg over past 60psi at the 3:00 position. When i unplug the sending unit on the engine, it will at least stop trying to push the gauge the any further.

The electric oil pressure gauge uses the same type of meter movement as the fuel gauge and can be tested using the same basic procedures.

With the sending unit wire unplugged, the gauge should read maximum pressure (or a bit beyond). Temporarily grounding the sending unit wire should make the gauge read zero (or close to it).

Originally Posted by Bmstd

Has anyone else experienced this? I used a new Standard PS154T (single terminal) sending unit and am questioning that this is any good or if i may have further issues or possibly damaged the gauge.

That is the correct sending unit to be using with a 60 PSI gauge. You can make sure the sending unit is okay by testing it with an ohmmeter. With the engine off (zero pressure) you should get a reading in the zero to 27Ω range (that's with the meter connected between the sending unit's terminal and ground). And with the engine running you should get a higher resistance reading ... up to about 90Ω depending on the actual oil pressure.

Originally Posted by Bmstd

Just as FYI, i have another gauge set i have tried as back up to validate the set i am using.

If you are experiencing the same oil pressure gauge problems with both gauge clusters, that seems to indicate a wiring issue.

And speaking of that, I believe the 4.3 V6 that was originally in your truck would have been equipped with an electric choke. The factory electric choke wiring was routed through a 2-terminal oil pressure switch so the choke would only receive power with the engine running. That same switch was also used to operate the "OIL/CHOKE" light in the warning light clusters. So simply re-pinning the cluster end of that wire would still leave the other end tied into the electric choke circuit. That could possibly be back-feeding power to the sending unit circuit and causing the gauge peg to the 3 O'Clock position.

[motornut]

Originally Posted by Bmstd

OK, i have re-pinned all the wires and added the one additional positive and negative pin to the plug.

However, i am experiencing the oil pressure gauge is wanting to peg over past 60psi at the 3:00 position. When i unplug the sending unit on the engine, it will at least stop trying to push the gauge the any further.

Has anyone else experienced this? I used a new Standard PS154T (single terminal) sending unit and am questioning that this is any good or if i may have further issues or possibly damaged the gauge. Just as FYI, i have another gauge set i have tried as back up to validate the set i am using.

Thanks,

Thats what I used ,no "T" tho,no probs

[Bmstd]

Well, I tried to test the oil sending unit with my ohm meter but could not achieve readings consistant to what you have provided me as a guide mr. ray_mcavoy.

You are correct, the truck wiring originally used the dual terminal plug for the oil warning light and control for electric choke. The wiring harness also contained the extra plug in for the single terminal oil gauge sending unit which is the only thing i have connected. i am not using an electric choke at this time nor do i have the dual terminal sending unit installed. I checked both terminals of the oil light/e-choke plug and the tan wire is live when key is on. everything else is has no current. However, i noted that the large plug for oil light/e-choke also contains a blue wire running into the center of the plug. Not sure what this wire is doing since there is only two terminals on underside and i do not have an extra blue wire going to the gauge cluster plug. This may have been a wire that ran to the junction that picked up some feeds for the simple ecm that was in the truck. I did try to jump the big dual terminal plug and it definately made the gauge try to move further beyond the 3pm position.

Another descrepancy i found when re-pinning the plug for the cluster is that the terminal guide provided states that the oil light/gauge light wire is supposed to be blue, but the truck wiring for this is tan, and so is the wires that were sticking out of the plug from the donor gauge cluster.
My next move is to try to check resistance on the tan wire from gauge cluster to the plug for the sending unit.

Regardless, I have feeling i may have ruined the oil sending unit as I did pull a brain dead move and tried to plug in the test cluster to the truck wiring before re-pinning. However, i do need to confirm i have correct wire connected at position #5 that is going to the sending unit as first step.

Any additional thoughts or input you could provide would be greatly appreciated?

[chengny]

I checked both terminals of the oil light/e-choke plug and the tan wire is live when key is on. everything else is has no current. However, i noted that the large plug for oil light/e-choke also contains a blue wire running into the center of the plug. Not sure what this wire is doing since there is only two terminals on underside and i do not have an extra blue wire going to the gauge cluster plug.

If a person hasn't looked closely at the schematics, I can see why it would look like the plug goes to a 3 connector switch. There are 3 leads entering the back of that snap-on connector. A pink/white, a light blue and a dark blue.


The power supply is on the pink/wht (it maybe faded enough to look tan). The light blue (goes over to the choke heater) and the dark blue (runs back to the instrument panel to extinguish the "CHOKE"/low oil pressure warning light).

The light blue and the dark blue are spliced together inside the connector body which makes it look like a 3 stab plug. But it is just power in on the pink/wht and out on the two blues:





When the switch is closed the PNK/WHT supplies power to the LT BLU & DK BLU leads - which are connected to the choke heater and the negative side of the CHOKE light respectively.

So, when the engine is started - and oil pressure hits about 10 psi - the oil pressure switch contacts close, the choke heater begins to assist the choke opening and the CHOKE light on the dash goes out.

[ray_mcavoy]

Originally Posted by Bmstd

Well, I tried to test the oil sending unit with my ohm meter but could not achieve readings consistant to what you have provided me as a guide mr. ray_mcavoy.

Out of curiosity, what were the readings you obtained?

Originally Posted by Bmstd

Another descrepancy i found when re-pinning the plug for the cluster is that the terminal guide provided states that the oil light/gauge light wire is supposed to be blue, but the truck wiring for this is tan, and so is the wires that were sticking out of the plug from the donor gauge cluster.

GM made some changes to the wire colors sometime during the 1978 model year. That terminal guide you looked at must be using the older colors. Tan (as you found on your truck) is the correct color for the late 78 & newer oil sending unit circuit. Another color discrepancy you may have noticed is the fuel sending unit wire (that one changed from tan to pink)

Originally Posted by Bmstd

You are correct, the truck wiring originally used the dual terminal plug for the oil warning light and control for electric choke. The wiring harness also contained the extra plug in for the single terminal oil gauge sending unit which is the only thing i have connected.

I took a closer look at the GM wiring diagrams and it appears as though that single terminal plug & tan wire are present on all trucks. But with warning lights, it is simply left unused and the wire dead-ends at the firewall bulkhead connector.

On trucks with e-choke & warning lights, the tan wire in the cab harness is actually connected to the dark blue wire in the engine harness that goes to the 2-terminal e-choke/oil pressure switch.

On trucks with e-choke & factory gauges, GM placed the tan wire in the cab harness into a different cavity in the bulkhead connector. In that location, it does connect to the tan wire in the engine harness leading to the single-terminal plug for the oil sending unit. And they added a dark blue wire to the cab harness to operate the "CHOKE" light at the bottom of the speedometer in the gauge cluster.

So without also re-pinning the tan wire on the cab side of the bulkhead connector, the oil gauge will still be connected to the e-choke pressure switch plug instead of the oil sending unit plug.

[Bmstd]

ray_mcavoy... you have become my wiring mentor and hero. You were able to help me connect the dots so to speak.

After much thought and common sense applied (or as much as I have to offer) i had come to the same conclusion knowing the original oil warning light and the e-choke light was getting its feed from same connection. I was missing the diagram to help me understand that. With your help, i was able to figure out which wire in the bulkhead to re-pin,,, and now the oil gauge provides a reading.

Now for the new question,,, When i shut engine off, the oil gauge stays where it was at last. Then, when I cycle the key to on position, the gauge tries to return to zero, but once it gets to the mark by zero, it then stops for split second and bounces to the 3 o'clock position and stays until i start the truck. I have rechecked the ohm readings on the oil pressure sending unit and do get a 0Ω reading with engine not running and do get 51 Ω reading when engine is idling.

To add to this, the original trucks wiring for bulkhead to the cluster (warning lights) is lacking an additional 12v positive lead and and addtional ground lead. I ran extra lead for ground wire and used the bulk body ground terminal on drivers pillar (under dash) as ground source. for the addtional positive lead for now I jumped power from #3 location at cluster to #6.

Is this a case of damaged oil pressure gauge or possible wiring (ground) issue?

My goal is to go to local salvage yard and pull all the correct color wires with correct terminals from a donor vehicle and add to my existing dash harness. I am a bit OCD when it comes to doing conversions and want to have this correctly wired as GM would have done. I will be pulling and adding a single pink/black for 12v positive, black for ground and dark blue for e-choke light. This just sounds more simple that pulling a whole harness from an original gauge truck and swapping into this one. That and there are plenty of gutted out truck with dash wiring left.

1.) Please let me know your input and thoughts on the oil gauge still returning to the 3 o'clock position when key is on?
2.) Also would it be possible to send me a copy of wiring diagram and pin position for wiring at the bulkhead and fuse box so i can also make sure all my pin positions for both positive and negative between bulkhead, fuse box and cluster are also in correct positions? I want to leave nothing to chance.

I greatly appreciate you help to this point and really like these types of challenges. When all is complete, i intend to write this swap up for the forums so other do not need to go through this much searching again.

Thank you!!!

[ray_mcavoy]

Originally Posted by Bmstd

ray_mcavoy... you have become my wiring mentor and hero. You were able to help me connect the dots so to speak.

After much thought and common sense applied (or as much as I have to offer) i had come to the same conclusion knowing the original oil warning light and the e-choke light was getting its feed from same connection. I was missing the diagram to help me understand that. With your help, i was able to figure out which wire in the bulkhead to re-pin,,, and now the oil gauge provides a reading.

You're welcome! Glad I could help.

Originally Posted by Bmstd

Now for the new question,,, When i shut engine off, the oil gauge stays where it was at last. Then, when I cycle the key to on position, the gauge tries to return to zero, but once it gets to the mark by zero, it then stops for split second and bounces to the 3 o'clock position and stays until i start the truck. I have rechecked the ohm readings on the oil pressure sending unit and do get a 0Ω reading with engine not running and do get 51 Ω reading when engine is idling.

The part where the gauge stays at it's last reading when the engine is shut off is normal. The electric oil, temp, and fuel gauges use an air core meter movement that positions the needle using a couple of perpendicular electromagnetic coils. When the power is switched off, there is no magnetic field to position the needle so it either stays in it's last position or might move one way or the other due to gravity.

And obviously, the gauge wanting to return to zero with the key on, engine off is normal as well. But having it bounce back to the 3 O'Clock position is definitely unusual.

The resistance readings from your oil pressure sending unit appear to be normal.

Originally Posted by Bmstd

To add to this, the original trucks wiring for bulkhead to the cluster (warning lights) is lacking an additional 12v positive lead and and addtional ground lead. I ran extra lead for ground wire and used the bulk body ground terminal on drivers pillar (under dash) as ground source. for the addtional positive lead for now I jumped power from #3 location at cluster to #6.

Did you mean to say you jumped power from #6 to #4 (instead of #3). On the 76+ gauge clusters, #3 should be ground for the voltmeter and #4 should be switched ignition power for the voltmeter. As you discovered, both of those wires need to be added when converting from lights to gauges. Jumping them over to pre-existing power & ground wires on the cluster plug is perfectly acceptable. In fact, if you were to untape an original gauge type harness, you'd find factory splices in those wires that branch them off to the multiple locations on the cluster plug.

Originally Posted by Bmstd

Is this a case of damaged oil pressure gauge or possible wiring (ground) issue?

It could possibly be a problem with the gauge but I have never seen one exhibit that sort of behavior. To verify the wiring, you can remove the lens and front tin from the cluster (leaving the rest of the unit still in the dash & plugged in). With the front off, you can remove the oil pressure gauge and hook a multimeter (set to DC volts) up to the cluster terminals that provide power & ground to the oil gauge. Then monitor the voltage to see if anything strange is happening when you cycle the key and leave the key on with the engine off. Similarly, if you connect the multimeter (set to ohms this time) between the cluster terminals that provide the ground & sending unit connections to the oil gauge. If the wiring is okay, you should get readings that are very close to the ones you obtained when measuring the sending unit directly.

Originally Posted by Bmstd

My goal is to go to local salvage yard and pull all the correct color wires with correct terminals from a donor vehicle and add to my existing dash harness. I am a bit OCD when it comes to doing conversions and want to have this correctly wired as GM would have done. I will be pulling and adding a single pink/black for 12v positive, black for ground and dark blue for e-choke light. This just sounds more simple that pulling a whole harness from an original gauge truck and swapping into this one. That and there are plenty of gutted out truck with dash wiring left.

I'm the same way when it comes to wiring ... I try to keep with the "factory correct" theme of wire colors, terminals, and such whenever I make modifications or repairs. Pulling & swapping the entire dash harness really isn't all that difficult. But there were some year-to-year differences so finding an exact match to what you need might not be easy. Your plan to extract the necessary wires/terminals from a donor harness and install them into yours should allow you to make sure everything matches the specific needs for your truck.

Originally Posted by Bmstd

1.) Please let me know your input and thoughts on the oil gauge still returning to the 3 o'clock position when key is on?

As I wrote above, I'm not really sure what is causing that. Removing the gauge and taking measurements at the cluster's oil gauge terminals should help determine if it's a gauge or wiring problem though.

There are a few places in the cluster's circuit design that are subject to poor / loose connections. Namely the cluster plug terminals and printed circuit traces they make contact with. Along with the metal clips that make the connection between the printed circuit traces and the studs on the back of the gauge. And the nuts that secure the ceramic resistor to the back of the gauge. A problem at any one of those points can prevent the gauge from operating correctly so those would be worth checking.

Originally Posted by Bmstd

2.) Also would it be possible to send me a copy of wiring diagram and pin position for wiring at the bulkhead and fuse box so i can also make sure all my pin positions for both positive and negative between bulkhead, fuse box and cluster are also in correct positions? I want to leave nothing to chance.

Thanks to fellow member hatzie, you can download PDF copies of the GM wiring diagrams here --> http://67-72chevytrucks.com/vboard/s...d.php?t=545416