454 long block replacement and associated sensor issues

91 Jimmy

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Hey guys – Need some help. My 89 GMC K3500 Cab and Chassis 7.4l (454) spun a lobe on the camshaft. Since I really don’t know the history of the engine and I plan to keep the truck, I decided to buy a long block for it.

The truck, although an 89, is a square body since it’s a 1-ton C&C truck. Surprisingly, the truck is carbureted, which I assumed had been changed by the previous owner, but when I search the VIN it appears it came that way from the factory.

The previous owner took all the pollution stuff off of it and didn’t necessarily do a great job of it so, as all projects go, I decided to do a little more than swap all my stuff onto the new block. I bought new exhaust manifolds that do not have all the air injector pipes in them. I had the Quadrajet rebuilt as the carb was giving me issues from the beginning. I also bought a new Weiand Street Warrior intake manifold which is designed to work with the peanut port heads.

Because it’s an 89, but still a square body and not TBI, finding the correct parts is a PITA. Mostly I can get by with looking up parts for an 86 since those were still carbureted. But, that doesn’t always work….

My biggest problem at the moment is getting the correct/required sensors. I found this diagram on-line:
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If you look at Item 2 it calls for a temperature sensor with, what looks to be a “button” (not spade) connector. This is exactly what came off my old engine:
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However, all the parts stores and even Rock Auto only show a spade connection sensor. Amazon, has this one, that looks similar:
Amazon link

Although the “Amazonconfirmedfit” states that it does not fit “1986 (or 89) GMC3500”. It doesn’t tell me that the thread size is but the other dimensions (3.1) seems too big; mine measures 2” long. The thread on my old sensor is ½” pipe thread.

Should I get a sensor with a spade terminal and change the electrical connector on the harness? The connector that’s currently on the harness is for the “button” style and appears to be original so I’m confused on what to do….?

If you now look at items 3, 4, & 6 on the schematic, this is the set-up I have (#4 not shown in photo)
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The new #4 Oil Pressure Sensor/Switch(?) is larger than the one that was on it (photo to follow). The problem is the #3 Oil Pressure/Fuel Switch. As I understand it, this is used to verify oil pressure and if it’s not sufficient, it shuts fuel delivery off. This seems to be a TBI equipped engine feature since I assume it would have to be an electric fuel pump, yet my engine has a mechanical fuel pump. Does this serve a different function than I am assuming? Do I need it?

Next problem is the coolant sensor on the intake manifold. This broken part was on the stock manifold:
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91 Jimmy

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Part deux:
There was no wire going to it, but this one was near it.
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This is the sensor NAPA supplied which obviously does not fit the wire/connector that was close but not connected to the broken sensor:

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Besides that, neither the old broken sensor nor the NAPA suggested replacement fit the new intake manifold. They are both a 3/8” pipe thread, but the manifold has ½” pipe threads. Any advice on what to use here would be great.

Also, any wiring diagrams with colors could help me figure out what goes to what.

Thanks in advance for the help!
 

AuroraGirl

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Alaska 79 K-10

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The key is to match the sensors to your gauges, or warning lights. Use the senders your truck calls for. It doesn't matter what the engine is. It is the signal the senders give to the factory gauges. Sometimes you must adapt the correct senders to the donor engine.
 

Bextreme04

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I don't think that truck was originally carbureted. The sensors on that engine are for a TBI truck. The one in the intake manifold provides CTS input to the TBI computer and the one in the drivers side head goes to the gauge. A carbureted engine would not have the intake manifold coolant temp sensor. The extra oil pressure switch is also a TBI sensor. It doesn't cut the fuel pump off when it looses oil pressure, but instead keeps the fuel pump on if there IS oil pressure and the main pump relay has failed. This makes sure that the truck doesn't die while you are driving, but will instead show a relay failure once you shut the truck off.
 

AuroraGirl

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I don't think that truck was originally carbureted. The sensors on that engine are for a TBI truck. The one in the intake manifold provides CTS input to the TBI computer and the one in the drivers side head goes to the gauge. A carbureted engine would not have the intake manifold coolant temp sensor. The extra oil pressure switch is also a TBI sensor. It doesn't cut the fuel pump off when it looses oil pressure, but instead keeps the fuel pump on if there IS oil pressure and the main pump relay has failed. This makes sure that the truck doesn't die while you are driving, but will instead show a relay failure once you shut the truck off.
The AC delco catalog made me think that the other 454 option was a port fuel under a different vin. one was Vin M and the other.. H... Z? I cant remmeber. One said "EFI" the other just didnt, i assume TPI and PFI but not sure where or why
 

Bextreme04

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If it is a factory Carb truck, the oil pressure switch is likely only used for the electric choke....

The thing you pulled out of the intake manifold actually doesn't look like a sensor at all. Looks to be a TVS to me, which would be used to switch vacuum feeds to your emissions stuff based on engine temp. When its cold it keeps vacuum from running to the EGR or vacuum advance can on the distributor. When it warms up, it lets them operate as normal.
 

91 Jimmy

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Also, you should punch your VIN into this since 2 454 engines were offered that year:
That sight is awesome; thanks!

Here's the problem though:

This is the sender the catalog suggests based on my VIN. Assuming it's the same thread as what I have, should I just use this style and replace the connector to match the spade terminal?
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On the sensor up on the intake I'm in the same position as I started. It appears as though the sensor that NAPA gave me is correct, but the thread doesn't match my aftermarket intake manifold and there is no connector on the wiring harness anywhere near that location that would plug into this even if it did fit mechanically. Thoughts?
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The Oil Pressure Sensor looks like what I have:

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However, I can't find what is shown below as item 3 which my harness and current configuration clearly has. Thoughts?

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AuroraGirl

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Im not an expert on big blocks.... @Vbb199 i recall had a lot of 88/89 expertise but i could be wrong. But I would say your vin and AC delco website should match. if the part doesnt go into factory positions, there is not originality going on. if your truck cant find the intake cts pigtail, its somewhere. I dont think it could have beeen a tbi without that, since it cant calculate AFR otherwise. it may be taped into the harness somewhere. where that would be im not sure
 

Bextreme04

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Next problem is the coolant sensor on the intake manifold. This broken part was on the stock manifold:
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That is not a coolant temp sensor. That is a TVS(Thermal Vacuum Switch). If all of your emissions junk is removed, you don't need it. Just plug the hole and move on.

The spade sensor should be ok with the connector you have on it now, I believe that connector can push onto the spade or click onto the button from the side.
Part deux:
There was no wire going to it, but this one was near it.
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That connector looks like the one that should go to the oil pressure switch for the choke. Try checking for continuity between the wires and the choke wire to verify. If that is the case, you just need to get the same oil pressure switch with the oval end on it that would have come on an 89 TBI truck. If it is, then you might need this oil pressure switch. https://www.autozone.com/external-e.../duralast-oil-pressure-switch-ps215/19001_0_0
 

91 Jimmy

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That is not a coolant temp sensor. That is a TVS(Thermal Vacuum Switch). If all of your emissions junk is removed, you don't need it. Just plug the hole and move on.

The spade sensor should be ok with the connector you have on it now, I believe that connector can push onto the spade or click onto the button from the side.

That connector looks like the one that should go to the oil pressure switch for the choke. Try checking for continuity between the wires and the choke wire to verify. If that is the case, you just need to get the same oil pressure switch with the oval end on it that would have come on an 89 TBI truck. If it is, then you might need this oil pressure switch. https://www.autozone.com/external-e.../duralast-oil-pressure-switch-ps215/19001_0_0
Thanks! I am confused (sorry for my lack of knowledge here...) According to this illustration it should have been a coolant temp sensor?
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OK, I'll see if that connector will go onto a spade terminal as well for the sensor in the head.

The connector I showed here:
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is on the passenger side, on top of the intake manifold, and would presumably reach to the area where the coolant temperature sensor in the intake manifold on the schematic above shows. If the connector looks like an oil pressure switch for the choke (the choke is electric BTW) where would it thread into?
 

Bextreme04

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Thanks! I am confused (sorry for my lack of knowledge here...) According to this illustration it should have been a coolant temp sensor?
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OK, I'll see if that connector will go onto a spade terminal as well for the sensor in the head.

The connector I showed here:
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is on the passenger side, on top of the intake manifold, and would presumably reach to the area where the coolant temperature sensor in the intake manifold on the schematic above shows. If the connector looks like an oil pressure switch for the choke (the choke is electric BTW) where would it thread into?
Going back and looking at your pics, you already showed the choke oil pressure switch previously. It is the #3 switch and you show a pic of it screwed into the "T" fitting. The other side of that T has the big sending unit for the oil pressure gauge. If you already know where the connector is that goes to #3, then disregard my comment about that connector shown in your hand. It could go to other things as well.

That #1 Coolant temp sensor is for fuel injected 454's only(as far as I know). My L29 454(1997) has the same sensor that goes only to the ECU. The TBI trucks also use it for fuel enrichment based on intake temp. Your carbureted engine MIGHT have something similar, as some of the late model Quadrajets did have an electronic idle enrichment circuit, but I'm not familiar with that. Its possible there is some kind of electronic plug that is disconnected on the quadrajet... or maybe that connector went to EGR, or air pump, or something else that is no longer present. Either way... the broken item you keep showing as an intake temp sensor, is not a sensor at all.

Those broken nubs are vacuum ports and the thermal probe on the end just moves a slide up and down based on temp to allow the vacuum signal to pass through those ports or get blocked off. You can get them at your local parts stores as "Ported vacuum switch". Here's a similar two port one at oreillys: https://www.oreillyauto.com/detail/...itch/std0/pvs72?q=ported+vacuum+switch&pos=16
 

AuroraGirl

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this may help maybe?
 

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AuroraGirl

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1989 According to what i got it from. same as 88 according to it.
 

QBuff02

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You don't need #3 if your vehicle is now carbureted and running a mechanical fuel pump. That oil pressure/fuel pump switch is required with an electric fuel pump in the event of a crash and subsequent loss of oil pressure when the engine dies, it opens the circuit to the fuel pump so it shuts off and doesn't keep pumping fuel up to the engine and start/feed a potential fire. When you turn the key on with the original system, or any modern vehicle for that matter, you hear the fuel pump kick on for a few seconds and shut off, and then when you crank the engine and it fires up and the system/sensor/switch sees oil pressure it closes the circuit so the fuel pump will run while the engine is running. No oil pressure at that location= no electric fuel pump.
 

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