Question about Gauge Swap??

[BigRed]

The '78 truck I just got has actual gauges instead of the idiot lights like my '75 has. I'd like to put them in my '75. Do y'all know of any problems that may arise with the swap? I plan on just swapping out the whole gauage housing and everything in one piece.

Thanks!

[navybear70]

should be no problem, '81 is when they made the interior changes. But I'm sure one of the resident pros will be able to confirm that...or rebuke it.

[PHOENIX]

I recommend changing the sending units while your at it. I dont know how the idiot light systems work, but you may have to do this either way. You may have of run wire from the sending unit(s) to the gauges. Im not real sure, but that is what I would expect.

[BigRed]

Eddie, you don't think the wiring harness will have that sending unit wires already there and will work? The only thing I can think may be different would be maybe the voltmeter sending unit. It may be an ammeter unit on the '75.

[PHOENIX]

well what im thinking is maybe (and im probly wrong) but on the non guage units the sending unit itself sends the signal when things are bad. But in a guage unit the sending unit sends a signal all the time. Just my guess. But I really dont know.

but either way the wires will probly not be a problem

[79Silv4x4]

I agree, I think the sending units will be different.

[BigRed]

Thanks guys. Changing the sending unit for water temp should be easy enough. I swapped out the oil sending unit about 3 months ago and they only had one listed at the parts store. I'm guessing I probably got the right one for that. They sure didn't ask me if it was gauges or lights.

Also, this may be a dumb question, but where does the voltmeter get it's reading from? Does it come straight off the alternator?

[ElGracho]

There will be some significant wiring changes that will be required to use that cluster in your truck.

Let's look at each gage:
The voltmeter connects to the output of the alternator and a ground in the cluster, not a big deal. There is no "sending unit" but you won't want to use the existing wire that has a switch that closes on low voltage.

Oil pressure: the 75 cluster with a gage would have been mechanical which is why they only listed one "sending unit" for your 75, which is really just a low pressure switch for the idiot light. For the 78 cluster, you need a sender that matches the gage so go to the parts store and get one for a 78. You can probably use the existing wire between the sender and the gage (meter really) if you trace it.

Water temp: again, the existing was a switch and you need a variable resistance sender, just like oil pressure, go and get one that matches the 78 at the parts store. Also, you can probably reuse that wire.

Get schematics for the 75 cluster and the 78 cluster, you may have to rearrange or add wires to the connector on the cluster.

[BigRed]

Ok, thanks Joe. I'm confused slightly about the oil sending unit. The one I put in for my '75 is not mechanical. It is just a screw-in sender with a male terminal connetor on top. Then, the wire runs into the cab (and I'm sure to the gauge cluser). Do I still need to change anything out for that?

Also, on the voltmeter, you're saying I need to run a new wire from the altenator output to the gauge cluster?

Thanks again!!

[ElGracho]

BigRed, that thing you just changed for you oil pressure light is not really a sending unit. It is a swich, on when pressure is low (<5psig) off when pressure is high. The gage from the 78 needs a sending unit that has variable resistance for varying oil pressures. Binary vs analog if that floats your boat (submarine in my case).

Anyhow, you may not need to run a new wire to the alternator. You are looking for a good low resistance connection to your battery voltage when the ignition is on. There are lots of places you could tap into, I would have to look at a specific diagram to see if an existing wire would work. I would most likely look at what was factory on the 78 and try to make the 75 look like that.

[Captkaos]

Read this post for full guage info. http://67-72chevytrucks.com/vboard/s...threadid=34842

If you have questions, email crossy, his addy is at the bottom of the post.

[BigRed]

Thanks guys for all then info!

[tkeduk8r]

i have some good news.. i just did a swap very similar in my 74.... i used a cluster out of a 79 with all gauges cept tach and put it in my 74 that had the ididots... actually this swap is easier (i think) than going to a original 73-76 cluster cuz all you have to do is switch pin placement on the plug that snaps into the back of the cluster... and you get a voltmeter instead of a ampmeter which i prefer too... THE key in doing it is to trace and label all the wire routing in the circuit sheet on the back of the clusters... I do have it written dwon exactly which pins i switched, but i have to dig it out.. i was gonna post it but forgot till now... ill see what i can do... it is very easy.. all you will need after removing the cluster is a straight blade jewelers screwwdriver and a 1/4" or 7mm socket/ratchet, and thats just if you want to use the 75 speedoand fuel gauge... I swapped my 74 speedo and fuel gauge to the 79 cluster... ill try to post more later... OH and you will need the ELECTRICAL sending units for the oil pressure and temp gauge, and they DO have them available at parts stores, I would go somewhere where they have access to ACDelco stuff and they can find them for you... they did for me... good luck,
--mike

[BigRed]

Thanks tkeduk8r! Very useful info. If you find what you wrote down, please post.

[Captkaos]

Scott, all the electrical only guages I swapped out I just used a Haynes manual to swap the pins. There are several differneces in pin locations based on the year model.

[BigRed]

Okay Chris, I happen to have one of those handy.