Factory Guage Test

[cheezsnake]

Hi All, trying to test that the factory oil and temp center gauges are functional on my 71 before firing and breaking in the rebuilt engine. Fuel gauge is working. Center cluster lights are working. I read about 5V from the regulator to the oil & temp gauges, and 5V out at the leads to the sending units. I've read that you should be able to ground those leads and the gauge needle should jump. I ran a wire from the sending unit plug back into the car (so I can see what happens with the gauge), and then grounded the wire ... Nothing. Neither the temp or oil gauges move at all when I try this. Does this necessarily mean both those gauges are bad, or could there be something else going on? Short of running the engine, is there a better way to test these gauges. Thanks.

 

[midlife]

Remove the center 3 gauge cluster, and use a C or D cell battery and apply negative side to the sending unit post and the positive side to the CVR post. The gauge should peg. Do this briefly.

When you did the prior test, did you have the key in ACC? Do you have a good ground from the battery to the chassis? Did you remove the sending unit wire and ground that, or just the post with the wire still attached? The test is to ground the sending wire unattached to the sending unit.

 

[cheezsnake]

When I did the ground test, the key was in the on position. I removed the sending wire from the unit and inserted a grounding wire into the plug. The battery is currently grounded to the front of the block. I have seen comments about a separate ground wire from the block to the chassis. I don't have this. I don't think there was one when the motor was pulled. I've seen other comments on this, though, and I plan to add one.

Thanks for the idea with the small battery. I'll give this a try.

 

[cheezsnake]

Good call! I connected a C battery, as you suggested, and got both the oil and temp needles to move. They did not peg immediately. They actually moved very slowly, as I held the connection for a few seconds. Maybe that's the difference between the normal 5V from the regulator and only 1.5V from the C cell? Anyway, they did move. I'm hoping this means the gauges are OK?

As to why the sending wire ground test didn't work, could that be related to the missing chassis ground?

 

[midlife]

Good results! Gauges are fine. It could well be that the line that takes CVR output from the dash cluster to the center gauge console is bad; it may not make good connection at the dash cluster connector pin, etc. But...you said you get 5V at the input to these gauges, so you should be good.

Yes, you really need a good grounding strap from the passenger side rear head to the firewall (factory set-up). Some take an additional ground from the engine block to the fender apron. It could also be that you didn't perform the first set of tests just right...who knows?

Add the grounding wire and then don't worry about the gauges until you get the engine fired up and see if you truly get a signal or not.

When I do bench tests, I either use a 9V transistor battery or a 12V DC power supply, so the gauges deflect a bit more quickly. This is dangerous to the gauges, so I only briefly touch the leads.

 

[cheezsnake]

Thanks for the advice and help! I'll be adding a grounding wire next.

 

[Bill73Ragtop]

It might be worthwhile to temporarily hook up a stand-alone mechanical oil pressure gauge and temp gauge during startup and break-in on a fresh engine. Once you're satisified with the break-in, they make good diagnostic tools down the road. The factory gauges are more of an indicator that things are working than anything else. Just one of those thoughts....

 

[cheezsnake]

Thanks. Yeah, if I couldn't determine that the factory gauges were working, my plan B was to go buy a set of aftermarket gauges. Now that I know the factory gauges respond to current, and assuming my new sending units are good, I'm comfortable firing the motor with the factory gauges. Should at least be able to tell if there's a major problem that requires shutting the motor down.

 

[Don C]

I agree with Bill, gauges and tach under the hood. That's where you'll want to be on start up.

 

[cheezsnake]

Great news! Engine break-in went well today, and all the gauges worked! The tach even worked (thanks Midlife!). A minor problem with a leaking valve cover gasket, and took awhile to get the timing/idle correct. Overall, a very successful day!

 

[will e]

That's great news. Engine break in is always a tense time for me.