New Wiring and Gauges but no Temp

[dcoland70RR]

Hello,

So over the winter I re wired my entire 1970 roadrunner with the factory matching wire harness from M&H electronics in California that Year One sells. Did so after finding out some previous owner did some hack job far under the dash and some plugs were falling apart being 48 years old. So not too tough but took some time. Any how, new gauges, new wiring, new rallye gauge circuit board, new temp sending unit and temp gauge won't work. So if I put a grounded wire in the engine plug that goes on the temp sending unit and turn ignition power on the gauge goes all the way up then back down searching for a temp so as best as I understand that means the gauge works? But still run car for awhile and gauge never moves. All the other factory rallye gauges I installed all work fine. Ideas, maybe defaulty new temp sending unit?

 

[Lionized]

I'd say sending unit if all other gauges work.
New=/=Good

 

[beanhead]

Yeah.. I agree with Lionized those sending units are a crapshoot unfortunately. If you ground the wire directly to the block, and the gauge moves, that definitely points to the sending unit..

 

[rlem1977]

Hello,

So over the winter I re wired my entire 1970 roadrunner with the factory matching wire harness from M&H electronics in California that Year One sells. Did so after finding out some previous owner did some hack job far under the dash and some plugs were falling apart being 48 years old. So not too tough but took some time. Any how, new gauges, new wiring, new rallye gauge circuit board, new temp sending unit and temp gauge won't work. So if I put a grounded wire in the engine plug that goes on the temp sending unit and turn ignition power on the gauge goes all the way up then back down searching for a temp so as best as I understand that means the gauge works? But still run car for awhile and gauge never moves. All the other factory rallye gauges I installed all work fine. Ideas, maybe defaulty new temp sending unit?

The wire from the gauge to the sender should not be grounded. The gauge itself needs to be grounded. I do not have a rallye gauge diagram to reference where the grounds are for those dash clusters.

Also, how was the M&H harness to install? I have been looking at rewiring my Super Bee and have been weighing the M&H and American Autowire Classic Update. The AA is about $200 less.

 

[dcoland70RR]

Thanks,

I bought all the wire harnesses from Yearone with a 30 percent off promo that took off 30 percent on all. It was very simple to install because all colors and everything was IDENTICAL to my factory wire harnesses. Very easy just dash took a bit. But good cuz took all dash plates off and cleaned everything out and repainted it all with factory black interior paint. Glad at that time too I pulled heater box out from under the dash and took it apart and wow 48 years of dust and dirt.

 

[beanhead]

The wire from the gauge to the sender should not be grounded. The gauge itself needs to be grounded. I do not have a rallye gauge diagram to reference where the grounds are for those dash clusters.

Also, how was the M&H harness to install? I have been looking at rewiring my Super Bee and have been weighing the M&H and American Autowire Classic Update. The AA is about $200 less.

Grounding the wire in the engine bay is merely a test to isolate the location of the problem, it does not stay grounded...

 

[rlem1977]

Thanks,

I bought all the wire harnesses from Yearone with a 30 percent off promo that took off 30 percent on all. It was very simple to install because all colors and everything was IDENTICAL to my factory wire harnesses. Very easy just dash took a bit. But good cuz took all dash plates off and cleaned everything out and repainted it all with factory black interior paint. Glad at that time too I pulled heater box out from under the dash and took it apart and wow 48 years of dust and dirt.

Good deal, thanks.

 

[idrivemopar]

Just because its new, doesn't mean it works. Easy way to test the sending unit, put it in a vise upside-down, hook up a ohm meter to the lead out on the sending unit and ground on the vise, make sure you have continuity between vise and sending unit base, use a propane torch or lighter if you have either handy, and put the flame on the area that goes into the manifold and see if the ohm meter moves. If it doesn't, then you found the problem.

 

[wagonman]

Silly question but since the gauges are regulated to 5 volts should we see that at the sender wire? Also are you sure the sender is for a gauge and not a lite? You say when the wire was grounded it swung all the way and then back dont sound right. Try and introduce some resistance when doing that like a test lite, the bulb has resistance. See what it dose.

 

[dcoland70RR]

Thanks all, So Friday I called M&H Electronics who manufactured the wiring harnesses. He told me to ground the wire for the temp sensor to the battery while it was unplugged. He said then turn power on and if gauge sweeps all the way up then the gauge is fine. He said gauge should go back to zero when I turn key ignition off which it did. He then said to test the sending unit it sell take my voltmeter and he told me what setting to put it on but cant remember. He said take positive side of my meter to the side of sending unit post and then negative side of meter to top of sending unit post. He said what was the reading when car was cold, and it was .3 He said now warm the car up and turn the car off. Do the same test and that number should be slightly larger closer to one for a stronger ground when car is hotter cuz thats what makes gauge work. Well I warmed car up for long time, shut off and did same test and still read .3, top of sending unit not hot at all so is it suppose to be then?

 

[miller]

One question...on the sending unit, did you use a thread sealer on it?
Threads into the water pump housing is how it gets ground!

 

[dadsbee]

You should test all your senders before install ! My piss off this year was a brand new oil pressure sender that was grounded out giving full scale on the gauge. I tested the coolant temp and fuel level sender before install, but didn't do the oil. All three have the same approximate 10 to 70 ohm range to work correctly WITH A RALLYE DASH (as all three gauges are the same with different faces on them). A standard dash with oil idiot light is simply an on / off switch with zero oil pressure = switch closes to light the red light.

10 ohms is required for the full fuel/ high heat / high oil pressure region on the gauge needle and 70 ohms being the empty / cold / no oil pressure region. Put your meter on ohms and clip a lead to the wire attachment stub and the case. Cold it should read over 70 ohms. Leave it hooked up and warm it up. Ohms should head towards 10. If it reads less than 70 ohms cold then your sender is pooched.

 

[beanhead]

I like to test the temp senders cold to get the ohm value, then put them in boiling water to make sure the reading drops to the proper range (10-15 ohms like dadsbee stated above) it seems to be the same for the non-rallye gauges also(at least in the '68)

 

[dadsbee]

I like to test the temp senders cold to get the ohm value, then put them in boiling water to make sure the reading drops to the proper range (10-15 ohms like dadsbee stated above) it seems to be the same for the non-rallye gauges also(at least in the '68)

Was the oil pressure only I was alluding to that would be different in a standard cluster with just an "idiot" light. The fuel and engine temp senders are the same as Rallye's.

One other thing, if you want to test your gauge for accuracy. Instead of just grounding it to see it sweep, put a 10 ohm resistor between the gauges sender wire and ground and it should show exactly full on the gas gauge and same "full" position on the oil and engine temp gauges.

Other picture shows how to verify an oil pressure sender for rallye gauge.

 

[dcoland70RR]

Thanks for the help

 

[dcoland70RR]

Just because its new, doesn't mean it works. Easy way to test the sending unit, put it in a vise upside-down, hook up a ohm meter to the lead out on the sending unit and ground on the vise, make sure you have continuity between vise and sending unit base, use a propane torch or lighter if you have either handy, and put the flame on the area that goes into the manifold and see if the ohm meter moves. If it doesn't, then you found the problem.

Thanks, tried that to this brand new temp sending unit and no movement in the gauge. So clearly this new 9 dollar factory sending unit is defective, Thanks