Tachman Voltage regulator for gauges

[bcorbin 10]

I'm looking for someone who may have installed one of these.

 

As a purely preventative measure I purchase one of these and installed it this weekend and none of the gauges worked correctly afterwards. They would never go past the low indication for the given gauge. I ended up reinstalling the original mechanical regulator and they all work fine again.

 

I just did the calibration check of each of the gauges (10ohm and 73ohm) and they are all pretty close to where they should read. I also connected up the Tachman's regulator on my work bench and and applied the 10 ohm and 73 ohm load and the regulator appears to work fine. I even went a step further and loaded his regulator with 5 ohms and it still worked fine. In theory the regulator when installed would never see less than about 20 ohms so this one does pass the bench test. The difference between the bench and the car is that the car has an inductive load from the gauges. That I don't have an easy way to duplicate.

 

Any thoughts? OK besides if it works don't fix it.

 

Thanks

[69RavenConv 286]

My first thought would be check the ground connection. These devices are pretty simple; 12V goes in one pin from the ign switch, 5V comes out the other pin, and the case provides ground. Without a good ground, the output will just float around and could cause behavior like you describe. Since your gauges work a little, the 12V and 5V connections are probably ok.

 

You might try sanding the contact surfaces lightly and make sure everything on the new part mounts up like the old one, especially the spots where there's any metal-to-metal contact, which is where the ground comes from.

[69RavenConv 286]

I had another thought (two in one day is good for me:tongue_smilie:), I notice on his website that his unit shuts down if it's not grounded or overloaded, but he doesn't say what he considers an overload. You may want to check for an overload condition (you may have a short in a sender, or his regulator might be defective, or his design might be marginal). Maybe disconnect one or more gauges and see if the others start working.

[ForceFed70 14]

Mechanical regulator? Does such a thing exist?

 

Why not use an OEM style repalcement regulator?

[69RavenConv 286]

Mechanical regulator? Does such a thing exist?

 

Why not use an OEM style repalcement regulator?

 

Believe it or not, the original instrument panel voltage regulators are electro-mechanical. They're simple bimetallic points that actually switch the output between 12V and 0V fast enough to get an "average" of 5-6V at the output. The "tachman" and other aftermarket versions are solid state and give a steady 5V at the output.

 

I understand that Ford used this version into the early 1990's and the OEM parts will be the bimetallic style.

[jag 10]

This is just a tip on electrical contacts. Never sand paper them. You can end up leaving very small grains of sand in the contact and actually make the connection worse. What has always worked for me is a #2 pencil eraser. It takes the corrosion off without removing the conductive surface. I used to use this many years ago in electronic repair.

JAG

[bcorbin 10]

Thanks for the comments. The fact that the original regulator is mechanical and as far as I know 40 years old is what made me want to replace it with the solid state version even though the original seems to work fine. I was really looking for the protection offered by the new versions.

 

The Tachman's regulator actually simulates the old style where it too switches the 12 volts on and off to get an average of 5 volts. I looked at the output of his regulator on an O-scope and believe the problem with the one I have is that the duty cycle of the switching only averages out to about 1.5 volts. It switches the 12 volts on for 80 milli-seconds every 650 milli-seconds. When you do the math for that it only averages 1.5 volts. A simpler example would be that if it switched the 12 volts on for 1/2 second every second the average would be 6 volts.

 

Unfortunately the Tachman is not very interested in helping me troubleshoot this and prefers I just send it back for a refund. If he gets it back and it fails his test he is willing to send me a new one though. The electrical engineer in me wants to understand what is going on though and I really want to get a solid state regulator installed and working. If my math and understanding are mistaken and this unit is working as it should then I have a problem with the wiring in the car and want to fix it. I am just not 100% sure one one way or the other yet. I have a couple of other tests to try and then either way I send it back hopefully to be replaced.

Thanks again for any comments or suggestions.

[69RavenConv 286]

Let us know what you find out. I was toying with the idea of replacing mine with a 7805 regulator chip while I have the dash apart but, like you, my original is still working fine. It sounds like you're pretty savvy about electrical stuff, I'm really curious what you figure out.

 

Good advice about the pencil eraser too, jag. Thanks

[ForceFed70 14]

Let us know what you find out. I was toying with the idea of replacing mine with a 7805 regulator chip while I have the dash apart but, like you, my original is still working fine. It sounds like you're pretty savvy about electrical stuff, I'm really curious what you figure out.

 

 

Thanks for the explanation. :thumbup1:

 

I know a fair bit about electronics myself, but had always assumed these cars use a newer style regulator.

 

I would think that an LM7805 or similar would work really well. You'd probably need to figure out a heatsink tho.

[69RavenConv 286]

Yeah, it turns out those reproduction regulators the Mustang suppliers sell for $39 are just a $1.50 7805 mounted inside a case that looks like the original (apparently, the "tachman's" is different, like bcorbin described, I've never seen one in real life). Since it's hidden, who cares what it looks like? I figure you could just bolt a 7805 to a strip of aluminum sheet metal. The only tricky part is the connectors/pins. It would be nice to find a trashed instrument cluster to get the regulator and gut it -- I don't want to gut my perfectly good one.

[bcorbin 10]

I found some gauges on EBAY that are correct for my car and I received them yesterday. Connecting them all up on my workbench with the aftermarket regulator connected to a 12 volt battery I found that they all work fine and so does the Tachman's voltage regulator. That's bad news for me as it means I have to track down why it doesn't work in my mach 1. I probably won't tackle this for a while but if I find the problem I will post what I find.