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MG MGA - Crane XR700--positive or negative or either?

Just bought a '59 A 1600. It already has the Crane XR 700 in it. I'm switching from positive ground to negative ground and am wondering if the unit is polarity specific.

Crane has gone belly up. The Moss sales rep checked back into previous catalogs and stated that the unit was sold as being usable either way. But a friend who has one in his Frogeye states he thinks his was bought as polarity specific.

Anyone have a definitive answer?

JM Morris

I have the instruction sheet, which makes no mention of polarity. It does however frequently refer to connection to the coil+, showing Red to coil+ and yellow to coil-, Black to ground. Seems they would mention if it matters. All connection diagrams show Batt to coil+.

Fletcher R Millmore

Crane Cams is re-opened under new ownership effective April 2009. Here is the link to the instruction sheet. It appears that the same unit works for either ground scheme. Make sure you re-wire when you make the switch.
Chuck Schaefer

>> It appears that the same unit works for either ground scheme.

Look at the "12 VOLT POSITIVE GROUND TECH NOTE" at the top of page 18 of that document where it states...

"WARNING: Use this hookup only for special XR700 12 volt positive ground ignition part number 720-0231. Do not use standard XR700 12 volt units for positive ground!"

I'd take that as a pretty good indication that these units ARE polarity sensitive.
Andy Bounsall

I looked at this when I posted it on my web site two years ago, just reviewed it again to be sure. The same unit can be wired for either negative or positive earth setup. Compare the diagrams on pages 10 and 18 of the PDF instructions (noted in prior post). The module only needs + and + battery connections, and it cares not which one happens to be grounded to the chassis. The new pickup module in the distributor does not ground through the distributor body, so the entire electronic system is "floating" until one of the power leads is grounded.

The XR700 unit is designed for ballasted ignition system, needing a 1.6-ohm coil and 1.6-ohm ballast resistor. The electronic unit then normally runs on 6-7 volts and tolerates a momentary 12 volt input during crank starting (same as the ballasted ignition coil).

The XR3000 unit is designed to operate continuously on 12-14 volts. This unit would be better suited for the older cars with no ballast resistor and 12-volt coil.

To use the XR700 unit in the MGA you need to use a 1.6-ohm coil and add the ballast resistor so the coil and electronic unit run continuously on 6-7 volts. There will be no bypass wire for full system voltage during cranking. System voltage may drop to 10 volts during cranking, and coil & module input may drop to 5 volts, but it should still spark okay during cranking.

With the XR700 unit in the MGA, if you want to use the full voltage bypass for cranking you can connect to the output side of the starter switch, but then you need to add a diode (10 amps capacity) in series to prevent drain back through the starter motor during normal operation while running. Alternately you could install a relay triggered by the starter switch to connect full system voltage to the coil momentarily during cranking.
Barney Gaylord

Seems my instructions were for a specific unit to convert from Lucas electronic systems.
I tend to agree with Barney on the universal theory, but I note that the diagram for the "positive ground" unit shows Black as being connected to IGN switch power, R & Y same for all. This makes sense, where the Black to ground seemed not right for pos grd. It could still bite you if the neg grd units have an internal connection such that the case is in some way connected to Black. I would expect that pos grd units should be prominently marked as such.
I also saw at least one error, in that SW & CB are said to be + & - respectively in one case, and reversed in another. AFAIK, all coils marked so are SW = -, CB = +, having come from the pos grd era.
And, it is up to you to be certain that the coil is connected correctly before using coil markings to determine circuit polarity as wired.

Fletcher R Millmore

According to Crane Cam's website, the current 700-0231 offered fo sale is rated at:

8 to 18 volts, reverse polarity protected, negative or positive ground. Not compatible with 6 volt electrical systems as these may drop below 4 volts during cranking.

Some searching on the web implies there were design chnages over the years. Also that the older ignition systems were polarity sensitive. You had to specify ground polarity when ordered.

Since you don't know the origin of your unit and it is currently wired to Pos Ground, I would try to find the full P/N on the unit and/or a date of manufature and then contact Crane Cams to find out if your unitwill work on either ground system.
Chuck Schaefer

Excellent information! Chuck's last post is what had me concerned--ie, possible design changes. Also, I noted that there was a ballast resistor on the inner fender wall that was out of the loop--not being used. Looks like I've got some resistance checking to do--both for the ballast resistor and the coil. The coil appears to be relatively new.

My initial thoughts were to simply switch the two coil wires, but if I understand this correctly, I will be leaving the wires where they are (XR700 pos wire-red-to coil pos side and XR700 black-neg-wire to coil neg side. And then switch the coil ground wire from the pos side of the coil to the neg side of the coil. All assuming that the unit is universal polarity. Does this sound right?

Thanks so much!
JM Morris

Correction to previous post: I should have said: "XR700 yellow wire goes to neg post of coil", not the black wire.
JM Morris

Update: Went to the Crane website to check info on the XR700 and there is a footnote stating that the later models are universal (non polarity specific) which implies the earlier models WERE polarity specific. The model number on mine is nowhere visible. Will have to remove and check the bottom for a model number. And then hope Crane will answer their phone which is non toll free. They wouldn't answer as of noon today.
JM Morris

Update: Just talked with the Crane tech department. I asked how I could tell if the XR700 unit I have is universal polarity or polarity specific. He stated that if the unit is polarity specific (in my case, positive ground) it will be stamped on the bottom of the case "positive ground". Presumably, if the unit is negative ground only, it will be stamped thusly. So that solves it. I'll find out this weekend which one mine is by removing the unit and taking a look at the bottom of the case.
JM Morris

This thread was discussed between 10/03/2010 and 26/03/2010

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