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MG TD TF 1500 - Wiper Motor Wire

I know there has been much said already about the appropriate wire for use with the TD wiper motor. Check the archives. I've never been at all happy about the use of the ultra thin audio wire to feed the wiper motor. Both Moss & AS sell this wire & the reason for its popularity isn't because of its current carrying capability but simply because of the colour of the covering! This grey shielded audio wire is similar looking to the grey covered black & green pair used originally (although the tiny wires themselves from memory are red & yellow). For my money a case of form over function. The following is an extract written by a guy called Evan from Dayton Ohio who posted his blog on Monday February 05 2007 titled MG TD Auxiliary fuse panel installation:

"If you look at the stock arrangement on the TD/TF you will notice several problems. The most obvious is the wiper motor. The wire used to power the wipers is much smaller than the car's standard 14/0.30. The carrying capacity is very low, yet it is protected by a 35 amp fuse. In the event of a short, the wire will glow before the fuse blows. This is just one example of under protection. The bigger problem is the lack of fuse protection for the majority of the cars circuits. The main purpose of a fuse installation is to protect wiring and prevent fires".

Clearly if the original wire is reckoned to be undersized I shudder to think what Evan would make of the use of audio wire! It has been suggested that by twisting both wires together to make one conductor & using the shielding as the second conductor in order to beef up the current carrying capacity, that all is well. Clearly this is still way thinner than the original which Evan describes as much smaller than 14/0.30 gauge. In the event of a short this audio wire will burn long before the fuse blows. Could this set fire to the car? Probably. So would I use it? Definitely not! He goes on to say:

"It is important to select fuses that will provide good protection. Dave DuBois helped me determine good starting values for each circuit. You can see the values on the wiring diagram above. Unfortunately, we could not find good data for current draw, so educated guesses were used. I recently acquired a heavy duty digital ammeter. I will be testing the cars circuits for current draw in the near future. Watch for a follow up article with better values... Thanks to Dave Braun, Jim Budrow, Dave DuBois and Dave Rome for their help with this project".

I've not seen the follow up to this blog & would be very keen to see what the Daves have to say on the use of the audio wire. My solution was to use two 14/0.30 wires one black & one green rated @ 8amps & covered with a grey heat shrink. This solution both looks as good & performs as well if not better than the original. I think I might also fit an appropriate in line wiper fuse behind the dash, just for good measure. Food for thought.. The pic below shows something we should all strive to avoid. Cheers
Peter TD 5801



P Hehir

Peter - I spent my working years as an electronic technician, most of it in the world of communications systems. The picture you attached is what we would call tuning for maximum smoke.

Somewhere, I have a chart that gives the specification for current carrying capacity for a given wire size (for automobile use) at different lengths. I will try to dig that out and post it here. The wire size given in the chart is all AWG, but there is a conversion in the British Wiring catalog that compares the British and American wire sizes. One of the difficulties in selecting a fuse for a given circuit is the way standard fuses are rated. The older British fuses gave 2 values (17/35 for instance). The lower number is the current that the fuse will hold indefinitely without blowing, and the larger number is the current that will cause the fuse to blow instantaneously. Standard American fuses only give one value - that of the current where the fuse where the fuse will not blow at all (the same as the lower value of British fuses), but the current level that will blow the fuse instantaneously twice the current level printed on the fuse. This makes fuse selection for the layman difficult to determine. Fast blow fuses can be purchased, but they are usually considerably more expensive and usually very low current levels (used in meters and other test equipment)and they are a pain in the back side in that any slight current surge will cause them to blow, even though the surge will not harm the wiring.

I agree with your assessment of the wire for the windshield wipers in the TDs. Even the original wire for them was entirely inadequate for the current drawn by the wiper motor, thus the motor spins v-e-r-y s-l-o-w-l-y at best. I like you idea of larger wires encased in gray heat shrink. Cheers - Dave
D W DuBois

Thank you Dave. If you were to install a fuse behind the dash in an attempt to protect the 8 amp wiring I've used could you perhaps suggest a value? Cheers
Peter TD 5801
P Hehir

Just looking thorough my pics on the wiper motor & thought I'd post the item below. This an original NOS wiper motor that sat on the top shelf gathering dust & pigeon crap at my local auto electricians for about 50 years prior to me buying it about two years ago. What is really interesting is the wiring that came with the motor, which was still in the original box. Not the green & black pair as used on the TD (which of course was part of the harness) but it's the gauge of the wire that's well worth noting. It appears to be quite substantial suggesting it could handle a sizable current. If the pair were green & black I would probably have used them as I thought the period woven grey exterior wouldn't look too out of place. Cheers
Peter TD 5801

P Hehir

Why would one wire a circuit that carries no more than a 2 amp load with wire that can readily carry 8 amps? At stall the TD wiper motor only draws about 2 amps. The twisted pair of audio signal wires is more than adequate to carry this current with the shield used as the return. If I were fusing the wiper circuit (and I probably will one of these days) I would put in a 3 amp fuse. Bud (retired EE)
Bud Krueger

Peter,

Nice to see a pic of my TD3966. Thanks!

On that precious moment and in a split second I decided to take a pic. Good timing! No damage by the way, we managed to undo the battery in time. It was all a matter of seconds...

Thanks again, Jasper
JL Nederhoed TD3966

Jasper: You are now an TD/TF BBS meme :-)

http://en.wikipedia.org/wiki/Meme
Christopher Couper

Chris,

I am really honoured!

;-)

Jasper
JL Nederhoed TD3966

Peter - " If you were to install a fuse behind the dash in an attempt to protect the 8 amp wiring I've used could you perhaps suggest a value?"
If you can get a 3 amp fuse, I would go with that or a 5 amp fuse.

I was not aware that the original wire was contained in a braided sheath beneath the outer insulation. The wire that came in the TD we have came with just two very small gauge wires with no braid around them. upon restoring the car, I replaced it with some 18 AWG wire twisted together. Cheers - Dave
D W DuBois

Jasper I'd often wondered where that pic came from & I'm pleased I can now put a name to it. It's an amazing shot! Dave as I've not seen the original wire, (apart from the wire in my pic which is not shielded), I'm not convinced the original was shielded. Shielded wiring is used primarily to minimize electrical interference, which just isn't an issue with a TD wiper motor. I was wondering Bud how you arrived at the two amp figure & what you measured the rating of one of the audio wires at? In the applications this wiring was designed for the current draw is in the order of milleamps. Presumably you found it necessary to twist two wires together & use the shielding as the return (which is an unusual practice & one I'd not seen before), because you'd established that one wire was inadequate for the task? I also wonder why Evan & others believe the original wiper motor wiring was underrated? Cheers
Peter TD 5801
P Hehir

Peter, when I was rebuilding the wiper of 'the53' I measured the current being drawn. To determine the adequacy of the 'audio wire' for the application I measured the voltage across the cable from the source to the load. I didn't record the values, but I found that the drop from using the pair, twisted as a single wire, was inconsequential. It was not an issue of inadequacy. The wire was there so I took advantage of it. A single wire would have been adequate to operate the wiper motor. Hook up an ammeter and measure the draw yourself. Bud
Bud Krueger

Dave: You are correct. It was not contained in a ground sheath. Its just that today's wires with a similar profile have one so Bud and others decided to optimize the conductivity by bundling conductors and using the ground.

I found in the late 60's a source of the original wires, but red and black. It was a European phone cord wire. Similar to that curled wire we had on land phones then in the US but a little larger. I still use it on my car today. No problems carrying the load of the windshield wiper motor.


Christopher Couper

Bud;
I measured the current on my motor when I rebuilt it. As you noted earlier it is in the 2 Amp range, varying with position and load.

I also measured the voltage drop in the (As now) supplied wire and it it very small when the two wires are used in parallel against the shield as the ground conductor.

Jim B.
JA Benjamin

Bud when I connect up the motor in situ I'll take your advice & measure the current under load. Chris when Dave said "he was not aware etc" I understood him to mean he believed it was shielded, not that it wasn't. English is indeed a tough language. Jim when two wires are twisted together they are electrically connected "in series", effectively becoming just one wire, as each "leg" is not carrying a different load. Obviously the motor can & does function using the aftermarket wires but the BIG problem with underrated wiring is that in the event of a short the wiring itself becomes the fuse & combusts long before the 35 amp fuse blows! Could this cause a fire? Absolutely! The point of a fuse surely is to protect the wiring & more importantly prevent the car from going up in smoke. This is the point Evan was making. By increasing the capacity of the wiper wire & protecting it with an appropriate fuse, the potential for fire caused by a wiper short is eliminated. Cheers
Peter TD 5801
P Hehir

Prter. I concur with Bud. The draw on the wiper motor was just over 2 amp

Cheers

Bill Chasser Jr.
TD4834
W. A. Chasser Jr

Thanks Bill. But the problem still remains however. A 2 amp plus load protected by a 35 amp fuse using puny shielded audio wiring that was never intended for this purpose is, in my opinion, just asking for trouble. Cheers
Peter TD 5801
P Hehir

I didn't care for the Moss wire. I found that clear speaker wire had the highest ratio of copper to overall wire footprint. I think I used 14 gauge and it fit nicely. I could not think of any reason not to use it. Heat shrink gives it a better look where it exits the windshield surround. I ran a hot wire from the ignition switch to a small fuse panel on the wood beam behind the dash for the connection.

Jim
J Barry

Peter, when the two wires are joined together at each end they are in parallel, not in series. The current through each leg will be virtually identical. If you look at the insides of the wiper motor you'll see that the wires in the armature and the pole pieces are quite fine. The parameter that is important in motors is a thing called 'ampere-turns'. In this case the design is to have a small number of amperes and a lot of turns. The wires in the 'audio' cable are much larger than those in the windings. Bud
Bud Krueger

You might consider Jim Barry's solution and just use appropriately colored gray heat shrink to match the color of the original wire where it is visible...
Geoffrey M Baker

Bud, I'm not an EE but as an instrument fitter I did study electrical theory. I do know that the difference between series & parallel, in simple terms, is that in series the load lies between the + & - terminals & when connected in parallel the path is forked & there are separate loads on each of the legs. In this case as there is no separate load the wires are effectively in series, albeit separated by insulation. Take a length of insulated wire, cut it exactly in half, strip the insulation & then overlay the wires together. Place an earthed load at one end & the live terminal at the other & I'm sure you'd agree you have a series connection. This is exactly what you have done (except that you didn't strip the insulation), which in electrical terms makes absolutely no difference. The proviso is of course that the wires are of equal resistance & equal length. The current though each "leg" then WILL be identical. While they may appear to be connected in parallel they are in fact connected in series. Semantics I know but the use of the term parallel in this case is misleading. I hope this clarifies my use of the word series. I've pulled a couple of wiper motors apart & you are correct however about how fine the armature windings are. I was unable to find anyone here able to rewind one. Cheers
Peter TD 5801
P Hehir

“At stall the TD wiper motor only draws about 2 amps”. When I rewired my car I used Evan Ford’s article as a guide to add fuses. The 4-amp fuse he recommends blows after about half a cycle of the wipers. I haven’t gotten around to trouble-shooting it yet because I never plan to drive in the rain and it may never rain again in Southern California anyway, but it would still be nice to have a working system. Any suggestions about where I should start trouble-shooting?

Incidentally, I didn’t like the Moss wire either, so I reused the original wire which was still in serviceable condition.

Joe
Joe Olson

I am sorry Peter but they are in parallel.

Consider each wire as a resister, which it is, albeit a small one.

CASE 1
If I twist the two conductors on one side together and DO NOTHING ELSE, and connect the current source to the two remaining wires then the SAME current flows through the resistors and they are then in series. Each wire will have a voltage drop and if the wires have the same resistance the voltage drops will be the same and will be twice the drop as you would have if you used only one wire and left the other alone, unconnected.

cASE 2

However if I twist the conductor together at BOTH ends and connect the same current source as I had before to the (now connected) ends the current has two separate routes and it divides. Assuming both paths have the same resistance, 1/2 of the current flows through one wire and one half through the other wire.

Since the current has divided the voltage drop is 1/2 that of a single wire of 1/4 that of the two wires connected at one end only.

Note that I use a current source. In essence the resistance of the wiper motor being much larger than the resistance of the wire turns any voltage source into a current source (for a small additional resistance).

If I were to connect a voltage, say a battery to the wires the resistance would be so low that the wires could melt.


I suppose one could use a very low voltage source, say a few milivolts and still be OK.

However the wires connected together at both ends and then inserted into the current path are parallel resistors.
Only if the wires were connected together at one end and inserted into the current path would they be in series.

Jim B.
JA Benjamin

Peter, I am an EE (retired). The concept of parallel and series is really fairly obvious and simple. If two conductors are connected to the same node at each end they are said to be in parallel. Enough. Bud
Bud Krueger

OK Jim, but most of us would ignore the resistance of the wire as a load when considering the application we're discussing, as the only load we're concerned about here is the wiper motor. The point I was making is that effectively the wires are in series as there is no load on either leg (apart from the voltage dropped due to the resistance of the wire itself, which is equal in each leg). The path is terminal, wire, load, wire & earth. A standard series connection. But have it your way. Like I said. Semantics. Cheers
Peter TD 5801
P Hehir

I agree Bud. Enough.
P Hehir

This thread was discussed between 05/01/2015 and 07/01/2015

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